Synthetic model complexes for studying light-driven electron transfer in photosynthesis

Abstract

Mixed multimolecular layers of ethyl chlorophyllide a and lumiflavin were prepared and found to exhibit a difference absorption spectrum against multimolecular layers of pure ethyl chlorophyllide a, with a maximum at 698 nm and a minimum at 670 nm in the 520-800 nm range. Similarly, molecular complexes of chlorophyllic a and lumiflavin dissolved in aqueous phosphate buffer containing poly- N-vinylpyrrolidone exhibited a difference absorption spectrum against the uncomplexed components, with a maximum near 695 nm and a minimum near 660 nm in the same wavelength range. The maxima in both difference spectra can be bleached reversibly by red light. The bleached samples recovered in the dark at room temperature with half-lives of 46 and 75 msec, respectively. The photobleaching was accompanied by the generation of electron spin resonance signals which are interpretable in terms of the oxidized chlorophyll radical and the lumiflavin radical. The photochemical properties of these model complexes are compared with those of P700, and their possible relevance to the primary energy conversion mechanism in photosynthesis is discussed.