PHYSICO-CHEMICAL STUDIES ON COMPLEX ION FORMATION BETWEEN METALS AND IMIDES

Abstract

avenget the nitrogen bearing Uganda those offering sites simultaneously for salt formation and co-ordination deserve special mention, apart from their importance la biological syaternsfthese reactions are Interesting from theoretical atend point and a few of then have yet to be investigated to add to the present state of knowledge in this field* To one such class of compounds,belong the imldestboth aliphatic and aromatic,known for their tendency to give hydrolysable metal complexes difficult to be characterised. A systematic and comprehensive study of the products formed by the Interaction of heavy metal tone end complexlng agents containing lmlde groups was, therefore.considered worth undertaking. Before introducing the subject.it will not be out of place to give a brief resume of the reactions of metals with compounds closely related to imldes. Jpino amidst Of these the glycinates have been most extensively investigated (1*3). The nature of the complexes has been revealed by infra T9A spectroscopy. It has been found that the metal-nitrogen bonds are largely covalent whereas the oxygen-metal bonds are essentially ionic (4). Trans square planar configuration exists in the sine as well as the copper and nickel complexes.although a tetrahedral configuration la reported in some cases(5). Metal glycinates Involving carboxylate bonds (6*9) and linear complexes of Ag(I) and % (II) formed through amino groups are also known to exist. Other amino acids besides glycine such as al aaina, laucine, valine etc. also show these types of bindings (10-13). Interesting results have also been reported by Tanford (14) and -iarma (16) on cobalt-arginine and on copper-glutamic acid complexes. Creen and Ang (16) have tried to explain more complicated reactions like one between chromium and alanine by introducing the concept of partial chelation end partial co-ordination In the same reaction. kith hlstidlne and cysteine, the behaviour is far more complicated. Four possible structures have been visualisedi (1) a five membered ring complex involving the carboxylate and <£ •amino group} (li) a six membered ring involving the amino and the imidazole group}(ill) a seven mossberad ring Involving the carboxylate and the imidasole groups and (iv) a structure in which all the donor groups of hlstidlne are combined with the metal ion. The work of Edsall (17) on copper dlhlstldlnate and the hydrogen equilibrium studies on cysteine and homocysteine carried out by Baneseh (18)f Schmidt (19) etc. can be put forward in support of the above contentions. Evidence for the simultaneous combination through the sulphahydryl and amino groups of cysteine has been provided in the ease of Co(II) (30,21 ),2n(II) and Cd(II) (22). Complexes of cysteine with natal ions have attracted the attention of Koithoff and 3tricks (23),and a number of papers an this topic have been published in the early -3- flftles. Their investigations en the soluble Fe(ZI) and Fe(III) complexes of cysteine in the high pH range (^ 10) with the possible formulation! FeOH (IB)-"" an* '•(^V" i are most interesting. More recently Malik and Khan (24) investigated the interaction of 0r(III) ions with glycine,lyclne,leucine and aspartle acid,both ep^etrophotometrieally and by Bjerrum*s method. On the basis of spectrophotometry,they coneludedt (1) for the concentration 0.04 M,leucine and lysine form complexes in the molar ratlo(CrCl3t amino acids) it3 while for the concentration 0.0133 M,the ratio is it2 } and for coneentratlon 0.02 M, the ratio for lysine is li2 and that for leucine is lt3 (il) for concentrations 0.02 M and 0.0133M,aspartle acid forms a complex in the molar ratio lil. The Information on combining ratios as obtained by Job's method was then successfully employed to determine the stability constants of a large number of amino acids, applying Albert's (26) procedure, the following values of stability constants in KC1 medium were obtained* km MammUtal mmmtJaamtl Glycine Valine Leucine atparaglne -Alanine X 16.30 14.60 16.3 14.0 16.2 IX 18,30 17.55 18.6 16.5 18.9 Lysine Arglnlne 14.7 14.1 11. * 4> f . A X XI Methionine Serine 14,60 14,0 17.10 17.1 Paottamst Moat of the work on metal complexes of peptides cited in the literature,deals with glycyi-glyelnate. Monk (26,27) gave a value of log K a 6,04 for Cu(II) glyeyl-glycinate as against P,62 for the corresponding glycinate