Spectroscopic evidence for the effect of the ortho H2O spin on the electron transfer in photosynthesis

Abstract

A spin mechanism for electron transfer control in the reaction center of purple bacteria in photosynthesis is proposed. Rotation and conversion of the ortho/para spin isomers of two H2O molecules located near the special pair of the reaction center are treated as the sources of the coherent modulations of transient kinetics. Modulation of the collective wave function of the reaction center electrons by the total proton spin of ortho H2O is a key feature allowing the molecule to play the role of a gate controlling the electron transfer. The iron atom in the reaction center with the gradient magnetic field is treated as a catalyst removing the strict forbiddenness of H2O ortho/para conversion. It is shown that the modulation of the reaction center stimulated emission kinetics observed in the field of femtosecond pulses coincides with the rotational transitions of ortho/para H2O. Influence of the effect of the electric field (Stark effect) on the level displacement and ortho/para conversion rate is discussed.