The road not taken: a theoretical view of an unexpected cryptochrome charge transfer path

Motivated by recent progress in electron paramagnetic resonance spectroscopy, we describe hole transfer along a chain of tryptophan amino acids within the cryptochrome protein of Synechocystis sp.: surprisingly, despite a close sequential and structural similarity to E. coli DNA photolyase, the charge transfer paths and the final sites of charge localization are different for these two enzymes. We study this phenomenon using atomistic simulations and electronic structure computations as a theoretical basis, and we take a new look at the concepts of charge transfer and introduce a modification of Marcus' theory that incorporates dynamic polarization effects. Only this variant of theory describes the population of the correct branch on the subnanosecond time scale. Based on our numerical analysis, we further suggest that the Asp372-Arg374 salt bridge acts as a novel stepping stone in the charge transfer reaction.