312 GENERAL DISCUSSION Dr. H. A. Dewhurst (Edinburgh Utziversity) (cotnniimicated) With regard to GENERAL DISCUSSION Dr. Dale’s interpretation of enhanced radiation effects an alternative mechanism has been proposed to account for the enhanced oxidation of ferrous sulphate produced by the addition of aliphatic alcohols l o aerated ferrous sulphate solutions.1 Here the enhanced yield has been attributed to a competitive reaction between the Fe2+ ion and the organic solute for the OH radicals with the production of an organic free radical. This organic free radical reacts with dissolved oxygen to form an organic peroxide which ultimately brings about an enhanced yield. A I Deuhur\t .I. Cheiji. t ’ / ~ ~ ‘ i i c ~ 1951. 19 1329. GENERAL DISCUSSION hit. Physiol.1951 59 442. 10 Bacq rind I-lerve Bull. Acari. Roj,. Me'd Belg. 1952 18 13. 313 similar type of mechanism has been postulated by Hart 2 to explain the enhanced oxidation of ferrous sulphate in the presence of formic acid. This peroxide mechanism could also account for the increased reduction yield found by Fricke and Brownscombe 3 for aerated dichromate solutions containing aliphatic acids ; and also for the increased ceric ion reduction yield produced by various organic solutes which was reported by Clark and Coe.4 IL may well be that these results are of biological significance because it is well known that many radiobiological reactions are greatly enhanced by the presence of oxygen. In this connection ii is noteworthy that a peroxidic mechanism has been indicated to account for the post irradiation effect observed with aerated solu- tions of deoxyribonucleic acid.5 Dr.W. M. Dale (Christie Hospitul Manclzestw) (communicated) Dr. Dewhurst's suggestion that an organic peroxide formed from an aliphatic alcohol is the cause of an enhanced radiation effect i s very plausible and could be valid in some cases. It is however known that alcohol in oxygenated systems can also suppress radiation effects i.e. can act as a protective substance.6 7 8 It seems therefore relevant to ask why alcohol does not always react as an oxidative agent via a peroxide. Prof. 2. M. Bacq (Liigc Belgium) said The biologists are much interested in this phenomenon of chemical protection against ionizing radiations studied by Dale and his collaborators.Recently we found that many amines (simple ali- phatic or aromatic amines) have a remarkable protective action. But the best seems to be P-mercaptoethylamine (cysteinamine) HS-CH2-CH2-NH2.91 dose of 700 r kills all the mice of my pure stock. If one injects 3 mg of p-mer- captoethylamine to 20 g mice before irradiation 95 out of 100 animals survive after 700 r. Dr. Philpot at Harwell has been kind enough to repeat those experiments and he has confirmed my results (6 mice weighing40 g received 6 mg of the amine all survived to a dose of 950 r). 6-mercaptoethylamine is a non- toxic substance very suitable for human use. The oxidized -S-S-derivative is unexpectedly as active as the reduced form. Both the thiol and the disulphide 10 A are completely inactive when injected immediately after irradiation.A possible explanation is that these amines inhibit the action of free radicals on living matter. Amines protect the oxidation of ferrous ions to ferric. The disulphide seems more active than the reduced compound in low concentration and less active in higher concentrations. In our experimental conditions (3 ml of 10-3 M ferrous ammonium sulphate in 0-8 N sulphuric acid saturated with air) a constant dose of 20,000 r (1,000 r in 35 sec 50 kV focal distance 4 cm) oxidizes 25 ;{ of the ferrous ions. The presence of /%mercaptoethylamine 5 x 10-4 M rcduces the percentage of oxidized ferrous to 20 %. The same Concentration of S-S/2 reduces the oxidation to 10 %. If one raises the concentration of cysteinamine to 2-5 >.10-3 M the oxidation is only 2.5 % (= 90 :( protection); in order to obtain the same protection with the corresponding disulphide one has to reach a concentration of 8 " 10 -3 M/2. 2 Hart J . Amer. Chenr. Soc. 1952 (in press). 3 Fricke and Brownscornbe J. Amer. Chem. Soc. 1933 55 2355. 4 Clark and Coe J . Ciiem. Physics 1937 5 97. 5 Butler and Conway J . Chem. Soc. 1950 3418 ; 1952 834. 6 Fricke Hart and Smith J . Ciiern. Physics 1938 6 229. 7 Hollaender Stapleton and Burnett Isotopes in Biochemistry a Ciba Foundatioii Conference (Churchill Ltd. London 1951) p. 96. 8 McDonald Aim. Report (Dept. Genetics Carnegie Institution of Washington Cold Spring Harbour N.Y. 1949-50). 9 Bacq Herve Lecomte Fischer Blavier Dechamps Le Bihan and Rayet Arch.GENERAL DISCUSSION 314 The -SH substance is oxidized to S-S (up to 6 x 10-4 M) by irradiation in the same conditions (fig. 1 ) ; it reduces 50 % of the ferric ions at a concentration of 10-2 M (fig. 2a). If one adds 0.25 M ferric + 0.75 M ferrous solution to the same irradiated solution of mercaptoethylamine no reduction of ferric ions takes place (fig. 26). Thus the action of ,B-mercaptoethylamine cannot be explained by a simple reduction of the ferric ions formed during irradiation. El SH /f3 / 2 3 4 5 6 ;7 8 9 FIG. 1 .