Unprecedented insight into the phototriggered unfolding of a polypeptide has been gained from a multiscale simulation connecting nonadiabatic ab initio molecular dynamics to classical molecular dynamics in a three-stage manner. An intramolecular H-transfer mechanism that saturates one of the S. radicals of the cleaved S-S bridge and thus prevents recyclization has been observed. This chemical quenching mechanism may be the key to resolving the controversy surrounding the S-S reformation rates.