J. CHEM. SOC. DALTON TRANS. 1986 1Stability and Structure of some Binary and Mixed-ligand Complexes of Zinc(ii)M. Sivasankaran Nair *Department of Chemistry, Madurai Kamaraj University P. G. Extension Centre, St. John 3 College Campus,Pala yamkottai- 627 002, Tamilnadu, IndiaM. Sankaranarayana Pillai and S. K. RamalingamDepartment of Inorganic Chemistry, Madurai Kamaraj University, Madurai- 625 02 I, Tamilnadu, IndiaThe stability and structure of ZnBH, ZnB, ZnB,H,, ZnB,H, or ZnB, types of binary complexes in theZn' t-~~-2,3-diaminopropionic acid (dapa), -~~-2,4-diaminobutyric acid (daba), and -oL-ornithine(Orn) (B); and ZnABH,, ZnABH, or ZnAB types of mixed-ligand complexes in the Zn"-histamine/L- histidine (A)-dapa,Aaba, and -0rn (B) systems are discussed from the computer-based analysis of the pH titration data at 37 "C and I = 0.1 5 mol dm-3 (NaCIO,).In both the binaryand mixed-ligand systems, dapa, daba, and Orn (B) appear t o be tridentate. The results clearlydemonstrate that in the ZnBH, ZnB,H,, and ZnB,H binary complexes and ZnABH mixed species thesite of protonation is the terminal amino-group of ligand B. In the ZnABH, complexes, one protonis attached to the ligand A and other to the ligand B. In the Zn"-histidine (A)-dapa,-daba, and- Orn (B) systems, the results suggest the histamine-like mode of binding for L-histidine (A). Morethan the statistical order of stabilities was observed for most of the mixed-ligand complex speciesdetected in the present investigation.Considerable attention has been paid in recent years to theinvestigation of the complex forming properties of potentiallytridentate ligands such as histidine, serine, threonine, trypto-phan, tyrosine, and diaminocarboxylic acids because suchstudies are closely connected with peptide and complex chem-i~try.'-~ Since the third donor group in the diaminocarboxylicacids of general formula NH2(CH2),CH(NH,)COOH, wheren = 1 (2,3-diaminopropionic acid), n = 2 (2,4-diaminobutyricacid), n = 3 (ornithine), and n = 4 (lysine), is the nitrogen atomwhich is able to co-ordinate with hydrogen ions at intermediatepH values, there is often significant competition betweenhydrogen and metal ions for co-ordinating with the thirddonor group, resulting in a number of complex equilibria.Thusinvestigations on the metal complexes of diaminocarboxylicacids have been carried out by several worker^.^,^-'^ The firstreports on these lines were made by Albert.s Copper(I1) com-plexes of diaminocarboxylic acids have been wellBrookes and Pettit investigated the diaminocarboxylic acidcomplexes of Co" and Ni" also. However, no detailed studieshave been carried out on the Zn" binary complexes ofdiaminocarboxylic acids. The present paper deals with thestability and structure of Zn" complexes with threediaminocarboxylic acids, namely ~~-2,3-diaminopropionic acid(dapa), ~~-2,4-diaminobutyric acid (daba), and DL-ornithine(Om). Again, bearing in mind the outstanding biologicalsignificance of the metal complexes containing imidazole and itsderivatives, the Zn"-A-B mixed-ligand systems [A = L-histi-dine, histamine, or imidazole; B = dapa, daba, or Om] werealso investigated by pH titrimetry at 37 "C and at I = 0.15 moldmP3 (NaCIO,).ExperimentalThe methods of preparation and determination of Zn(C104),and of other reagents are described earlier.