-Oxidation of p-mercaptoethylamine at different concentrations by a constant dose of X-rays (20,000 r ; 50 kW ; focal distance 4 cm ; 1,000 r/35 sec) ; acidity of solution 0.8 N HzSO4.ordinates concentration of S-S/2 (cystinamine) ; abcissae concentration of /3-mercaptoethylamine (SH). FIG. 2.-Ferric ions reduction of a solution containing the same ferrous-ferric ions concentration as an irradiated ferrous solution (0.25 x 10-3 M ferric; 0.75 x 10-3 M ferrous ; 0.8 N H2S04). Dosage by o-phenanthroline or- dinates % reduction abscissae concentration of P-mercapto- ethylamine. (a) F-mercapto-ethylamine (b) irradiated (20,000 r) 8- mercap to-ethylamine. GENERAL DISCUSSION 315 Dr. A. J. Swallow (Birmingham University) said Dr. Bacq has mentioned that certain substances will protect animals against lethal doses of radiation. Cysteine is quite a good protective substance and I have irradiated pure solutions of cysteine in order LO investigate the reactions occurring.11 Cysteine is oxidized to cystine and in concentrated solutions in the presence of oxygen there is a yield of 24 molecules of cysteine lost for 32.5 eV absorbed.This is a rather large yield and is probably due to a chain reaction. I have suggested the following mechanism RSH + OH -+ RS + H2O RS + RSH -+ RSSR + H H + 0 2 -+ HO2 11 Swallow J. Chem. SOC. 1952 1334. H02 + RSH -+ RS + €3202. Other workers have detected hydrogen peroxide in the system.12 In the absence of oxygen the yield for concentrated aqueous solutions is about 3. Dr. Dale said that he had also irradiated cysteine and he confirmed that large yields were obtained. In his experiments the yield fell rapidly when more than 10 % of the cysteine was oxidized.The ionic yields and the reactions given above are for the early part of the reaction where yield is still proportional to dose. Dr. George wondered whether the reaction RS + RSH -+ RSSR + H was not endothermic. This reacrion is endothermic but even so may possibly occur. The irradiations were at a low dose rate and a high yield was obtained only with concentrated solutions of cysteine. If the reaction RSH + OH 2- RS + H20 occurs then the thiol radical so produced will have many collisions with cysteine molecules before meeting any other possible reactants. I have not yet investigated the effect of temperature on the reaction. Dr. J. Weiss (Durham University Newcastle) (communicated) It would be interesting to know how much cystine is actually formed in this reaction because it is very likely that in the presence of oxygen there are also some oxygen-containing products formed.If that is the case the actual chain mechanism will be rather different from the one suggested by Dr. Swallow which also meets with some difficulties from a thermodynamic point of view. Dr. W. M. Dale (Manchester) said With regard to Dr. Stein’s remarks on methylene blue we can in general confirm his conclusions. However the problem is rather more complicated than would appear at first because while the rate of bleaching of methylene blue in water is the same in oxygenated as in evacuated solution in gelatine as pointed out by Stein the evacuated system is bleached more quickly. Furthermore if you add thiourea to the aqueous solution it acts protectively on the dye in both oxygen-containing and oxygen-free solution while in gelatine the protective power is more doubtful.Experimental circumstances make a clear- cut decision between anaerobic and aerobic conditions difficult because of the induction period in the presence of oxygen and the decrease in diffusion rate of oxygen into the gel may give rise to a condition of partial lack of oxygen in the interior of the gel. I don’t know whether Dr. George was under the impression that we had worked with catalase. However that may be carboxypeptidase is insensitive to H202 and does not contain iron. Dr. M. Magat (Paris) (partly communicated) I think the results of Dr. Minder can be kinetically interpreted along the lines developed by Dainton and Miller 13 12 Rotheram Todd and Whitcher AECD UCLA-119 195 1.13 Proc. Znt. Congr. Pure Appl. Chem. (London 1947). 316 and Chapiro,l4 i.e. as a competition between the recombination of the free radicals and the attack of one of them on the organic chlorine compound with extraction of chlorine e.g. If 1 GENERAL DISCUSSION RCl -1- H - R* -i- HCl. ( 1 ) Since such a reaction would require a relatively high activation energy a rather large concentration of RCI would be necessary to catch all the free radicals produced and so obtain a yield independent of the RCl concentration and of the nature of the compound used. As was correctly pointed out by Dr. Minder the concen- trations would be so high that direct effects as well as secondary reactions would become important.The kinetics is so involved that it seems hardly possible to use this type of reaction for determining the G value of the solvent. However at low concentration the initial slope of the curve should be proportional to the concentration of C-C1 bonds and to the rate constant kl. For two different compounds in the same