Radiation Decomposition Mechanism of Betaine and Choline Chains /CRYSTALLINE orthorhombic choline chloride (Ch.Cl) \^A at room temperature is very sensitive to ionizing radiation1 and the choline ion breaks down mainly into trimethylamine and acetaldehyde2:HO-CH2-CH2-N (CH3)3 Cl - N(CH3)3CH3-CHO 4- HC1 Lemmon et al.9, after investigating the effects of ionizing radiation on Ch.Cl and its analogues, concluded that: (i) choline bromide is about one-third as sensitive as the chloride. None of many other analysed analogues, including choline iodide (Ch.I), shows abnormal radiation instability; (ii) Ch.Cl susceptibility to radiation damage is a function of its crystal structure, as shown by its contrasting stability in solution; (iii) the high values of molecular decomposition per 100 eV indicate a chain mechanism for the solid-state reaction.The crystal structure of the orthorhombic phase of Ch.Cl was published by Senko and Templeton4. Data on the cubic disordered reversible phase, stable over 73 C, were given by Collin5, who suggested that the transition to the disordered cubic phase and the subsequent increased stability to radiation furnished further evidence that the decomposition was highly stereospecific. From their crystallographic evidence the authors were not able to find a mechanism which accounted for the foregoing facts. Serlin6 reported that for y-radiation the decomposition of crystalline Ch.Cl per 100 eV absorbed was higher at 50 C than at 20 C, but at 150 C the solid Ch.Cl was much more stable than at room temperature.Finally, Lemmon et al.1, although still unable to find an explanatory mechanism, produced the following relevant evidence from their isotopic investigations of the radiation decomposition of crystalline Ch.Cl: ic(i) the carbinol group of the ethanol moiety becomes the aldehyde group of the resultant aldehyde; (ii) no hydrogens are transferred to or from the trimethylamino-group; (iii) the hydrogens of the ethanol moiety are highly mobile; (iv) intermolecular hydrogen transfers take place". Because X-ray diffraction data obtained under irradiation conditions provide a reliable quantitative picture of the behaviour of the Ch. and Cl~ ions during the collection of the diffraction measurements, I have made an intensive examination of the combined crystallographic evidence from the structures of tetramethylammonium (TMA)9"10 halides, muscarine iodide11,, pentamethonium iodide12, choline chloride4, hydroxylammonium chloride13, hydrogen chloride monohydrate14 and acetylcholine bromide (Canepa, Pauling and Sorum, work in preparation). As a result of this examination, a mechanism was found for the radiation decomposition of Ch.Cl, the attenuated decomposition of Ch.Br, the insensitivity of Ch.I to radiation, the temperature effect on the rate of their decomposition and a similar mechanism for the radiation decomposition of betaine hydrochloride, based on the following evidence:Ch.Cl orthorhombic structure can be defined as an infinite molecule with cations and anions bound by a mixture of weak electrostatic forces and even weaker hydrogen bonds. The dipole moment of the CH3 groups15"16 acts as a dielectric constant reducing the coulombic forces between the Nquaternary group and the Cl~ anions and will be dealt with in a later communication. In the published Ch.Cl structure4, because the Cl~ ... O distance is 3-03 A, instead of 2-95 as in hydrogen chloride, 2-91 and 2-99 in tropolone hydrochloride, and 2-99 in hydroxylammonium chloride, the conclusion was reached by Senko and Templeton that the hydrogen bonds, if present, were very weak. Nevertheless, a calculation with their published co-ordinates (see Figs. I and 2) shows the angle C4C5O == 102-5, instead of the expected 108-110, and the N. . . O distance 3-26 only, as compared with the shortest of the six N.... Cl~ electrostatic co-ordination distances: 4-08, 4-22 and 4-28 A; moreover the non-bonded Ca . . . . C5 carbons, 3-3 apart instead of a minimum of 4 A, are the seat of repulsive forces. The foregoing three factors indicate a very significant electrostatic binding between the Ngroup and the alkyl oxygen. A comparative calculation with the public co-ordinates of muscarine iodide gives an even shorter N.... O distance of 3-07 A, and the shortest of its six N.... I- distances as 4-49 A, suggesting a stronger electrostatic binding in the