*-" All the ligandsused were obtained from Fluka.Doubly-distilled water wasused for the preparation of all the solutions. The pH titrationswere carried out at 37 "C under a nitrogen atmosphere (freedfrom oxygen and CO,) with the apparatus and proceduredescribed A constant ionic strength of 0.15 moldm-3 was maintained by the addition of sodium perchlorate.TaMe 1. Values of protonation constants (ref. 14) for histamine and L-histidine ligands and their parent binary stability constants with Zn" at37 "C and I = 0.15 mol dm-3 (NaCIO,). Standard deviations are givenin parenthesesA - Parameter Histamine L-Histidinelog BHA 9.39(8) 8.96(3)log PHzA 15.34( 1) 14.96( 5)log PH,A - 17.3769)log PZnA 5.39(3) 6.41(2)log PZnAH 11.91(5) -log PZnAzH2 -log h n A 2 H - 17.47(9)1% KZz",2 5.06 5.3322.80( 12)log PZnAl 10.45(4) 11.74(2)Table 2.Stability constants for the parent binary Zn"-dapa,-daba, and-0rn (B) systems at 37 "C and I = 0.15 mol dm-3 (NaClO,). Standarddeviations are given in parenthesesBAI \Parameter dapa daba Orn* From ref. 8.9.37(2)15.98(3)17.37(5)13.61(4)2 5.70( 3)13.70(2)--4.246.96--9.93(2)18.02(4)1 9.8 8( 6)14.22(4)27.04(16)2 1.47(4)-13.44(4)4.297.187.05-10.22( 1)18.85(2)20.99(4)14.56(2)6.69( 3)27.83( 1 1)--4.347.397.87-Calculations were made with the aid of the MINIQUAD-75computer program l 3 on an IBM-370 computer.Variousmodels were fitted to the data and the model selected was thatwhich gave the best statistical fit, consistent with chemical logic2 J. CHEM. SOC. DALTON TRANS. 1986Table 3. Stability constants for the Zn"-histamine (A)-dapa,daba,and -0rn (B) mixed systems at 37 "C and Z = 0.15 mol dm-3 (NaClO,).Standard deviations are given in parenthesesBAf \dapa daba- 12.87(8)- 19.65( 15)25.44(8) 26.22(14)- 7.48- -- -- 1.85- 6.78- 14.26- 0.04- 1.8113.53 14.3111.83 12.00- 0.08 + 0.09-Om11.56(3)19.33(3)26.59(7)6.174.87- 0.527.7713.94- 0.620.3814.6812.03+0.12-Table 4.Stability constants for the Zn"-L-histidine (A)-dapa,daba,and -Om (B) mixed systems at 37 "C and Z = 0.15 mol dm-3 (NaCIO,).Standard deviations are given in parenthesesBAI \Parameter dapa daba Om-19.03(2)25.66( 1 6)---12.625.420.6212.056.632.82- 0.99- 11.69(15)- 20.30(2)26.81(15) 27.10(2)- 5.285.00- 1.41- 13.89- 5.74- 0.67- 1.0312.59 12.54- 6.803.78 3.57---to the range of titration data without giving any systematicdrifts in the magnitudes of various residuals. At high pH values,hydroxo-complexes were often present. Since these data couldnot be fitted satisfactorily to any simple model, points above theonset of a systematic drift in residuals were omitted.The resultsobtained are reported in Tables 2 4 . Though the binarystability constant data of Zn"-histamine and +-histidine (A)systems have been reported previo~sly,'~ they have beenincluded in Table 1 as these constants have been used in thepresent study for the computation of mixed-ligand stabilityconstant data. The species distribution plots obtained for thebinary systems and two mixed-ligand systems are given inFigures 1-3. The charges of all the complex species reportedin this paper are omitted for clarity.Results and DiscussionBinary Complexes of Zinc(I1) with dapa, daba, and Om.-Ourresults from the detailed titration studies indicate that the Zn"-dapa (B) system contains ZnBH, ZnB,H,, and ZnB, as themajor species, while the Zn'l-daba (B) system showed thepresence of ZnBH, ZnB2H2, ZnB,H, and ZnB, complexes inaddition to the species HB, H2B, and H3B.In the Zn"-Orn (B)system, the species HB, H,B, and H3B along with the complexes/"" \ \ l c80c 60NaJ0Cc40a200PH\ 5bFigure 1. Distribution diagram for the Zn"-B (1 : 2) systems: (a) dapa,(b) daba, (c) Om. (1) Unbound Zn, (2) ZnBH, (3) ZnB, (4) ZnB,H,, ( 5 )ZnB,H, and (6) ZnB,ZnBH, ZnB, and ZnB,H, were detected. The ZnBH species inall the above systems were found to be favoured at low pH(Figure l), indicating that the extra proton in this complexspecies can attach to any one of the two amino-groups,preferably to the terminal amino-group of the ligand B.In orderto characterise the metal-ligand binding, the parameter log Pwas computed using equation (1). The value obtained in all thethree systems (Table 2) compares favourably with the value ofthe overall formation constant for the ZnA glycine complex.This clearly suggests that an a-amino-carboxylate chelation