solvent and under identical irradiation conditions the ratio of the slopes is roughly given by K = 4 e x p (-) - RT AE* where n and n’ are the numbers of C-Cl bonds in each molecule and AE+ is the difference in activation energies which is about 0-3 AD where A D is the difference of the C-C1 bond dissociation energies. The compounds listed in fig. 3 of Dr. Minder’s papers belong to 4 different groups (i) c2cl(j (ii) RCC13 (iii) Y - C ~ H ~ C I ~ and (iv) P‘CfjH4C12.Comparing the slopes of groups (ii) and (iv) one gets AE* = 1 kcal and AD N 3 kcal a reasonable value for the difference between aliphatic and aromatic C-C1 bonds while the AE* between groups (i) and the (ii) is N 0.4 kcal. In general an increase in the bond energy when going from group (i) to group (iv) seems in keeping with the rest of our knowledge on the subject. Dr. W. Minder (Radium Institute Berne) said We compleLely agree with Dr. Magat that the curves fig. 1 and 2 of Dr. Maget’s and co-worker’s paper and fig. 3 of our paper are representing exactly the same things. Nevertheless the interpretations are somewhat difl‘erent. In our curves we plotted the yield of perhaps the most probable reaction of different solutes capable of the same reaction against concentration in the same solvent.Our yields are within experimental error proportional to the number of C1 atoms in the solute molecules both for the aliphatic and for the cyclic compoimds. The difference between the two yields per C1 atom is of course due to the different binding energies. Table 1 of Dr. Magat and co-worker’s paper shows the decomposition yields of one and the same solute (diphenyl picryhydrazil) in different solvents. If we were to try to interpret our results in a similar manner it would be neces- sary to assume the liberation of all the Cl atoms which are bound to one molecule of the solute by one radical produced from the solvent alcohol or the production of different numbers of radicals in the same solvent if different solutes are present.Furthermore it would be difficult to understand the fact that relatively large con- centrations of water in the solvent do not in any way influence the number of active radicals. Dr. W. Minder (Radium Institute Berne) said ; The total radiation dose within a spherical vessel containing a suitable radioisotope solution can be calculated exactly 15 if the decay properties of the isotope is known. We have compared such a calculation with the results for the oxidation by self-irradiation of a ferrous sulphate solution (0.005 N FeSO4 -1- 0-8 N H2S04) containing 0.1479 mc of Rb86 14 this Discussion. 15 Minder and H. Schindler Strahletttker 1952 86 602.GENERAL DISCUSSION 317 per cm3. The total radiation dose administered to the solution in such an ex- periment during the time t is D = KMG s’ exp (- At) dt = KMG -(1 - exP (- W) 0 x where K is the overall dosage rate (in r/mc hj M the total amount of the radio- isotope in mc X the decay constant and G the so-called geometrical factor which takes into account the form size and absorption and which is in every case a rather complicated mathematical expression. Comparing our irradiation results with the calculations (the irradiation was made in a closed vessel of 700 ml without adding 0 2 and the oxidation was deter- mined by titration with KMn04) we have found a total dose (t = a) of 193,000 r by calculation and 200,000 r by oxidation of Fe3+.This value was obtained with an ionic yield (Fe3+ per 32.5 eVj of 3. This yield is in good agreement with our earlier experiments with y-rays 16 and X-rays 17 and with the value gwen by Wright 18 for a 0.0025 N solution after oxygen removal. Dr. J. F. Suttle and Dr. J. W. Schulte (Los Alamos New Mexico) (communicated) For the past several months we have been irradiating dry pure chloroform solu- tions with gamma rays from a C060 source. The results to date indicate that good linearity is obtained up to 4 x 105 r when chloride ion in the irradiated solution is determined potentiometrically with silver nitrate. For 4 x 105 r approximately 0.18 mequiv. of chloride ion is found per ml of irradiated chloroform. However to obtain any significant decomposition by irradiation it is necessary that oxygen be present.We find it interesting that the authors of this paper make no mention that even traces of oxygen might have decided influence on the system. Prof. Milton Burton (University of Notre Dame Indiana) said In this Discus- sion we have witnessed two kinds of difficulty regarding expression of radiation yield. The first and more important is an experimental discrepancy in the G value for Fez+ -+ Fe 3+ in 0.8 N sulphuric acid solution dependent on the source of the information. Although the difference is presently unexplained we may confidently expect that an explanation will soon be forthcoming. The second type of difficulty results from the method of expression of yield. The custom persists of reporting M/N in cases where N is not and probably never can be experimentally known.The number of ion pairs is calculated on a variety of bases depending on logic prejudice