Proton transfer reactions in macrocyclic complexes and in metal-peptide complexes

Abstract

The rates of proton transfer reactions at the γ-carbon atom in macrocyclic tetraazadiene complexes and at the nitrogen atom in metal peptide complexes are several orders of magnitude slower than the reaction rates of typical oxygen and nitrogen acids with similar pK_a values. The Brönsted plots for the macrocyclic complexes deviate little from a slope of 0.5 over a wide range of ΔpK_a values, while the metal peptides undergo a relatively fast transition from α values of unity to zero, without reaching the diffusion limiting rates. In order to fit the data to the Marcus theory it is necessary to include, in addition to the reorganizational barrier (λ/4), a term W′_R, which represents the solvent reorganization necessary to initiate hydrogen bonding after the encounter acid-base species are formed. The W′_R term is in addition to the work necessary to form the encounter species, but it is independent of ΔpK_a. The metal-macrocyclic complexes have large λ/4 values and small W′_R values, while the metal-peptide complexes have small λ/4 values and large W′_R values.