isinvolved resulting in a five-membered chelate ring in the ZnBH(B = dapa, daba, or Om) complexes with the proton residingon the respective terminal amino-groups.Of the several complex species that contain Zn" and dapa,daba, or Orn ligands (B) in the ratio 1 : 2, ZnB,H, is the oneformed at the lowest pH values, accounting for a maximum ofca. 38% at pH 5.5 in the Zn'I-dapa system, ca.3% at pH 4.3 inthe Zn'-daba system, and ca. 4.5 in the Zn"-Orn system (Figure1). Here it may be mentioned that since the percentage of Zn" inthe form of ZnB,H, in the ZnII-daba and -Om (B) systemswas found to be very low, the chemical models excluding thesecomplex species were also tested. However, for obtaining thebest-fit statistical model the inclusion of these complex specieswas found to be essential. The less preference for the formationof ZnB,H, species in the systems with B = daba or Orn is alsoevident from the values of their formation constants (Table 2)with very high standard deviations. As discussed above, sincethe extra proton in the ZnBH (B = dapa, daba, or Om)complexes is attached to the corresponding terminal amino-groups of the ligand, it may be easily concluded that in theZn(BH), complex also, the protonation sites are the terminalamino-groups of the two ligands. This becomes clearer if it isnoted that the log P' values in Table 1 derived from equation (2)are comparable to the values of the overall formation constantJ. CHEM.SOC. DALTON TRANS. 1986 3for the ZnA, glycine complex. Thus in the ZnB2H2 (B = dapa,daba, or Om) complexes, the two ligands bind the metal in alog p' = 1% PZnB,H, - 2 log PHB (2)glycine-like mode resulting in two five-membered chelate ringswith the two protons residing on their terminal amino-groups.Only in the Zn"-daba system, the best-fit model containedZnB,H species, accounting for ca. 55% of the total metal at pH7.5. Since the site of protonation is the terminal amino-group ofdaba in its ZnBH complex, it may be concluded that in itsZnB,H complex also the proton is attached to the terminalamino-group of one among the two daba ligands.The logK$:!yH value of 7.05 is of the order expected for the tridentatebinding of B- in the Zn(BH)B complex. Thus in the ZnB,H(B = daba) complex, one among the two ligands binds themetal with all three donor groups and the other ligand bindsthe metal in glycine-like mode with its terminal amino-groupbeing protonated.It is surprising to note that the Zn"-Orn system showed thepresence of ZnB species, while the ZnB, complex was notdetected. The reverse was the case in the Zn"4apa and 4 a b a(B) systems. About 20% of the total metal was found to bepresent in the form of ZnB in the Zn"-Om system.Albert5reported a value of 4.10 log units at 25 "C for the ZnB-Orncomplex and concluded that Orn binds Zn" in a glycine-likemode. But the present investigation gives a value of 6.69 logunits, indicating the possibility of the binding of Orn in atridentate manner though it would involve one seven- and onefive-membered chelate rings. However, it is not surprisingbecause in the Nil' and Co" complexes containing thepotentially tridentate ligands such as Om, lysine, or argininealso, these ligands at high pH range bind the metal with all thethree co-ordination groups, although the chelate rings formedby the two nitrogen donors would be abnormally large.6However, in the Cu"-Om, -lysine, or -arginine binary systemsthese ligands bind the metal in a glycine-like mode.6-8 In theZn"-Orn system in the present investigation the ZnB complexwas found to be favoured at high pH (Figure 1).Thus,analogous to the NIB and COB (B = Om) complexes, it isjustified that Orn binds Zn" in a tridentate manner. Since Orn istridentate in its ZnB complex involving one five- and one seven-membered chelate ring, one would expect the ZnB, (B = Om)complex species to contain two five- and two seven-memberedchelate rings which must be less favoured due to steric reasons.At pH 7.5, the respective amounts of the total Zn" present inthe form of ZnB, species in the Zn"-dapa and d a b a systems are99 and 26%. The log PZnB, values of 13.70 and 13.44respectively in the above two systems (Table 2) are higher thanthose values expected for the bidentate binding of both ligands,indicating that both the ligands must be tridentate in theirrespective ZnB, complexes.However, since the log PH,B valueof 19.88 for daba is higher than that of 17.37 for dapa, oneshould expect a higher log P value for the ZnB, (B = daba)complex than that for the dapa complex. However, the results inTable 2 indicate the opposite trend. This may probably beaccounted for by considering the steric factors associated withthe two six- and two five-membered chelate rings in the ZnB,(B = daba) complex in comparison with the four five-membered chelate rings in the ZnB, (B = dapa) complex.Mixed-ligand Syslerns ofZinc(ir).-Six mixed-ligand systems,namely Zn"-histamine (A)-dapa, -daba, or -0rn (B) and Zn"-L-histidine (A)--dapa,-daba, or -Om (B) are discussed in thissection (see Tables 3 and 4 and Figures 2 and 3).The Zn"-imidazole (A)-dapa,-daba, or -0rn (B) systems were alsostudied, but no appreciable complexing was revealed. The Zn"-histamine (Awaba or -0rn (B) mixed-ligand systems showed-60 -CNa,e 6" 40-CVa,La -III I204 5 6 7PHFigure 2. Distribution diagram for the Zn"-histamine ( A p a p a (B)system (C, = 2.948 xmol dm-3). (1) Unbound Zn, (2) ZnAH, (3) ZnA, (4) ZnA,, (5) ZnBH,(6) ZnB,H,, (7) ZnB,, and (8) ZnABH,C, = 4.492 x C, = 4.484 xN = I \ ?-I8PHFigure 3. Distribution diagram for the Zn'l-L-histidine (A)-dapa (B)system (C, = 2.948 x lC3, C, = 3.054 x lP3, C, = 2.988 xmol dm-3).(1) Unbound metal, (2) ZnA, (3) ZnA,H,, (4) ZnA,H, (5)ZnA,, (6) ZnBH, (7) ZnB,H,, (8) ZnB,, (9) ZnABH,, and (10) ZnABHthe presence of three mixed complexes (ZnABH,, ZnABH, andZnAB), while in the Zn"-histamine (A)dapa (B) system onlythe ZnABH, species was detected. In all the three mixed-ligandsystems with histidine as the primary ligand (A), the ZnABH,type of mixed species was detected. In addition to this species,the system with B = dapa showed the presence of ZnABHspecies while in the system with B = Om, ZnABH and ZnABwere also detected. During the computation of mixed-ligan4 J. CHEM. SOC. DALTON TRANS. 1986complex stability, the stability constants for the binarycomplexes of zinc(1r) with the ligands A and B estimated underidentical conditions (Tables 1 and 2) were treated as non-refinable parameters.Stability and Structure of ZnAB Mixed Complexes.-TheZnAB complexes in the Zn"-histamine (A)-daba and -0rn (B)systems and Zn"-histidine (A j O r n (B) systems were found tobe favoured above pH 6.5 and there is a steady increase in theirformation with rise in pH.The log PZnAB values of 11.56 and1 I .69 respectively in the systems Zn"-histamine (A)-Orn (B)and Zn"-histidine (A)-Orn (B) are comparable within the limitsof experimental error. This indicates that histidine in its ZnABspecies binds in a histamine-like manner. This is also reflected inthe log K",:, and log Kz,"iB values in Table 4 for theZn"-histidine (A)-Orn (B) system.If histidine were tridentate inthe ZnAB species also, as is the case with the ZnA histidinebinary species,14 one should expect the log K$;:, and logPZnA values to be comparable. However, the log valueof 5.00 is ca. 1.4 log units less than the log PZnA value for theZn"-histidine system (Tables 1 and 4). This clearly indicates adifference in the bonding of histidine in the mixed and binarycomplex species. The log K2:fB value of 5.00 is nearly identicalto the log P value of 5.39 for the ZnA histamine complex.Therefore it can be