UV-visible absorption spectrum of FAD and its reduced forms embedded in cryptochrome protein
Cryptochromes are a type of flavoproteins proposed as candidates to explain magnetoreception of animals, plants and bacteria. The main hypothesis is that a biradical is formed upon blue-light absorption by flavin adenine dinucleotide (FAD). In protein milieu, the oxidized form of FAD can be reduced, leading to four derivate redox forms: anionic and neutral semi-reduced radicals, and anionic and neutral fully reduced forms. All these forms have a characteristic electronic absorption spectrum, with a strong vibrational resolution. Here, we employ a normal mode analysis at the electrostatic embedding QM/MM level of theory to compute the vibrationally resolved absorption spectra of five redox forms of FAD embedded in plant cryptochrome. We show that explicit account of vibrational broadening contributions to electronic transitions are essential to reproduce the experimental spectra. In the case of the neutral radical form of FAD, the absorption spectrum is reproduced only if the presence of a tryptophan radical is considered.