admitted inadequacy of information or on purely formal advantage. Unfortunately readers who get only raw M/N values from summaries or abstracts sometimes hunt vainly through an article for details as to the basis of calculation. This latter difficulty is really an avoidable nuisance. Although occasionally an author can profitably include consideration of M/N in his reasoning in many cases nothing is contributed to our knowledge of mechanism by use of this nota- tion. Consequently irrespective of individual prejudices it is desirable that we join in a notation which has singularity of meaning.I have discussed this matter with several participants in the Discussion and we have jointly arrived at con- clusions which I have summarized as follows. G This notation (i.e. 100 eV yield) should be used whenever molecules pro- duced or converted and actual energy input are actually measured. Of course errors may occur in both determinations but such values are subject to check and the methods (if adequately described) are subject to criticism. G’ Sometimes energy input is not directly measured but is computed from other data e.g. from observations with a dosimeter. In such case the author should indicate both his methods and his conversion factors unambiguously and 16 Minder and Liechti Experientia 1946 2 410. 17 Minder Radiol.Clin. 1951 20 286 1951. 1 * Wright this Discussion. GENERAL DISCUSSION 318 use the symbol G’ to indicate that the 100eV yield is calculated rather than measured. G20 Yields may be measured in terms of another actinometer ; e.g. the highly favoured FeS04 actinometer. In a recent paper which I had the good fortune LO read prior to publication Johnson and Allen expressed yields in an aqueous system in terms of a comparison with Fezf -+ Fe3+ in such an actinometer. Since they explicitly indicated their method it will be possible at some fuwre time to establish their G’ values. Simple numerical values would have been useful if difficulty of interpretation could have been simultaneously avoided. It is sug- gested that in the future when the FeS04 + 0.8 N H2SO4 actinometer is used as a standard the conditions of use be explicitly stated G(Fe2+ -f Fe3+) be taken as 20 until an accepted value is established and the symbol G20 be used for expression of the calculated 100 eV yield.G (. . . ) For convenience of typography we suggest that the change to which G refers be indicated where necessary in parentheses after G ; e.g. G (H202 produced) or G (Fe2+ -z Fe3+). G 1.5 The papers of both Dr. Hardwick and Dr. Minder emphasize the signi- ficance of microscopic G value for molecules converted or produced per 100 eV in an increment of path where the energy of the effective particle is not significantly changed. Unlike the usual G values this is not an average value over the whole length of path. It tells of the chemical effect produced by a particle in a micro- scopic portion of its path when it has a very specific energy.We suggest use of subscripts such as shown where units of energy are in MeV. In any case we join in the urgent suggestion that explicit statements be made as to the quantities actually measured. Dr. N. Miller (Edinburgh Universify) (partly conmunicated) Prof. Burton’s suggestion that methods of expressing radiation yield be standardized is a timely one. I think that the moment is also appropriate for the standardization of units of dose and dose rate. Most workers in the field would agree that units of dose based on ionization such as the roentgen or rep are less satisfactory than absolute units such as eV/ml ergs/g. It should be emphasized however that full experi- mental details of all physical dose measurements should be included by authors on publication.With regard to the relative merits of the units eV/ml and ergs/g it may be pointed out that the former involves the use of very large numbers whereas the latter is currently receiving considerable support among medical physicists. Prof. Burton has made the further suggestion that yields observed in X- or y-ray induced reactions in aqueous media be expressed relative to that observed for ferrous sulphate oxidation under the same experimental conditions. Such yields could then readily be adjusted to conform with any future changes in the accepted yield value for the ferrous sulphate system. I think that such a procedure is quite satisfactory at low dose rates but that utitil more work is done on this method of dosimetry at high dose rates and general agreement on the yield value in this region is reached an upper limit should for the time being be set on the dose rate at which such a procedure is considered reliable.A figure of 1000 r/min would for the moment be a safe or even con- servative upper limit. Very probably this limit will be raised or even consider- ably raised when more becomes known about the general kinetics of ferrous sulphate oxidation at high dose rates. It seems almost certain however that the upper limit for the use of ferrous sulphate will prove to be lower than that for ceric sulphate or other systems which are not oxygen-dependent. 