concluded that in the ZnAB mixed species inthe 2n"-histidine (A)-Orn (B) system, histidine binds in ahistamine-like manner. The same conclusion may further beconfirmed from the fact that the log Ki,n2B value of 5.28 islower than the log K;iB value in the Zn'I-Orn (B) binarysystem by ca.1.4 log units. This is because, for the computationof log K:iiB, equation (3), the value of log (jZnA used wasthat for the tridentate binding of histidine (A) in its ZnA binarycomplex, though it is bidentate in the ZnAB mixed species. If theallowance for this difference in binding of histidine in its binaryand mixed complex species is made, then the log Ki,niBvalue in Table 4 clearly demonstrates that Orn is tridentate inthe ZnAB species in the Zn"-histidine (A)-Orn (B) system.Again, the log K ; t t B values of 7.48 and 6.17 respectively inthe Zn"-histamine (A)-daba and -0rn (B) systems (Table 3) arevery close to those expected for the tridentate binding of dabaand Orn.The log K$ZB,, value of 4.87 in the system withB = Orn is of the order expected for the bidentate binding ofhistamine (A) in its ZnAB complex. This parameter could not becomputed for the system where B = daba because the stabilityconstant datum for the ZnB (B = daba) complex could not beobtained in the present investigation (Table 2). However,comparison of the log PZnAB value of 12.87 in this system withthe value ( 1 1.56) obtained in the Zn"-histamine (A)-Orn (B)system clearly suggests that histamine is bidentate in the formersystem also. Since the log PH values are in the orderOrn > daba (Table 2), one should expect a higher log PZnAB (orlog K;:;,) value for the Zn"-histamine (A)-Orn (B)system compared to that for the Zn"-histamine (Awaba (B)system.The opposite trend (Table 3) observed may be accountedfor by steric factors since one five-, one six-, and one seven-membered chelate ring are present in the ZnAB species in theternary system with B = Om compared to the one five- and twosix-membered chelate rings in the ZnAB species in the systemwith B = daba. Thus the results on the ZnAB complexes in theZn"-histamine (A)-daba and -0rn (B) systems and Zn"-histidine (A)-Orn (B) system show that this complex specieswould have an octahedral structure with one of its faces beingoccupied by the secondary ligand (B), two other sites of theother face would be occupied by the histamine or histidine (A)ligand, and the third site of this face would be completed by thesolvent water molecule.In order to characterize the stability of the ZnAB mixedspecies with respect to the corresponding binary analogues, theparameters A log KZnAB [equations (4) and (511 and log XZnAB[equations (6) and (7)] are computed.On statisticalZnA + ZnB ZnAB + Zn (4)ZnA, + ZnB, e 2 ZnAB (6)log XZnAB = log PZnAB - (log PZnA2 + log PZnB,) (7)grounds,'*l5 in general considerably less negative A log K andmore positive log Xvalues indicate the marked stabilities of themixed complexes. The values expected '*' statistically for thesetwo parameters for Zn" mixed-ligand complexes are -0.6 and + 0.6 respectively. The A log KZnAB value of - 0.52 for the Zn"-histamine (A)-Orn (B) system falls within the statistical order.The value of log XZnAB could not be calculated in this system aslog P for the ZnB, (B = Om) complex could not be obtained inthe present investigation.The log XZnAB value of 1.85 for theZn"-histamine (A)-daba (B) system is very much higher thanthat statistically expected ' 9 ' ( + 0.6) indicating the preferencefor the formation of ZnAB ternary complexes compared to theformation of ZnA, or ZnB, binary complexes. The A log KZnABvalue could not be computed in this system as the stabilityconstant datum for the ZnB (B = daba) complex could not beobtained (Table 2). The value of - 1.41 for A log KZnAB for theZn"-histidine (A)-Orn (B) system is very highly negative. Thismay probably be accounted for by considering the fact that forthe computation of this parameter [equation ( 5 ) ] , the log PZnAvalue used was that for the tridentate binding of histidine,though it is bidentate in the ZnAB mixed species as discussedearlier. If allowance is made for this difference in binding ofhistidine in the binary and mixed species, one can easilyconclude from the A log KZnAB value that the ZnAB mixedspecies in this system is markedly stabilized.Stability and Structure of ZnABH Mixed Complexes.-TheZnABH species in the Zn"-histamine (Awaba and-Om (B)and Zn"-histidine (Awapa and -0rn (B) systems were found tobe favoured above pH 5.5.With regard to the site of protonationin these ZnABH species, it seems that the proton is attached tothe secondary ligand, B, possibly to its terminal amino-group asis the case with ZnBH or ZnB,H, (B = dapa, daba, or Om)complexes.This would become more apparent if a comparisonis made between the pK;,A,H and pK&BH values. Forexample, in the Zn"-histamine (A)-Orn (B) system thepgnABH value of 7.77 compares favourably with thepKznB, value of 7.87. In the system with B = daba, since thepKgnBH value is not available (Table 2), the same type ofcomparison is not possible. However, if the log KEiBH valueof 14.26 for this system is compared with the log (jZnBH value of14.22, it becomes clear that in both the mixed and binary speciesthe same type of protonation site is involved which means thatas in the case of ZnBH (B = daba) in the ZnABH mixed speciesalso the proton is attached to the terminal amino-group ofdaba (B).The log KZz",BH values in the Zn"-histidine (A)4 a p a and -0rn (B) systems (Table 3) follow the trend oflog PZnBH values in Table 2, demonstrating that the extra protonin the ZnABH complex species in these two ternary systems isattached to the secondary ligand (B), possibly to its terminalamino-group. The log KZZn,BAHBH values of 5.42 and 5.74respectively in the above two systems are slightly greater thanthe log P value of 5.39 for the ZnA (A = histamine) complexJ. CHEM. SOC. DALTON TRANS. 1986 5but far less than the log P value of 6.41 in the ZnA(A = histidine) complex. This indicates that the most probablemode of binding of histidine (A) in the ZnABH complexes in theabove two systems is histamine-like. We now consider how toaccount for the slightly higher log Kg:i:H valuescompared to that expected for the histamine-like mode ofbinding of histidine.In the light of the discussion given above,the electrostatic interaction between the free -COO - group ofthe histidine (A) ligand and the protonated terminal aminogroup, -NH3+ of the dapa or Orn (B) ligands in the ZnABHcomplexes would certainly give additional stabilization to thesecomplexes and thus high log K:::!, values. The logK:&H values [equation (8)] in the Zn"-histidine (A)-dapaand -0rn (B) systems also suggest a different mode of binding ofhistidine in the ZnABH mixed complexes and in the ZnAhistidine binary complex. If histidine were tridentate in theZnABH mixed complexes also as in the ZnA histidine binarycomplex, one should expect comparable values of logPZnBH and log Kg,"iBH (Tables 2 and 4).However the formerparameter, in both the systems under discussion, is ca. 1 log unitless than the latter suggesting a difference in binding of histidine(A) in the ZnA binary species and ZnABH mixed species; in thelight of the above discussion, it can be easily concluded thathistidine should be histamine-like in the ZnABH complexes,while it is tridentate in the ZnA binary species.14In order to define the parameters A log Kand log Xin the caseof ZnABH, the exact protonated ligand species must be takeninto consideration. From the foregoing discussion, it can be seenthat the above parameters can be defined by equations (9)-(12). The A log KZnABH value of -0.04 and log XZnABH values' ofZnA + ZnBH e ZnABH + Zn (9)ZnA, + ZnB,H, 2 ZnABH (12)1.81 in the Zn"-histamine (Awaba (B) system are higher thanthe respective statistically expected values demonstratingthe marked stabilities for the mixed complexes.However, in theZn"-histamine (A)-Orn (B) system, the A log KZnABH value of-0.62 and log XZnABH value of 0.38 do not deviate much fromtheir corresponding statistically expected values. The A logKZnABH values in the Zn"-histidine (A)-dapa and -0rn (B)systems (Table 4) are highly negative. This may again beaccounted for by considering the same factors as describedabove for explaining the highly negative A log KZnAB values inthe Zn"-histidine (A)-Orn (B) system.Stability and Structure of ZnABH, Mixed Complexes.-TheZnABH, complexes in the Zn"-histamine/histidine (A)-dapa,4aba, and -0rn (B) systems were found to be favouredabove pH 4.00.Regarding their solution structures, it may bepredicted with a fair amount of certainty that of the twoprotons therein, one would be attached to the hist-aminelhistidine (A) ligand and the other to the dapa, daba, orOrn (B) ligand. This becomes more obvious if a comparison ismade between log Kgzz!H2 and log PZnAH and alsobetween log K::tFH, and log PZnBH. For example, the logK$XitH2 values in Table 3 in all the Zn"-histamine (A)-dapa,-daba, or -0rn (B) systems (the respective values are11.83, 12.00, and 12.03) compare favourably with the log PZnAHvalue of 11.91 in the Zn"-histamine (A) system.Again the logK$:iiH2 values of 13.53, 14.31, and 14.68 (Table 3) in themixed systems with B = dapa, daba, or Orn are nearly identicalto the corresponding log PZnBH values of 13.61, 14.22, and 14.56in the Zn"-dapa,-daba, and -Om (B) binary systems (Table 2).As set out in the introduction, the extra proton in the ZnAH(A = histamine) complex is attached to the primary amino-group and the extra proton in the ZnBH (B = dapa, daba, orOm) complexes is attached to their terminal amino-groups.The same structural characteristics can be assigned to theZnABH, complexes in the Zn"-histamine (A)-dapa,+iaba, or-0rn (B) systems also. Regarding the site of protonation in theZnABH, complexes in the Zn"-histidine (A)-dapa,-daba, or-Om (B) systems, the log K$:i:HZ values in Table 4 arenearly identical to each other in all the three systems suggestingthat one proton must be attached to the histidine (A) ligand.Anatural consequence is that the other proton must be attachedto the dapa, daba, or Orn (B) ligand, possibly to their terminalamino-group as is the case with the ZnBH or ZnB,H, (B =dapa, daba, or Om) complexes as described in the introduction.However, this type of binding cannot be confirmed bycomparing the log Kg:ifH2 values because log P valuesfor the ZnAH complex are not available (Table 1). It has alreadybeen reportedl4 that in the ZnA,H and ZnA,H, (A =histidine) complexes, only the imidazole nitrogen of thehistidine is involved in co-ordination with the metal with thecarboxyl groups of the ligands remaining free, the extra protonsattaching to the primary amino-groups.The same type of bind-ing of histidine may be assigned for the ZnABH, species in themixed systems with A = histidine also. These structuralcharacteristics would be preferred because of the electro-static interaction between the -NH3+ groups in thehistidine (A) or dapa, daba, or Orn (B) ligands and the -COO-group in the histidine (A) ligand. This unidentate binding of theprotonated histidine ligand may further be confirmed by notingthat the log PZDABHz values in the Zn"-histamine (A) d a p a ,-daba, or -0rn (B) systems (Table 3) differ from those values inZn"-histidine (A)-dapa,daba, or -Om (B) systems (Table 4)only by 0.24.4 log units. These slightly higher log PZnABH2values in the systems with