Equilibrium and stereochemical study of diastereoisomeric dipeptides and their copper (II)- and nickel (II)-complexes in aqueous solution

Abstract

The aqueous solution equilibria of copper(II)- and nickel(II)- diastereoisomeric dipeptides such as L-alanyl-L-alanine, D-alanyl-L-alanine, L-leucyl-L-tyrosine and D-leucyl-L-tyrosine have been studied by potentiometric and spectrophotometric methods. For the alanylalanine systems, it has been found that the formation of CuL[superscript +] and Cu(H[subscript -1] L)L[superscript -] is favored for the D-L isomer over the L-L isomer, while the formation of CuH[subscript -1]L is favored for the L-L isomer, (where HL-[superscript +] represents the dipolar ionic species; H[subscript -1]L, the peptideproton dissociated form; and D-L, the D-alanyl-L-alanine isomer). Selectivity in the formation of diastereoisomers of CuH[subscript -1]LOH[superscript -] and (CuH[subscript -1]L)[subscript 2]OH[superscript -] was not observed. In the case of nickel(II) complexes, it was found that the formation of NiL[superscript +] and NiL[subscript 2] is favored for the D-L isomer while the formation of the NiHL[superscript 2+] and NiH[subscript -1]L forms is favored for the L-L isomer. Introduction of bulky groups on the peptide side chains, as in leucyltyrosine, produced large steric effects on the acid-base and metal complex equilibria of the diastereoisomers. In this case peptide proton dissociation from CuHL[superscript +] is greatly favored for the L-L isomer, as compared to the corresponding alanylalanine complex. Thus peptide proton dissociation of the L-L complex of nickel(II) was observed, while the corresponding reaction of the D-L complex was not detected. On the other hand, the formation of the 1 : 2 complex, Ni(HL)[subscript 2], of the D-L isomer was observed while that of the L-L isomer was not detected. ...