In photosynthetic organisms light energy drives electrons from a donor chlorophyll species via a series of acceptors across a biological membrane. This light-induced electron transfer process is remarkably efficient, indicating a near complete inhibition of unproductive charge recombination reactions. Unproductive charge recombination reactions can be inhibited if they occur in the, so-called, inverted region. However, inverted region electron transfer has never been demonstrated in any native photosynthetic system.

Here I will describe our recent studies using time-resolved visible and infrared (FTIR) spectroscopy to study solar energy conversion processes in native and (cofactor) modified photosystem I photosynthetic reaction centers. From these studies I will show that unproductive charge recombination in native photosystem I does occur in the inverted region.

Computational modeling of light-induced electron transfer processes in photosystem I indicate a decrease in photosynthetic quantum efficiency, from 98% to below 72%, if the unproductive charge recombination does not occur in the inverted region. Inverted region electron transfer is therefore shown to be an important mechanism driving the efficient solar energy conversion process in photosystem I.

The unproductive charge recombination reactions do not occur in the inverted-region in other photosystems, such as purple bacterial reaction centers. Photosystem I is highly reducing (compared to any other photosystem), and 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.