312 GENERAL DISCUSSION GENERAL DISCUSSION Dr.H. A. Dewhurst (Edinburgh Utziversity) (cotnniimicated) With regard to Dr. Dale’s interpretation of enhanced radiation effects an alternative mechanism has been proposed to account for the enhanced oxidation of ferrous sulphate produced by the addition of aliphatic alcohols l o aerated ferrous sulphate solutions.1 Here the enhanced yield has been attributed to a competitive reaction between the Fe2+ ion and the organic solute for the OH radicals with the production of an organic free radical. This organic free radical reacts with dissolved oxygen to form an organic peroxide which ultimately brings about an enhanced yield. A I Deuhur\t .I. Cheiji. t ’ / ~ ~ ‘ i i c ~ 1951. 19 1329 GENERAL DISCUSSION 313 similar type of mechanism has been postulated by Hart 2 to explain the enhanced oxidation of ferrous sulphate in the presence of formic acid.This peroxide mechanism could also account for the increased reduction yield found by Fricke and Brownscombe 3 for aerated dichromate solutions containing aliphatic acids ; and also for the increased ceric ion reduction yield produced by various organic solutes which was reported by Clark and Coe.4 IL may well be that these results are of biological significance because it is well known that many radiobiological reactions are greatly enhanced by the presence of oxygen. In this connection ii is noteworthy that a peroxidic mechanism has been indicated to account for the post irradiation effect observed with aerated solu-tions of deoxyribonucleic acid.5 Dr.W. M. Dale (Christie Hospitul Manclzestw) (communicated) Dr. Dewhurst's suggestion that an organic peroxide formed from an aliphatic alcohol is the cause of an enhanced radiation effect i s very plausible and could be valid in some cases. It is however known that alcohol in oxygenated systems can also suppress radiation effects i.e. can act as a protective substance.6 7 8 It seems therefore relevant to ask why alcohol does not always react as an oxidative agent via a peroxide. Prof. 2. M. Bacq (Liigc Belgium) said The biologists are much interested in this phenomenon of chemical protection against ionizing radiations studied by Dale and his collaborators. Recently we found that many amines (simple ali-phatic or aromatic amines) have a remarkable protective action.But the best seems to be P-mercaptoethylamine (cysteinamine) HS-CH2-CH2-NH2.91 10 A dose of 700 r kills all the mice of my pure stock. If one injects 3 mg of p-mer-captoethylamine to 20 g mice before irradiation 95 out of 100 animals survive after 700 r. Dr. Philpot at Harwell has been kind enough to repeat those experiments and he has confirmed my results (6 mice weighing40 g received 6 mg of the amine all survived to a dose of 950 r). 6-mercaptoethylamine is a non-toxic substance very suitable for human use. The oxidized -S-S-derivative is, unexpectedly as active as the reduced form. Both the thiol and the disulphide are completely inactive when injected immediately after irradiation. A possible explanation is that these amines inhibit the action of free radicals on living matter.Amines protect the oxidation of ferrous ions to ferric. The disulphide seems more active than the reduced compound in low concentration and less active in higher concentrations. In our experimental conditions (3 ml of 10-3 M ferrous ammonium sulphate in 0-8 N sulphuric acid saturated with air) a constant dose of 20,000 r (1,000 r in 35 sec 50 kV focal distance 4 cm) oxidizes 25 ;{ of the ferrous ions. The presence of /%mercaptoethylamine 5 x 10-4 M rcduces the percentage of oxidized ferrous to 20 %. The same Concentration of S-S/2 reduces the oxidation to 10 %. If one raises the concentration of cysteinamine to 2-5 >. 10-3 M the oxidation is only 2.5 % (= 90 :( protection); in order to obtain the same protection with the corresponding disulphide one has to reach a concentration of 8 " 10 -3 M/2.2 Hart J . Amer. Chenr. Soc. 1952 (in press). 3 Fricke and Brownscornbe J. Amer. Chem. Soc. 1933 55 2355. 4 Clark and Coe J . Ciiem. Physics 1937 5 97. 5 Butler and Conway J . Chem. Soc. 1950 3418 ; 1952 834. 6 Fricke Hart and Smith J . Ciiern. Physics 1938 6 229. 7 Hollaender Stapleton and Burnett Isotopes in Biochemistry a Ciba Foundatioii 8 McDonald Aim. Report (Dept. Genetics Carnegie Institution of Washington, 9 Bacq Herve Lecomte Fischer Blavier Dechamps Le Bihan and Rayet Arch. 10 Bacq rind I-lerve Bull. Acari. Roj,. Me'd Belg. 1952 18 13. Conference (Churchill Ltd. London 1951) p. 96. Cold Spring Harbour N.Y. 1949-50). hit. Physiol. 1951 59 442 314 GENERAL DISCUSSION The -SH substance is oxidized to S-S (up to 6 x 10-4 M) by irradiation in the same conditions (fig.1 ) ; it reduces 50 % of the ferric ions at a concentration of 10-2 M (fig. 2a). If one adds 0.25 M ferric + 0.75 M ferrous solution to the same irradiated solution of mercaptoethylamine no reduction of ferric ions takes place (fig. 26). Thus the action of ,B-mercaptoethylamine cannot be explained by a simple reduction of the ferric ions formed during irradiation. El SH /f3 / 2 3 4 5 6 ;7 8 9 , FIG. 1 .