N.... O system of muscarine. Thus, there is in Ch.Cl structure a balance of forces between the pulls exerted by Nand by Cl~ on the oxygen in the electrostatic system (A) which: .... N.... O-H .... Cl- .... (A) "-3-26 - x - 3-03 ->accounts for the longer N. . . . O distance (3-26 A), that is, weaker electrostatic binding, and for the longer O-H .... Cl- (3-03 ). A comparison of the d-r values of Ch.Cl and TMAC1 in fable 1 shows that repulsive forces between anions increase with the ionic co-ordination (see Ch.Cl and TMAC1) but not with anionic radii (see TMA halides). Therefore, it is likely that the 11 per cent increase of unit cell volume during the Ch.Cl transition from orthorhombic to the cubic phase is due to the disorder of the new phase and to a change from the distorted six-fold to the distorted eight-fold electrostatic co-ordination, the latter increasing the value of N.... Cl- distance from 4-08 to, say, 4-38 as in TMAC1 (see Table 1). This decreased binding energy in the expanded N. . . . Cl- distances of Ch.Cl cubic form allows a stronger N.... O electrostatic interaction which should cause the N.... O distance to decrease from 3-26 to at least 3-07 A. This is the JST. . . . O distance in the sixfold distorted muscarine iodide and possibly in the four-fold co-ordinated acetylcholine chloride. Because of the increased N. . . . Cl- and decreased N. . . . O distances a weakening of the hydrogen O-H .... Cl- is implied in the expanded unit cell of cubic Ch.Cl. Fig 1. Chin Ch.Cl orthorhombic Phase. Z atomic co-ordinates given. Ca. . . C4, 2-65 A; T. . . C6, 2-58 A; O . . . C4, 2-45 A; C6-O, 1-39 A Fig. 2. N.. . . Cl- and Cl" . . . . 01" distances in Ch.Cl orthorhombic phase The consequence of the foregoing is the greater stability of the Ch.Cl cubic phase to radiation decomposition. This will be better understood after a close examination of the atomic thermal displacements under X-ray irradiation in the compounds listed in Table 2.The temperature factor is Bi = 8n;2ui2, where irepresents the isotropic mean square displacement of the atoms in the X-irradiated crystal structure and i = O, N, Cl, etc., depending on the chemical composition of the structure. Relevant also to the radiation decomposition mechanism is the greater mobility of the hydrogen atoms (higher values of B) relative to X-irradiated heavier atoms which occur in most X-ray crystal structures. If the thermal displacements ui of the orthorhombic Ch.Cl (see Table 2 following) were really isotropic, then the maximum additive displacements of N.... Cl-, N. . . . O and O .... Cl- would be cio js -0-48 A, tNwo -0-5 A, ci 4- uo -0-53 A. Table 1Ch.I Ch.Br. orthorhombic Ch.Cl Ch.Cl cubic TMACl tetragonal TMABRtetragonal TMA I"tetragonal First partNo. molecules per cell 4 4 4 4 2 2 2 Unit cell volume = V 877-4 817-7 763-0 857-0 334-8 333-1362-5 Shortest distance (N . . . halide) = d - 4-08 4-38? 4-38 4-37 4-56Halide radiusr 2-15 1-95 1-81 1-81 1-81 1-95 2-16 d-r - - 2-27 2-57? 2-57 2-42 2-40Electrostatic co-ordination - - 6 8 8 8 8 Second partCationic volumeinterstices = v- ( F/4 - anin volume) 177-12 173-38 165-91 189-41 58-86 52-23 48-4V = Vhalidf - "iodide 0 -3-74 -11-2112-2910-4653-83 0 Degree of N. . . halide compression ~ Partial Marked Table 21 Atom Cl O N Ci Cs C3 C4 C5 Choline chloride (H co-ordinates not given)orthorhombic at V E o "a 5 6 4 57 577 room temperature J0.252 0.276 0.225 0-297 H 3-0 0-195Hydrogen chloride l Atom Cla C10 Oa O^ monohydrate at ^0-036 -0-007 0-041 -0-016 N' 1-5 0-138-35 C J u 0-0214 O 1-5 0-138Atom Cl B 1-5u 0-138 Hydroxylammonium chlorideThe evidence as to the relevance of thermal displacements in the oscillatory behaviour of the loosely bound tetratomic system (A) under irradiation is provided by the resultant products of the radiation decomposition. The already strained bond N - C4 of 1-6 length, joining the two branches of the gauche CHchain, is stretched further if the atoms C5 and O of one branch close up towards Nand C2 on the other branch due to thermal displacements assisted by the high energy of the y-radiation (see Fig. 1). This compresses the N. . . . O and C2 . . . . C5 distances so that the N - C4 bond breaks. The increased Ninduction on the O - H group repels the highly mobile hydrogen atom towards either the hydrocarbon chain (i) or the Cl- (ii). (i) (CH3)3CH2- CH2 (CH3)3NCH3- CHOHC1 (ii) (CH8)3 NCH3 - CHOHC1 In either, a pressure wave is set up and transmitted through the highly compressed orthorhombic structure of Ch.Cl (volume 763 A3) due to the van der Waals forces originating from the molecules produced in the radiation decomposition because these require a van der Waals space greater even than that available in the expanded cubic phase of Ch.Cl (volume 857 A8).The already existing thermal displacements, the Y-radiation and the new pressure wave further compress all the N.... O and C2 . . . . C5 systems in the contiguous Chchains, resulting in additional decomposition of the chain reaction type. Acetylcholine Conformation in Aqueous SolutionTHE great stability to irradiation of the Chcations in the aqueous solutions of Ch.Cl is accounted for by: (i) The coulombic forces between the hydrophobic Ngroup and the hydrated Cl- anions are reduced as compared with that in the crystalline state, since besides the CH3 dipoles surrounding the Nthere are the water dipoles around the Cl~. Thereby the N.... Cl~ distances in solution are greater than in the crystalline phases. (ii) This strengthens the binding energy of N. . . . O, shortening its distance to at least 3-07 as in muscarine iodide crystal structure, leaving the O-H system too weak to compete with the water molecules for the Cl-anions. Because in aqueous solution the chain Chdoes not contain the irradiation sensitive oscillatory system (A) it is far more stable to the y-rays. Moreover, in the aqueous medium the high-energy y-radiation impact on the Chchains can be kinetically absorbed by recoil movements of these cations. (iii) This Chchain stability towards irradiation in solution is the first strong indirect evidence relating the forces acting on Chin the solid state to those acting on Chin solution. Thus the N.... O distance of Chin solution is similar to that of Ach. in Ach.Br crystal structure and gives to both cations because of their hydro-phobic Ngroup a quasi-zwitter ion or quasi betaine character only in aqueous solution. This completes the analysis of the infra-ryd evidence of Ach. in aqueous solution of Canepa and Mooney17, where a ring conformation due to the Ngroup and the carbonyl oxygen of Ach. was ruled out. The N.... O conformation in the Ach. solid state and in aqueous solution is relevant to its adsorption at the cholinergic receptor and its hydrolysis. This provide-another striking example of the relations between ths forces acting in the solid state and those acting in the adsorption state18: I shall deal with these in a subsequent communication. Decomposition of Ch.Br compared with the stability to irradiation of Ch.I. It can be seen from the first part of Table 1 that the repulsive forces between the anions co-ordinating the Ngroup in the TMA halid.es decrease when the radii of the anions increase; as a consequence, the volume of the rigid cationic TMAgroup, plus inter-ionic interstices, is minimum for TMAI. Conversely, the second part of Table 1 shows that in the crystalline state the available amount of cationic volume in the Ch. halide is maximum for Ch.I and minimum for Ch.Cl. It follows from the negative values of 8v that the N.... halide compression is greater for Ch.Cl than for Ch.Br; therefore, the corresponding N. . . . O distance in the oscillating system (B): BT. . . . O-H .... Br- (B) is intermediate between 3-26 and 3-07 A, that is, the N. . . . O bridge joins the branches of the gauche chain of Chin the Ch.Br crystal structure with a rigidity intermediate between that of Ch.Cl and that of Ch.I, which accordingly reduces the Chthermal displacements and with it the rate of radiation decomposition of Ch.Br. The value of the temperature factor B = S-n:2i2 is minimum when the Ch.Cl molecule is in the ground state (zero point energy), and increases steadily with the temperature for a given crystalline phase. The almost zero thermal displacement at liquid air temperature explains the stability of Ch.Cl in such conditions, whereas the growth of the rate of radiation decomposition must coincide with rising temperature.It is possible to suggest the decomposition mechanism of betaine hydrochloride on similar lines to (B); the oxygen charge, being located between the CH3 groups, where their dielectric constant is a minimum, is strongly attracted towards the centre of the Ngroup. I thank Dr. C. G. Smith for his help and advice .