Microscopic dissection of chirality-induced spin selectivity in cryptochrome

Chirality-induced spin selectivity (CISS) has been proposed as a mechanism that enhances magnetic field sensitivity of radical pairs in magnetosensitive proteins, such as cryptochrome, through the recruitment of net spin polarization during radical pair formation and recombination. However, existing studies have been largely limited to radical pairs undergoing one-step electron transfer recombination or phenomenological CISS models. Here, we develop a microscopically derived model of CISS acting during both formation and recombination within a radical pair in a multi-step donor–bridge–acceptor system representative of the flavin–tryptophan electron transfer chain in cryptochrome. We find that both spin polarization and coherence are generated during the step-wise electron transfer, governed by the interplay of exchange coupling and transfer rates through the bridge. However, the microscopically derived recombination operator preferentially probes coherence, thereby limiting the extent to which CISS effects enhance orientation-dependent sensitivity. Although CISS can substantially modify the overall reaction yield, the orientation-dependent changes in reaction yield remain modest in absolute terms, although they can be significant relative to the orientation-averaged yield, particularly in rapid and asymmetric recombination regimes indicative of the quantum Zeno effect. These results suggest that previous phenomenological predictions of CISS-enhanced magnetosensitivity may have overestimated its effect in boosting the magnetosensitivity of biological radical pair systems, while supporting a more modest, yet physically consistent, role for microscopically derived CISS effects, particularly in regimes where the quantum Zeno effect is operative.