Thermodynamics of metalloprotein electron transfer reactions

Abstract

Thin-layer spectroelectrochemical methods have been used to determine the formal reduction potentials of several electron transfer metalloproteins in the temperature range 5–40°C (5–26°C for plastocyanin). Measurements were made using OTTLE cells in a nonisothermal configuration. Electron transfer reaction entropies (ΔSºrc = Sºred - Sºox)at 25°C (pH 7, μ = 0.1 M) derived from these measurements are as follows: cytochrome c (horse heart), −14.9 ± 1.2; cytochrome c2(Rhodospirillum rubrum) −9.6 ± 1.2; cytochrome c551(Pseudomonas aeruginosa) −16.2 ± 1.2; HiPIP (Chromatiumvinosum), −10.0 ± 1.2; azurin (Pseudomonas aeruginosa) −16.1 ± 1.2; stellacyanin (Rhus vernicifera) −4.2 ± 1.2; plastocyanin (Phaseolus vulgaris) −2.4 ± 1.2 eu. In all cases the ΔH° values are negative. The fact that the ΔSºrc values are more negative than inorganic complexes with low inner sphere electron transfer barriers is discussed in terms of changes in protein-solvent interactions that could accompany reduction. Either increased solvent ordering in the protein interior or a more compact protein structure in which solvent is excluded could account for the observed loss in entropy in the reduced solution state. © IUPAC