We suspect contributions from changes in the amide I band, which are indicative for changes in the secondary structure of aureochrome 1, and overlap with the chromophore mode. Furthermore, the stretching vibration of C4=O4 oscillates with a frequency of 1712 cm21, the in-plane bending vibration of N3-H at 1374 cm21 and the ring vibration involving mainly n, n, n, n, d at 1246 cm21. In aureochrome 1, the atoms O4, N3 and O2 of ring III of the FMN form hydrogen bonds with the side chains of N286, N296 and Q317, residues that are highly conserved in LOV domains. Thus, the vibrations of ring III are influenced by the strength of the hydrogen bonds. The observed shift by 3 to 5 cm21 to lower wavenumbers corresponds to an increase of the strength of the hydrogen bond network Chlormethiazole hydrochloride formed with the chromophore. The adduct state is characterized by a covalent bond between the C4a of the isoalloxazine ring of FMN and the sulfur of a nearby cysteine. