Thermodynamics of the interaction of oxygen with Co(II)-bishistidine and Co(II)-bisbenzylhistidine

Abstract

Oxygen binding by many Co(II)-complexes in aqueous solutions proceeds via a two-step mechanism: Co(II) + O 2 ← K 1 → Co(III)O 2 −1; Co(III)O 2 −1 + Co(II) ← K 2 → Co(III)O 2 −2Co(III). The values of the overall binding constant K 1K 2 for most complexes is known, but the values of K 1 and K 2 are not. In this paper we describe the determination of both constants for the classical Co(II)-bishistidine and Co(II)-benzylbishistidine complexes, by means of a combination of polarographic and spectrophotometric measurements. The overall binding constants for the two complexes at 20°C are 1.7458 × 10 7 M −2 and 9.4050 × 10 6 M −2, respectively. The lower K 1K 2 value of Co(II)-bisbenzylhistidine seems to find its origin in a decrease of the value of the oxygen binding constant K 1 (K 1 = 393 ± 173 M −1 for Co-bishis, and 182 ± 93 M −1 for Co-bisbenzylhis). The thermodynamic constants ΔH and ΔS for the oxygen binding and dimerization steps of Co(II)-bishistidine have been determined from the temperature dependency of K 1 and K 2. The favorable enthalpy of oxygen binding (ΔH 1 = −20.5 ± 2.7 kcal/mol) is almost entirely negated by a very unfavorable entropy term (ΔS 1 = −78.8 ± 12.3 e.u., standard state of 1 atm O 2). The intrinsic oxygen affinity of this complex (P 50 = 1438 mm of Hg at 20°C) is almost negligible. Oxygen binding is completely driven by the formation of the binuclear μ-peroxo complex, (ΔH 2 = −16.1 ± 2.7 kcal/mol and ΔS 2 = −33.6 ± 9.1 e.u.).