A = histidine compared to those inthe corresponding systems with A = histamine may beaccounted for by considering the additional stabilization forthe ZnABH, complexes in the former systems due to theelectrostatic interaction as discussed above.The A log KZnABH, values calculated via equations (13) and(14) in the Zn"-histamine (A)-dapa, -daba, or -0rn systemsZnAH + ZnBH ZnABH, + Zn (13)A log KZnABH2 = log PZnABH2 - (log PZnAH + log PZnBH) (14)(Table 3) are much less negative suggesting enhanced stabilitiesfor the ZnABH, complexes in all these systems. However, thisparameter could not be calculated for the mixed systems withA = histidine as the stability constant for the ZnAH(A = histidine) complex is not available (Table 1).However,the parameter log XZnABH2 was calculated for all the threesystems with A = histidine [equations (15) and (16)] and isvery much higher than the corresponding statistically expectedvalue ' 9 ' ' of +0.6, indicating the marked stabilities of theZnABH, complexes. However, this parameter could not becalculated for the ZnABH, species in the systems withA = histamine because the stability constant for the ZnA,H,species is not available (Table 1)6 J. CHEM. SOC. DALTON TRANS. 1986The distribution of various binary and mixed complexes (aspercentages of total metal) as a function of pH has beencalculated for all the mixed-ligand systems under study andsuch plots obtained for Zn"-histamine (Awapa (B) and Zn"-histidine (A)dapa (B) are given in Figures 2 and 3 respec-tively.Though marked stabilities compared to the statisticalcase were observed for most of the mixed species detected,in none of them did the maximum amount of total Zn"found in the form of mixed complex species exceed thestatistically expected 50% with regard to the parent binaryspecies. Of course this is not surprising because usually theamount of mixed species with zinc never exceeds 50% and moreoften it is nearer 30%. Now it appears to be more appropriate tocompare the mixed-ligand complex formation tendency of Cu"and Zn" in the presence of histamine/histidine (A) and dapa,daba, or Orn (B) ligands. The data available in the literature l oon Cu"-histamine/histidine (Awapa,-daba, or -0rn (B)mixed systems show that the CuABH,, CuABH, and CuABtypes of complexes in these systems have very high stabilitiescompared to the binary complexes and their formation wasfound to be appreciable in most of the systems, accounting formore than 60% of the total metal.However, the results onsimilar Zn" mixed-ligand systems reported in this paper showthat the statistical stability of the ZnABH,, ZnABH, and ZnABcomplexes is not that much higher than in the correspondingCu" complexes. Also the extent of Zn" mixed-ligand complexformation is not appreciable when compared with thecorresponding Cu" mixed-ligand systems. These show that Zn"mixed-ligand complex formation in the presence of thehistamine/histidine primary ligand (A) is less importantcompared with similar Cu" mixed-ligand formation.Thisprobably can be accounted for by considering the fact that in the2n"-histamine/histidine (A) binary systems, the complexformation is favoured by the n-acceptor property of theimidazole group in both the ZnA and ZnA, complexes, unlikethat for the similar Cu" systems where this property appearsonly in the CuA and for steric reasons not in the CuA,complexes. 7 * 1 So, in the Cut'-histamine/histidine (A)-secondary ligand (B) systems, generally there would beextensive mixed-ligand complex formation while in the Zn"mixed systems containing histamine/histidine primary ligand(A), the formation of mixed complexes is less favoured and theformation of bis-complexes due to histamine/histidine ligandswould be predominant.AcknowledgementsOne of us (M. S. 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