-Oxidation of p-mercaptoethylamine at different concentrations by a constant dose of X-rays (20,000 r ; 50 kW ; focal distance 4 cm ; 1,000 r/35 sec) ; acidity of solution 0.8 N HzSO4. ordinates concentration of S-S/2 (cystinamine) ; abcissae concentration of /3-mercaptoethylamine (SH).FIG. 2.-Ferric ions reduction of a solution containing the same ferrous-ferric ions concentration as an irradiated ferrous solution (0.25 x 10-3 M ferric; 0.75 x 10-3 M ferrous ; 0.8 N H2S04). Dosage by o-phenanthroline or-dinates % reduction abscissae, concentration of P-mercapto-ethylamine. (a) F-mercapto-ethylamine, (b) irradiated (20,000 r) 8-mercap to-ethylamine GENERAL DISCUSSION 315 Dr. A. J. Swallow (Birmingham University) said Dr. Bacq has mentioned that certain substances will protect animals against lethal doses of radiation. Cysteine is quite a good protective substance and I have irradiated pure solutions of cysteine in order LO investigate the reactions occurring.11 Cysteine is oxidized to cystine and in concentrated solutions in the presence of oxygen there is a yield of 24 molecules of cysteine lost for 32.5 eV absorbed.This is a rather large yield, and is probably due to a chain reaction. I have suggested the following mechanism : RSH + OH -+ RS + H2O RS + RSH -+ RSSR + H H + 0 2 -+ HO2 H02 + RSH -+ RS + €3202. Other workers have detected hydrogen peroxide in the system.12 In the absence of oxygen the yield for concentrated aqueous solutions is about 3. Dr. Dale said that he had also irradiated cysteine and he confirmed that large yields were obtained. In his experiments the yield fell rapidly when more than 10 % of the cysteine was oxidized. The ionic yields and the reactions given above are for the early part of the reaction where yield is still proportional to dose.Dr. George wondered whether the reaction RS + RSH -+ RSSR + H was not endothermic. This reacrion is endothermic but even so may possibly occur. The irradiations were at a low dose rate and a high yield was obtained only with concentrated solutions of cysteine. If the reaction RSH + OH 2- RS + H20 occurs then the thiol radical so produced will have many collisions with cysteine molecules before meeting any other possible reactants. I have not yet investigated the effect of temperature on the reaction. Dr. J. Weiss (Durham University Newcastle) (communicated) It would be interesting to know how much cystine is actually formed in this reaction because it is very likely that in the presence of oxygen there are also some oxygen-containing products formed.If that is the case the actual chain mechanism will be rather different from the one suggested by Dr. Swallow which also meets with some difficulties from a thermodynamic point of view. Dr. W. M. Dale (Manchester) said With regard to Dr. Stein’s remarks on methylene blue we can in general confirm his conclusions. However the problem is rather more complicated than would appear at first because while the rate of bleaching of methylene blue in water is the same in oxygenated as in evacuated solution in gelatine as pointed out by Stein the evacuated system is bleached more quickly. Furthermore if you add thiourea to the aqueous solution it acts protectively on the dye in both oxygen-containing and oxygen-free solution while in gelatine the protective power is more doubtful.Experimental circumstances make a clear-cut decision between anaerobic and aerobic conditions difficult because of the induction period in the presence of oxygen and the decrease in diffusion rate of oxygen into the gel may give rise to a condition of partial lack of oxygen in the interior of the gel. I don’t know whether Dr. George was under the impression that we had worked with catalase. However that may be carboxypeptidase is insensitive to H202 and does not contain iron. Dr. M. Magat (Paris) (partly communicated) I think the results of Dr. Minder can be kinetically interpreted along the lines developed by Dainton and Miller 13 11 Swallow J. Chem. SOC. 1952 1334. 12 Rotheram Todd and Whitcher AECD UCLA-119 195 1. 13 Proc. Znt. Congr. Pure Appl.Chem. (London 1947) 316 GENERAL DISCUSSION and Chapiro,l4 i.e. as a competition between the recombination of the free radicals and the attack of one of them on the organic chlorine compound with extraction of chlorine e.g. If 1 RCl -1- H - R* -i- HCl. ( 1 ) Since such a reaction would require a relatively high activation energy a rather large concentration of RCI would be necessary to catch all the free radicals produced and so obtain a yield independent of the RCl concentration and of the nature of the compound used. As was correctly pointed out by Dr. Minder the concen-trations would be so high that direct effects as well as secondary reactions would become important. The kinetics is so involved that it seems hardly possible to use this type of reaction for determining the G value of the solvent.However at low concentration the initial slope of the curve should be proportional to the concentration of C-C1 bonds and to the rate constant kl. For two different compounds in the