EPR and ENDOR of radicals of chlorin- and bacteriochlorin-quinone model compounds for electron transfer in photosynthesis

Abstract

A series of covalently linked (bacterio)chlorin-quinones were synthesized as model compounds for light reactions in photosynthesis. By replacing the more commonly used porphyrin by chlorins, a closer match to the photosynthetically active chlorophylls is achieved. Eight compounds are presented, in which the quinone is linked directly or via a cyclohexylene spacer to the meso position 5 or 10 of the chlorin. The electronic structure of donor and acceptor portions were investigated by EPR and electron–nuclear double resonance (ENDOR) of the cation radicals of bacteriochlorin and chlorin and of the cation and anion radicals of the (bacterio)chlorin-quinones. Semi-empirical molecular orbital calculations (methods: AM1 or PM3 on the neutral compounds, RHF-INDO/SP on the radicals) were performed for the assignment of hyperfine coupling constants, to determine the conformations of the model compounds and to estimate electron transfer properties. The directly linked quinone has a local effect on the electronic structure and conformation of the chlorin. The magnitude of this effect depends on the linking position. Linking the quinone to the cyclohexylene spacer has no measurable effect on the electronic structure of the chlorin. The electronic structure suggests that ratios of charge separation to recombination rates could differ by factors of 20 for different linking positions for the acceptor due to the electronic matrix element. Copyright © 2000 John Wiley & Sons, Ltd.