OORKEESTUDIES ON THE REACTION OF AMINO ACIDS WITH COBALTAMMINES AND CYANOFERRATES

Abstract

The reaction of metals with amino acid is not only of academic interest but is also of great physiological importance. Amino acids are the building units of all proteins and enzymes and are intimately associated with metal ions in biological systems. For example, it has been established that a number of metal cations act as either activators or accelerators in enzymatic reactions (1,2). Recent studies have been directed to know the exact role of metal ions in proteins and enzymes. This has been possible by carrying out fundamental studies on these reactions by taking amino acids as the models and it is expected that precise information on the role of metals in biological systems would be made available on the basis of these studies. Amino acids are the products of hydrolysis of polypeptides and enzymes (proteins) and their hydrolysis is either acid-catalysed or metal ion-catalysed The latter, however, is very interesting as it involves a mechanism with metal ion coordinated with various donor groups in the enzymes as an intermediate. A widely accepted explanation of this phenomenon is that the associated metal protein complex represents the active form of the enzyme. The metal serves as an activating prosthetic group. In the class of ligands, bound simultaneously with nitrogen and oxygen atoms to the central metal ion, the amino acid anions form a numerous and coherent subclass. These ligands are characterised by the simultaneous presence of amino and carboxylate groups. The biologically important species are nearly all a-amino acids(R)CH(NHp.)COOH with-COO"and -WA.? bound to the same carbon atom. These neutral amino acids occur as zwitterion, fH3-CH(R)C007 with a large dipole moment. In the normal case, the a-amino acid forms a stable five membered ring as a bidentate ligand but it links sometimes as a unidentate ligand (3,4) also. Metal-Amino Acid Complexes The amino acid anions, NHpCH(R)CCO, have two effective donor atoms in a-amino and carboxyl groups. In addition, the side chain, R, sometimes may have donor atoms which may complete with the former two in coordination. Amongst the metal amino acid complexes, the glycinates have been studied most extensively. Having the common amino and carboxyl groups as coordina ting species, the stability of glycinates has been studied by comparison with oxalate and ethylenediamine com plexes. It was observed that the affinity of Co, Ni, Zn and Cu is greatest for ethylenediamine and least for 9 oxalate, showing their preference for bonding with nitrogen, than to oxygen (5-7). It has been shown by infrared spectra that the N-metal bonds in crystalline Cu and Ni glycinates are largely covalent while the oxygen-metal bonds are essentially ionic (8). Similar studies on the crystalline Zn11 complex led to'the conclusion (9) that linear sp metal orbitals were involved in the bonds between Zn11 and the a-amino nitrogen atom. As a result, the Zn11 complex has the same trans TT square planar configuration as the Cu1 and Ni _ II complexes. The ionic character of the Oxygen Zn bond is related to the non-participation of these bonds in sp3 hybridisation (tetrahedral configuration). The complexes of glycine with alkaline earth metals (lO,ll) involve the carboxylate bond and have the form M-00CCH2NH3. Such complexes are also formed by dipolar forms of glycine and alanine with Cu ' and ZnIi:(l2,13). Glycine, although forms chelates with most of the metal ions, yet shows the texidency of forming linear complexes in which the coordination takes place only with the amino group, e.g., Ag and Hg11 complexes. Flood and Loras (5) investigated such complexes and observed that although the carboxylic group weakens the Ag-N bond, the donor properties of nitrogen are balanced by coulombic effect of the charge on the carboxylate group. h Complexes with other monoamLno-monocarboxylic acids like alanine, leucine, valine etc. are known in the literature (6,14-16), but the behaviour is much the same as that of glycine. Amino acids with a third reactive group show a different behaviour in complex formation. Tanford and Shore (7) have shown that Co11 combines with arginine with log k1? log k<? and log k3 equal to 3.87 3»20 and 2.08 respectively. It was shown that the difference between the log ^ values of arginine and alanine (4.27) could be attributed to the electrostatic repulsive effect of the guanidinium group. These authors further observed that cobalt11 combines with aspartic acid with values of association constants falling III between those of glycine and alanine Ct " , under certain circumstances, combines with alanine in more than one way* giving a complex in which amino acid is partially chelated and partially coordinated through carboxyl group. QCO CH3. HC \ NHp + 0C0CH(CH3)NH3 0C0CH(CH3)NH3 ++ Metal complexes of histidine (17) present a more interesting behaviour than cited above. For example, it has been seen that histidine combines with a metal ion 5 through either (a) the usual five membered ring involving the carboxylate and a-amino group, (b) a six membered ring involving the amino and the imidazole group (c) a seven membered ring involving the carboxylate and imidazole groups or (d) a structure in which all the donor groups of histidine are combined with the metal ion. All these structures are possible except, for (c) where the possibility of the reaction is negligible except when the medium is highly acidic, rendering the a-amino group inert. Evidence of combination with a metal ion through (b) is forthcoming from the work of Edsall and coworkers (18). They found that in Cu -dihistidinate, Cu is bound to two II imidazole groups and to two amino groups. That Cu reacts with the imidazole ring of histidine, is further•supported by the fact that both histidine and imidazole are degbAled by dilute hydrogen peroxide at 0° to 25°. The possibility that different structures resulting from (a), (b), (c) and (d) (given above) may exist at the same time means that independent constants must be worked out describing the formation of each type. It has been done in the case of hydrogen ion equilibrium of cysteine and homocysteine (19,20). Cobaltammines: The transition metals of eight group in general e and cobalt in particular, form a large number of octahedral complexes and among them the ammines occupy a.unique position. Owing to a large number of atoms and groups (21) which may be attached to cobalt(III), the cobalt ammines are numerous and their variety, their interesting reactions and possibly also their striking colours have fascinated many chemists. Many of these ammines are easily prepared (22) and often possess very high stability towards chemical reagents. Ammonia is much more firmly bound to cobalt(ill) than to cobalt(ll). Ammoniacal solution of cobalt(ll) has a great tendency to be oxidized to cobalt(lll) state which is affected even by atmospheric oxygen. On passing air in solution of cobalt(ll) containing NH4OI and NH4OH, orange yellow crystals of [Co(NH3)g]ci3, hexammine cobalt(ill) chloride (leuteocobaltic chloride) are formed. In addition to this yellow compound, it is possible to isolate a red salt [Co(NH3)5(HgO')]Cl3, aquopentammine cobalt(III) chloride (reseocobaltic chloride). Yet another purple red anhydrous compound is obtained with a composition C0C13.5NH3 formerly called purpureocobaltic chloride. This compound has one chlorine bound to cobalt and is accordingly formulated as [Co(NH3)5Cl]Cl?, chloropentammine cobalt (III) chloride. Another important compound exists ? as CoCl3.4NH3, which contains two chlorine bound within the complex with the formula [Co(NH3)4'Clr1]Cl dichlorotetrammine cobalt(ill) chloride. It exists in two isomers, the cis form (blue violet) and the trans-form (green), originally known as violeo and praseocobaltic chlorides. In addition to these, there occur numerous complexes with formulae ranging from [Co(NH3)6]"+ to [Co(NH5)gX4] ,where X represents neutral or anionic atoms or groups. These cobaltammines exhibit all types of stereoisomerism, structural isomerism and polymerism. In fact these were the compounds for which the existence of optical isomerism was first detected among inorganic compounds by Werner in 1911. The aquopentammine cobalt(ill) ion is acidic in nature, i.e. it splits off proton and passes over into the corresponding hydroxo ion. [Co(NH3)5H20] + Hg0 > [Co(NH3)50HJ + HgO Various cobaltammines also exist as polynuclear complexes. One of the constituents of the first product obtained by the oxidation of ammoniacal cobalt (II) chloride by oxygen is a sparingly soluble violet black salt of the formula. . . JCo KNHHj9, -- CCoo"1-LJ-~-HHgc(O ^Cl ~\ Cljj trichloro-aquohexammine-/*-amidodicobalt( III) chloride . 8 The first product obtained as above from Co(ll) nitrate is a compound [(ra3)5CoIII-0-0-CoIII(NH3)5](N03)4.2H20 decammine-ju-peroxodicobalt(lll) nitrate which crystallises in brown black prisms. This is converted, by the action of nitric acid into an. intensely green salt [(NH3)5CoIIT-0-0-CoIY(NH3)5](N03)5 decammine-p-peroxocobalt(lII,IV) nitrate. This compound and other salts derived from same cation are interesting in that they contain cobalt in both +3 and +4 states. This structure was confirmed by Gleu(23) (1938) and Malatesta (23)(1942). In addition to these binuclear complexes, there exist tetranuclear brown black complexes of the type [Co^>Co(NH3)4)5] Xb dodecammine hexoltetra cobalt(ill) salts. There also exist a series of red brown cobalt salts with the same empirical composition as above but of only half the molecular weight.