Photopysical and photochemical excitation and relaxation dynamics of LOV domains of phot from Chlamydomonas reinhardtii

The absorption and emission behaviour of flavin mononucleotide (FMN) in the wild-type light, oxygen and voltage-sensitive (LOV) domains LOV1 and LOV2 of the photoreceptor phot from the green alga Chlamydomonas reinhardtii is studied. FMN is non-covalently bound to the protein binding pocket of the LOV domains (LOV-ncb). Only a small amount of FMN is not bound in aqueous solution (LOV-ads). Blue-light photo-excitation of non-covalently bound FMN generates a non-fluorescent intermediate flavin-C(4a)-cysteinyl adduct, and concomitant blue-light adduct excitation converts it partly back to non-covalently bound FMN, thus hindering complete bound FMN transfer to the adduct form. The LOV domains with non-covalently bound FMN consist of two conformations, one with fast FMN–Cys adduct back recovery time to bound FMN (LOV-fast), and one with slow recovery time (LOV-slow) due to different activation barriers. Prolonged blue-light irradiation of the flavin-C(4a)-cysteinyl adducts reduces their ability to recover back in the dark. The photo-adduct formation in the LOV1 domains proceeds via triplet formation, while for the LOV2 domains the photo-adduct formation is thought to proceed via singlet excited-state electron transfer and singlet excited-state triplet formation.