Study of some mixed ligand complexes containing biologically imporant ligands

Abstract

Since the various biological activity of living tissues depend upon the interaction of ligand with metal ion, it is of considerable interest to study the coordination behaviour of amino acids and vitamins as biologically important ligand. The present communication deals with the coordination behaviour of some mixed ligand complexes (ternary complexes), where some vitamins and amino acids were used as ligand. In this research work potentiometric determination of stability constant, potentiometric titration curves, species distribution curves, electronic spectral analysis were studied. The stability of ternary complexes have been determined in terms of .6.logK=logKMA-logKMvalue i.e. the difference in the MAL ML tendencies of L to bind with the free metal ion and the metal ion already bound to another ligand. From statistical consideration .6.logK is eXp'ected to be negative. In the present investigation • different kinds!bf ternary complex of [MAL] type have been studied. These are as follows: (1) [MAL] where, M=Cu (11), Ni(ll) and Co(ll); A=Thiamine (Th), L=Ox, gly, Tyr, a. -ala, ph-ala, Tryp. (11) [MAL] where, M=Cu (11), Ni(ll) and Co(ll); A= Riboflavin (Ribo); L=Ox, gly, Tyr, a -ala, ph-ala, Tryp. The stability constants of ternary complex were determined by carrying out pH-metric titration in aqueous media. The calculation was carried out by using SCOGS Computer Programme. The protonligand formation constant and formation constant of the binary complexes were first refined. These value were used as fixed. , parameters for the refinement of the formation constant of the ternary complexes. It is observed that for the ternary complexes of Cu(ll) and Co(Il), L',.logK value is less negative than corresponding Ni(iI) ternary complexes. This is due to the presence of Jahn-Teller distortion in Cu(ll) and Co(ll) Complexes. It is observed that lllogK is positive for complex [MAL], where L is oxalic acid, M=Cu(Il), Co(ll) and less negative for Ni(ll) complex. This is because of electron repulsion between the' metal ion d1t electron and lone pair electron of Oxalic acid. More over as oxalate ion has two negative charges, it is more strongly attracted by [MAt2 than other neutral ligands. It is observed that lllogK value is less negative for phenylalanine, tyrosine and tryptophane. This is due to intermolecular interligand interaction between non-coordinated side chain,--which stabilizes the ternary complexes leading' to less negative lllogK value. In case of riboflavin an extra stabilization of tyrosine and tryptophane is also observed. This is due to hydrogen bonding between tyrosine, tryptophane and riboflavin. Another reason for greater stability of Cu(ll) and Co(ll) complexes as compare to those complex of Ni(ll), is due to the distorted octahedral and square planar geometry of Cu(ll) and Co(ll). II This creates a geometry favourable for the co-ordination of the , secondary ligand where ligand-ligand repulsion is.minimum. More over larger size of Cu(ll) and Co(ll) fascilate the accomm~dation of ligand more easily than Ni(ll). Hence in the present investigation, the order of the stability of the ternary complexes are as follows :- (Cu(A)L» (Co(A)L» (Ni(A)L) Finally [M(Ribo)L] complex is more stable than [M(Th)L] complex. This is due to size of the chelate ring. ~Logl<J,value of [M(Ribo)L] is higher for formation of five membered ring with metal ion. But [M(Th)L] form six membered ring. [ The stability of ternary complexes have been confirmed by , UV spectral studies. Here absorption spectra of[M(Ribo)L] I appeared at longer wave length than [M(Th)L]. This indicates greater splitting of d orbital and also greater stability of [M(Ribo)L] complexes over [M(Th)L] complexes.