and attributed the difference to the differing basicities of <* -amino nitrogen atoms. The author found a value, log K. • 1,24, for calcium glyeyl-glycinate which was eo close to the log K^ value for calcium acetate(l.O) that the binding of calcium to the peptide was considered to take place through carboxylate group, 0reewald(2S) has mentioned the possibility of the peptide complexes undergoing ionlsation In the high pH range, Debbie and Kermack (29) have shown that the solution prepared from cuprous chloride and dipolar glycyl-glyclne In the ratio It 2 may take up four moles of alkali during the titration ( pH 11). This behaviour was explained by assuming the co-ordination between Cu(II) and peptide nitrogen,resulting in the ionlsation of two protons• CHg - COO* JHg - COO Monayek at al.,(30) have assigned the same structure for the complexes of glycyl-glycine and diglycyI-glycine with Co(II) and Ni(II). Proteinst The literature on metal protein complexes -5- dealing with both qualitative and quantitative studies is so vast that it is difficult to summarise it in a few pages. However, an attempt will be made to describe a few important reactions relevant to the present studies. Proteins offer several functional groups for combina tion with metals,depending c« the pH of the medium.Binding of the metal with the a«ino group is difficult to realise even in the moderately low pH range. For example, the binding capacity of zinc at pH 7 is r9ty small, Gurd and Goodman (31) have carried out extensive studies on the binding of zinc with serum albumin employing tquilibrium technique. From th«ae studies, they concluded that the binding of &i(II) with albumin in the pH range 5,6 to 7.5 is almost due to sixteen imidazole groups of the histidine residues and that vary little Elnc is bound at more acid pH values. The significance of the imidazole group for the binding of sine was also confirmed by Tsnford(32) and Cohn (33). Strong evidence of binding through the amino group of the protein is obtained in strongly basic medium. Mehl et al.(34,35),Klote md Curme (36) had shown that each metal was bound to four peptide nitrogen atoms in the oase of copper complexes. Iron complexes hava been mainly studied with blood plasma. Competitive binding of the ferrous ions with slderephliin and B - pseudoglobulin In plasma protein (IY) were carried out by Cohn(37), On the basis of the similarity In the spectrophotometrle behaviour of the iron complexes of the protein and asparaginic acid, he concluded that the metal was bound to the protein through a linkage similar to hydroxamlc sold (38), at already mentioned under amino acids,the chromium complexes of proteins present a number of difficulties due to the existence of polynuclaar complexes in the aqueous solution of the metal it-self, Gustavson (39),^mythe and Schmidt (40) have provided a satisfactory reaction mechanlsn for the interaction of basic chromic salt with collagen. According to them, the initial reaction la an Ionic interaction of oatlonlc chromium complexes with the charged carboxyl groups of the protein,resulting in the formation of a covalent-coordinate bond, -ince several chromium atoms are prmmt in the polynuclaar cor piex and in view of the secondary aggregation of the fixed chromium complexes by further hydrolysis, possibilities for a multipoint interaction of one chain like chromium complex with several carboxyl groups of collagen lattice exist. The interaction of metals, especially Hg(II),with the «*&H grmp of the protein is also worth mentioning. The mercuric ion or Its mono-aikyl derivatives couple with the single sulphydryl group of serum albumin to give complexee (41,42), with albumin, a dimer is formed when one mole of Hg(II) is pfamxX for every two moles of albumin, while with higher ratio of the metal to albumin, a monomer Is obtained, illver also forms similar complexes with proteins and both the metals can, there fore, be employed to estimate the •81 groups of proteins ( 43,44 ), mrfm Experimental evidence is available to prove that there exist relative avidities (45) of proteins for metal ions. For example, aiderophllln, the metal binding component of plasma, and conalbumin show a marked affinity for b >th iron and copper (46,47) in comparison to other proteins. The nature of these specific sites, although not clearly understood (48), has been attributed to the a HH side chains,formed due to the displacement of i by the combining metals.