same solvent and under identical irradiation conditions the ratio of the slopes is roughly given by - AE* K = 4 e x p (-) RT where n and n’ are the numbers of C-Cl bonds in each molecule and AE+ is the difference in activation energies which is about 0-3 AD where A D is the difference of the C-C1 bond dissociation energies. The compounds listed in fig. 3 of Dr. Minder’s papers belong to 4 different groups (i) c2cl(j (ii) RCC13 (iii) Y - C ~ H ~ C I ~ , and (iv) P‘CfjH4C12. Comparing the slopes of groups (ii) and (iv) one gets AE* = 1 kcal and AD N 3 kcal a reasonable value for the difference between aliphatic and aromatic C-C1 bonds while the AE* between groups (i) and the (ii) is N 0.4 kcal.In general an increase in the bond energy when going from group (i) to group (iv) seems in keeping with the rest of our knowledge on the subject. Dr. W. Minder (Radium Institute Berne) said We compleLely agree with Dr. Magat that the curves fig. 1 and 2 of Dr. Maget’s and co-worker’s paper and fig. 3 of our paper are representing exactly the same things. Nevertheless the interpretations are somewhat difl‘erent. In our curves we plotted the yield of perhaps the most probable reaction of different solutes capable of the same reaction against concentration in the same solvent. Our yields are within experimental error proportional to the number of C1 atoms in the solute molecules both for the aliphatic and for the cyclic compoimds.The difference between the two yields per C1 atom is of course due to the different binding energies. Table 1 of Dr. Magat and co-worker’s paper shows the decomposition yields of one and the same solute (diphenyl picryhydrazil) in different solvents. If we were to try to interpret our results in a similar manner it would be neces-sary to assume the liberation of all the Cl atoms which are bound to one molecule of the solute by one radical produced from the solvent alcohol or the production of different numbers of radicals in the same solvent if different solutes are present. Furthermore it would be difficult to understand the fact that relatively large con-centrations of water in the solvent do not in any way influence the number of active radicals.Dr. W. Minder (Radium Institute Berne) said ; The total radiation dose within a spherical vessel containing a suitable radioisotope solution can be calculated exactly 15 if the decay properties of the isotope is known. We have compared such a calculation with the results for the oxidation by self-irradiation of a ferrous sulphate solution (0.005 N FeSO4 -1- 0-8 N H2S04) containing 0.1479 mc of Rb86 14 this Discussion. 15 Minder and H. Schindler Strahletttker 1952 86 602 GENERAL DISCUSSION per cm3. The total radiation dose administered to periment during the time t is KMG D = KMG s’ exp (- At) dt = -(1 0 x the solution in - exP (- W), 317 such an ex-where K is the overall dosage rate (in r/mc hj M the total amount of the radio-isotope in mc X the decay constant and G the so-called geometrical factor which takes into account the form size and absorption and which is in every case a rather complicated mathematical expression.Comparing our irradiation results with the calculations (the irradiation was made in a closed vessel of 700 ml without adding 0 2 and the oxidation was deter-mined by titration with KMn04) we have found a total dose (t = a) of 193,000 r by calculation and 200,000 r by oxidation of Fe3+. This value was obtained with an ionic yield (Fe3+ per 32.5 eVj of 3. This yield is in good agreement with our earlier experiments with y-rays 16 and X-rays 17 and with the value gwen by Wright 18 for a 0.0025 N solution after oxygen removal.Dr. J. F. Suttle and Dr. J. W. Schulte (Los Alamos New Mexico) (communicated) : For the past several months we have been irradiating dry pure chloroform solu-tions with gamma rays from a C060 source. The results to date indicate that good linearity is obtained up to 4 x 105 r when chloride ion in the irradiated solution is determined potentiometrically with silver nitrate. For 4 x 105 r approximately 0.18 mequiv. of chloride ion is found per ml of irradiated chloroform. However, to obtain any significant decomposition by irradiation it is necessary that oxygen be present. We find it interesting that the authors of this paper make no mention that even traces of oxygen might have decided influence on the system. Prof.Milton Burton (University of Notre Dame Indiana) said In this Discus-sion we have witnessed two kinds of difficulty regarding expression of radiation yield. The first and more important is an experimental discrepancy in the G value for Fez+ -+ Fe 3+ in 0.8 N sulphuric acid solution dependent on the source of the information. Although the difference is presently unexplained we may confidently expect that an explanation will soon be forthcoming. The second type of difficulty results from the method of expression of yield. The custom persists of reporting M/N in cases where N is not and probably never can be, experimentally known. The number of ion pairs is calculated on a variety of bases depending on logic prejudice