Electron-transfer reactions studied by laser-induced optoacoustics: Learning about chromophore-medium (protein) interactions*

The ability of time-resolving enthalpy (∆H) and structural volume (∆V) changes in the nanoto μs time range offered by laser-induced optoacoustic spectroscopy (LIOAS) opens the possibility of a stepwise thermodynamic analysis of chromophore-medium interactions upon photoinduced reactions in biological systems. We applied LIOAS to biological photoreceptors, as well as to model systems, with the purpose of understanding the origin of ∆V in electron-transfer (ET) reactions in those systems. The linear correlation between the counterion-dependent volume changes and ∆H for the free-radical formation upon ET quenching of erythrosin dianion triplet, 3Er2–, by Mo(CN)8 4– and of Ru(bpy)3 2+ by MV2+ is interpreted in terms of an enthalpy–entropy compensation owing to the strong influence of the counterions on the water hydrogen-bond network in which the reactants are embedded. The relatively large entropic term determined for radical formation thus originates in water rearrangements during the process. The increasing contraction in acetonitrile, propionitrile, butyronitrile, and valeronitrile for the ET quenching of 3Zn-tetraphenylporphin by 1,4-benzoquinone is understood by considering the increasing interaction strength between the electron-pair donor nitriles and ZnTPP+. Thus, in polar environments, specific chromophore-medium (solvent or proteins) interactions, in addition to electrostriction, should be considered to explain the time-resolved ∆V and ∆H values.