Indian Journal of Chemistry Vol.34A, May 1995,pp. 365-369 Stability and structure of copper(II), nickel(II) and zinc(II) heterobinuclear complexes containing some biologically important ligands M Sivasankaran Nair*, P Thillai Arasu, S Sheik Mansoor, P ShenbagavallizyxwvutsrqponmlkjihgfedcbaZYXWVU & M A Neelakantan" Department of Chemistry, Manonmaniam Sundaranar University, Schaffter Hall, St. Mark's Road, Palayamkottai 627 002 Received II March 1994;revised 6 September 1994;accepted 31 October 1994 The stability constants of heterobinuclear complexes formed in aqueous solution for the biologically important ligands, viz., (i) dopa, (ii) dopamine (dopm) and (iii) L-histidine(his) with Cu(JI)-Ni(I1), Cu(JI)-Zn(ll) and Ni(II)-Zn(II) and (iv) L-cysteine (cys) and (v) D-penicillamine(pen) with Ni(I1)-Zn(lI) have been evaluated by computer based analysis of the pH titration data. The binary systems Ni(IJ)jZn(I1)-dopa and dopm have also been studied. The heterobinuc1ear species of the types MAM' or MA2M' have been detected with dopa, dopm, cys and pen ligands, while in the his ligand systems MA2HM', MA2M', MA2H _1M'andMA 2 H- 2M'mixed metal species have been found to be present. A qualitative attempt has been made on the comparison of the log 13 values, to study the Irving-Williams order of stabilities. The higher stabilities of mixed metal complexes over the mixed ligand system is also discussed. The complexes of amino acids and peptides are important in the study ofbiofluids, particularly when hyper accumulated metal ions are present for physiological or pathological reasons'.zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA In vivo, the effective metal ion concentrations are interrelated and their mutual influence may be one of antagonism or stimulation". However, the causes of these influences are not yet satisfactorily understood. In view of these observations and also by considering the fact that investigations on the mixed metal systems compared to the mixed ligand systems are sparse, we report in this paper the heterobinuc1ear complex formation of (i) dopa (ii) dopamine (dopm) and (iii) L-histidine (his) with Cu(II)-Ni(II), Cu(II)-Zn(Il) and Ni(II)-Zn(II) and (iv) L-cysteine (cys) and (v) D-penicillamine with Ni(IJ)-Zn(II) in aqueous solution. The ligands L-dopa and dopm are well known for their use in neuro-transmission process- and in the treatment of Parkinson disease". The imidazole group of histidine plays a fundamental role in several metal protein and metal-enzyme reactions in living organisms>. Through the soft mercapto sulphur atom, penicillamine and cysteine participate in a variety of redox and acid-· base reactions with metal ions. This paper is in continuation of our earlier work":" on mixed metal complex systems. t Department of Chemistry, National Engineering College, Kovilpatti 627716 Materials and Methods All the ligands used were of Fluka products of puriss quality. The metal(II) perchlorate stock solutions were prepared and estimated as described earlier" - 10. The ionic strength was adjusted to 0.15 mol dm - 3 with NaCl0 4 . Other details on the potentiometric pH titrations, and data analysis and calculations have been earlier reported":". All the calculations have been done with the aid of MINIQUAD-75 computer program 11 on a HCL-MAGNUM II computer and the species distribution plots have been obtained using the computer program'? AUTOPLOT. The auxiliary binary stability constant data under the present experimental conditions (Table I) required for the computation of the mixed metal complex stability have been taken from our earlier work?' 13 - 16. However, the Ni(II)/Zn(II)-dopm and Ni(ll)/ Zn(II)-dopa systems have been reinvestigated under the present experimental conditions. Results and Discussion Binary Ni(Il)/Zn(Il)-dopm and dopa systems From the computer analysis of thehgfedcbaZYXWVUT pH titration data, the binary species NiAH, NiA and NiA2H in the Ni(II)-dopm(A) system, and ZnAH and ZnA types of species in the Zn(II)-dopm(A) system have been detected in addition to HA, H2A and H3A species. The log ~ values of MA species in the