admitted inadequacy of information or on purely formal advantage.Unfortunately readers who get only raw M/N values from summaries or abstracts sometimes hunt vainly through an article for details as to the basis of calculation. This latter difficulty is really an avoidable nuisance. Although occasionally an author can profitably include consideration of M/N in his reasoning in many cases nothing is contributed to our knowledge of mechanism by use of this nota-tion. Consequently irrespective of individual prejudices it is desirable that we join in a notation which has singularity of meaning. I have discussed this matter with several participants in the Discussion and we have jointly arrived at con-clusions which I have summarized as follows. G This notation (i.e. 100 eV yield) should be used whenever molecules pro-duced or converted and actual energy input are actually measured.Of course, errors may occur in both determinations but such values are subject to check and the methods (if adequately described) are subject to criticism. Sometimes energy input is not directly measured but is computed from other data e.g. from observations with a dosimeter. In such case the author should indicate both his methods and his conversion factors unambiguously and G’ 16 Minder and Liechti Experientia 1946 2 410. 17 Minder Radiol. Clin. 1951 20 286 1951. 1 * Wright this Discussion 318 GENERAL DISCUSSION use the symbol G’ to indicate that the 100eV yield is calculated rather than measured. G20 Yields may be measured in terms of another actinometer ; e.g. the highly favoured FeS04 actinometer.In a recent paper which I had the good fortune LO read prior to publication Johnson and Allen expressed yields in an aqueous system in terms of a comparison with Fezf -+ Fe3+ in such an actinometer. Since they explicitly indicated their method it will be possible at some fuwre time to establish their G’ values. Simple numerical values would have been useful if difficulty of interpretation could have been simultaneously avoided. It is sug-gested that in the future when the FeS04 + 0.8 N H2SO4 actinometer is used as a standard the conditions of use be explicitly stated G(Fe2+ -f Fe3+) be taken as 20 until an accepted value is established and the symbol G20 be used for expression of the calculated 100 eV yield. G (. . . ) For convenience of typography we suggest that the change to which G refers be indicated where necessary in parentheses after G ; e.g.G (H202 produced) or G (Fe2+ -z Fe3+). G 1.5 The papers of both Dr. Hardwick and Dr. Minder emphasize the signi-ficance of microscopic G value for molecules converted or produced per 100 eV in an increment of path where the energy of the effective particle is not significantly changed. Unlike the usual G values this is not an average value over the whole length of path. It tells of the chemical effect produced by a particle in a micro-scopic portion of its path when it has a very specific energy. We suggest use of subscripts such as shown where units of energy are in MeV. In any case we join in the urgent suggestion that explicit statements be made as to the quantities actually measured.Dr. N. Miller (Edinburgh Universify) (partly conmunicated) Prof. Burton’s suggestion that methods of expressing radiation yield be standardized is a timely one. I think that the moment is also appropriate for the standardization of units of dose and dose rate. Most workers in the field would agree that units of dose based on ionization such as the roentgen or rep are less satisfactory than absolute units such as eV/ml ergs/g. It should be emphasized however that full experi-mental details of all physical dose measurements should be included by authors on publication. With regard to the relative merits of the units eV/ml and ergs/g, it may be pointed out that the former involves the use of very large numbers, whereas the latter is currently receiving considerable support among medical physicists. Prof. Burton has made the further suggestion that yields observed in X- or y-ray induced reactions in aqueous media be expressed relative to that observed for ferrous sulphate oxidation under the same experimental conditions. Such yields could then readily be adjusted to conform with any future changes in the accepted yield value for the ferrous sulphate system. I think that such a procedure is quite satisfactory at low dose rates but that utitil more work is done on this method of dosimetry at high dose rates and general agreement on the yield value in this region is reached an upper limit should for the time being be set on the dose rate at which such a procedure is considered reliable. A figure of 1000 r/min would for the moment be a safe or even con-servative upper limit. Very probably this limit will be raised or even consider-ably raised when more becomes known about the general kinetics of ferrous sulphate oxidation at high dose rates. It seems almost certain however that the upper limit for the use of ferrous sulphate will prove to be lower than that for ceric sulphate or other systems which are not oxygen-dependent