Abstract
The temperature-jump method has been used to determine the complexation rate constants for the reactions of L-carnosine (HL) with copper(II) at 25° and ionic strength 0.1 M. It had been previously postulated that L-camosine is a bidentate ligand in the lower pH range forming a complex containing a seven-membered chelate ring. The complexation reactions observed are Cu2+ + H2L+ ^ CuHL2+ + H+ and Cu2+ + HL ±=; CuHL2+. The rate constants for the former reaction are ki = 8.9 X 102M-1 sec-1 and k_i = 5.0 X 104M-1sec-1; for the latter they are k2 = 3.5 X 106M-1sec-1andZ:_2 = 35sec-1. These rate constants are consistent with a steri- cally controlled substitution (SCS) mechanism. The complex CuHL2+ can release a proton, producing CuL+; this latter complex can also release a proton to form CuA. The rate constants for CuHL2+ ^ CuL+ + + are 230 sec-1 for the forward reaction and 1.0 X 107 M~l sec-1 for the reverse process. For the reaction CuL+ ^ CuA + H+, the values are 360 sec-1 for the forward reaction and 5.0 X 107 M-1 sec-1 for the reverse process. Both reverse reactions are several orders of magnitude slower than the diffusion-controlled limit. This is attributed to intra- molecular changes which are concurrent with proton attack.