Reactivity of Excited States of Flavin and 5‐deazaflavin in Electron Transfer Reactions

Abstract

Quenching of the excited states of lumiflavin and 3-methyl-5-deaza-lumiflavin by methyl-and methoxy-substituted benzenes and naphthalenes in methanol was investigated. The observed difference in the reactivity of acid and neutral lumiflavin triplets is explained thermodynamically by applying the Michaelis cycle, as being due to the higher reduction potential of the acid triplet. In this connection the pK values of lumiflavin triplet (pKM = 6.5) and semiquinone (pKM = 11.3) have also been determined in methanol. The difference in the reactivity between the singlet and triplet states of lumiflavin is found to be greater as predicted by the difference in excitation energy. The reactivities of the excited states of flavin and 5-deazaflavin differ only slightly in contrast to the marked difference in the ground state reactivities of electron transfer reactions. This is explained in terms of the model of Rehm and Weller. The pH dependence of the electron transfer quenching of 5-deazaflavin triplet was investigated in water, yielding a triplet pK of 2.5. In contrast to the flavin, this triplet pK does not significantly differ from the pK of the 5-deazaflavin ground state. From this, different sites of protonation are deduced for the photoexcited triplet states of flavin and 5-deazaflavin.

Dye Binding and Photodynamic Action

“…The discrepancy can be interpreted in terms of the exciplex formation instead of radical pairs. A similar result has been obtained with flavin [124].…”

EFFECT OF TEMPERATURE AND pH ON THE QUENCHING OF TRIPLET LUMIFLAVIN IN THE PRESENCE AND ABSENCE OF FERRI‐ AND FERROCYANIDE

“…The effects of different quenchers on the photoreduction of the lumiflavin triplet state have been studied at different p H (Traber et al, 1981). Quenching of triplet lumiflavin through a collisional complex by ferrocyanide ions occurs as a result of electron transfer, whereas quenching by ferricyanide is a result of energy transfer, and both processes are functions of complicated parameters (Visser, 1984;Traber et al, 1981).…”

“…The decay kinetics and quenching of the triplet state of lumiflavin by ethylenediaminetetraacetic acid (EDTA)* (Archer et al, 1977;Shigehara and Tsuchida, 1977;Albery and Archer, 1977) and ferriand ferrocyanide ions have been studied because lumiflavin (Lf) is photoreduced in the presence of a suitable reducing agent, yielding the semireduced form (Lf-) or leuco form (Lf=) (Naman and Tegner, 1986;Naman, 1988). Through a suitable system the energy stored in the charged-separated product can be converted to electrical energy, the so-called photogalvanic effect (Mercedes et al, 1984;Traber et al, 1981;Pan et al, 1982).…”

Energy Conversion in the Decay of Triplet Lumiflavin in the Presence of Ferri‐ and Ferrocyanide

“…Also the reactivity of triplet state in the presence of amino acids in electron transfer reaction has been explained by several workers (Traber et al, 1981;Rosa, 1985). The decay kinetics of the triplet excited state have been studied using conventional flash photolysis (Naman and Tegner, 1986) and laser flash photolysis techniques and this state has been quantitatively observed to be formed in high yields under a variety of conditions (Knowles, 1968;Edmondson, 1977;Moore, 1977 andGrodowski, 1977).…”

“…The second longer lived species has been observed in the flash photolysis of lumiflavin and this has been shown to be the semiquinone radical (semireduced lumiflavin) (Schreiner et al, 1975). The normal and anionic semiquinone forms of flavines have been extensively studied over the past years by a variety of physical and chemical techniques (Traber et al, 1981;Rosa et al, 1985). In recent years flavin has been used in photochemical and photogalvanic systems utilizing solar energy for production hydrogen and (to a minor extent) HzOz (Yamase, 1981;Rosa, 1984).…”