The thermodynamic parameters for the formation of the free radicals upon electron transfer quenching of the flavin triplet state (3FMN) by tryptophan and tyrosine, ΔFRH and ΔFRV, were obtained in aqueous solution by the application of laser-induced optoacoustic spectroscopy at various temperatures. The ΔFRH and ΔFRV values include the electron transfer and charge separation steps plus the protonation of the FMN anion radical and the deprotonation of the amino-acid cation radical. A linear correlation was found between the ΔFRH and ΔFRV values for each of the amino acids in phosphate buffers of [CH3(CH2)3]4N , Li , NH4 , K and Cs . The compensation between ΔFRH and ΔFRV within the salt series, and the independent evaluation of the Gibbs energy for electron transfer ΔETGo afforded the entropy change, ΔFRS, for the reaction, different for the two amino acids. The values of ΔFRH, ΔFRV and ΔFRS in each buffer are mainly determined by the changes in strength and probably number of hydrogen bonds between the reacting partners and water produced along all steps leading to the radicals FMNH· and A·. The ΔFRV values linearly correlate with the tabulated entropy of organization of the water structure for the five cations, ΔSo(cat). The entropy change upon formation of the free radicals, ΔFRS, quantitatively correlated to the ΔFRV value, drives the separation of the ion pair after the electron transfer reaction in the case of highly organizing cations. The ratio X = T ΔFRS/ΔFRV = (55 ± 9) kJ cm−3 for Trp as 3FMN quencher is smaller than X = (83 ± 9) kJ cm−3 for Tyr as quencher. These values are discussed in conjunction with the Marcus reorganization energy, as calculated from the Gibbs activation energy of the electron transfer process, which is independent of the salt present but different for each of the two quenchers.
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Vol. 82 • No. 1