COPPER(2) CHELATION KINETICS . ALPHA-ALANINE, BETA-ALANINE, AND HISTIDINE

Rate constants for the formation of Cu2+ complexes with α- and β-alanate and the histidine zwitterion have been measured by temperature-jump and stopped-flow methods. The forward rate constants for α-alanine (1.3 X 109 and 1.5 X 108 M−1 sec−1) illustrate “normal” kinetics for Cu2+. Those for the mono and bis complexes of β-alanine are substantially smaller (2.0 X 108 and 8 X 106 M−1 sec−1), and for histidine smaller yet (1.3 X 107 and 3.0 X 106 M−1 sec−1). In addition, for β-alanine, a slow kinetic step following the complexation reactions was observed, probably associated with the formation of a partly half-bonded tris complex. For the formation of the five-membered chelate with α-alanine, the rate-determining step was ligand penetration, consistent with the Eigen-Tamm mechanism for the formation of a strong chelate. The substantially reduced rates for β-alanine and histidine are interpreted on the basis of ring closure as the rate-determining step, the effect being most pronounced for the formation of the histidine chelate. © 1969, American Chemical Society. All rights reserved.