Kinetic Stereoselectivity in Complexation Reactions of L-Leucyl-L-tyrosine and D-Leucyl-L-tyrosine with Nickel(II) in Aqueous Solution

Abstract

It is known that L-leucyl-L-tyrosine and D-leucyl-L-tyrosine show different pK values and stability constants with metal ions. To investigate kinetic effects of the optical activity of dipeptides, the rate constants of the following reactions were determined by temperature jump method
k1 Ni2++A- 4 NiA+ k2 NiA+ +A NiA2 k_ 2 (A= L-leucyl-L-tyrosine or D-leucyl-L-tyrosine).
The results, at 25° C and ionic strength of 0.15, were: L-leucyl-L-tyrosine, k, =2.2X 10 mol-'l'sec1, k_1= 1.3 sec', k2= 1.7 × 103 morl isec', k_2= 3.0 sec'; D-leucyl-L-tyrosine, k, 2.4 × 103 morl / sec', k_1=0.45 sec-1, k2-= 2.0 x 10' mol-1 /k2=2.4 sect. The value of formation rate constant k1 for D-leucyl-L-tyrosine is equal within experimental error to that for L-leucylL-tyrosine. But in dissociation reactions the value of k1 for D-leucyl-L-tyrosine was about one third of the value for L-leucyl-L-tyrosine. Kinetic stereoselectivity in complexation reactions of nickel(II) with these dipeptides appeared in the dissociation reaction of the complexes. These results were similar to the case of bi-histidine complexes of nickel(II), cobalt(II), zinc(II).