Electron-transfer reactions studied by laser-induced optoacoustics: Learning about chromophore-medium (protein) interactions
S. E. Braslavsky
Max-Planck-Institut für Strahlenchemie, Postfach
10 13 65, D-45413 Mülheim an der Ruhr, Germany
Abstract: The ability of time-resolving enthalpy (DH)
and structural volume (DV) changes
in the nano- to 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 DV in electron-transfer (ET)
reactions in those systems. The linear correlation between the counterion-dependent
volume changes and DH for the free-radical
formation upon ET quenching of erythrosin dianion triplet, 3Er2-,
by Mo(CN)84- and of Ru(bpy)32+
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
DV and DH
values.
* Lectures presented at the XIXth IUPAC Symposium on Photochemistry, Budapest, Hungary, 14�19 July 2002. Other presentations are published in this issue, pp.999�1090.
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