Time-Resolved Thermodynamics of Inter-Molecular Electron Transfer Between Water-Soluble Anionic Free-Base Porphyrins and Ubiquinone

Abstract

Inter-molecular electron transfer (ET) between ubiquinone (ubiQ) and biological molecules represents a critical process in cellular respiration. The biological importance of this process has stimulated research efforts to understand the fundamental mechanisms associated with proton-coupled ET involving ubiQ. In the present study the kinetics and time resolved thermodynamics associated with photoinduced intermolecular ET between the singlet states of two anionic porphyrins, meso-tetrakis(4-carboxyphenyl)porphyrin (4CP) and meso-tetrakis(4-sulfonatophenyl)porphyrin (4SP), and ubiQ are reproted. The anionic porphyrins facilitate charge separation between the porphyrin cation radical and the ubiQ anion radical. Addition of ubiQ to solutions of either 4SP or 4CP result in quenching of the porphyrin excited singlet state, τ(4SP/4CP)∼10-11ns and τ(4SP/4CP-ubiQ)∼8ns, with the quenching rate constants, kq, of 5x1010 M−1s−1 and 2x1010 M−1s−1 respectively. Time-resolved photoacoustic calorimetry (PAC) signals for both porphyrin species in the presence of ubiQ are monophasic (τ<50ns) with respect to a calorimetric reference at pH 9 and 7 and is independent of ubiQ concentration up to 1.4μM (initial concentration of 4SP/4CP ∼6-10μM). At pH 7 and 9 ΔV is independent of ubiQ concentration with a volume contraction of ∼2-3 mL mol−1 and expansion of ∼20 mL mol−1 respectively. Biphasic signals are observed for each species at pH 6: a fast phase <50ns and slow phase ∼300-400ns. The observed ΔH for the fast phase are independent of concentration within experimental error while those for the slow phase are reported as a function of ubiQ concentration.