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Inverted-region electron transfer as a mechanism for enhancing photosynthetic solar energy conversion efficiency

Significance Inverted-region electron transfer is widely suggested to be an important mechanism contributing to photosynthetic efficiency. However, this mechanism has never been demonstrated in any native photosynthetic system under physiological conditions. Here, inverted-region electron transfer is demonstrated in a native photosynthetic protein complex under physiological conditions. Furthermore, inverted-region electron transfer is shown quantitatively to be an important mechanism underlying the very high efficiency associated with solar energy conversion in photosystem I in situ. In all photosynthetic organisms, light energy is used to drive electrons from a donor chlorophyll species via a series of acceptors across a biological membrane. These light-induced electron-transfer processes display a remarkably high quantum efficiency, indicating a near-complete inhibition of unproductive charge recombination reactions. It has been suggested that unproductive charge recombination could be inhibited if the reaction occurs in the so-called inverted region. However, inverted-region electron transfer has never been demonstrated in any native photosynthetic system. Here we demonstrate that the unproductive charge recombination in native photosystem I photosynthetic reaction centers does occur in the inverted region, at both room and cryogenic temperatures. Computational modeling of light-induced electron-transfer processes in photosystem I demonstrate a marked decrease in photosynthetic quantum efficiency, from 98% to below 72%, if the unproductive charge recombination process does not occur in the inverted region. Inverted-region electron transfer is therefore demonstrated to be an important mechanism contributing to efficient solar energy conversion in photosystem I. Inverted-region electron transfer does not appear to be an important mechanism in other photosystems; it is likely because of the highly reducing nature of photosystem I, and the energetic requirements placed on the pigments to operate in such a regime, that the inverted-region electron transfer mechanism becomes important.