Theoretical examination of nuclear spin diffusion in light-induced spin coherences in photosystem I

Abstract

Microsecond coherence times are predicted in spin dynamics calculations of a donor–acceptor electron spin pair P+A1A− created upon light excitation of photosystem I (PSI). The effects of nuclear spin diffusion (NSD) due to the anisotropic protein environment on the predicted coherence times TM are studied. Closely positioned pairs and triples of protons located 5–8 Å from the electron spin are shown to largely control TM. Knowledge of the crystal structure of PSI allows for spin dynamics calculations where specific cofactors and amino acid residues are removed or replaced and the identification of anisotropic environmental features controlling electronic decoherence. Finally, we show that NSD alone cannot explain the >3× shorter experimentally observed coherence times and suggest that methyl groups at key protein sites may explain this discrepancy.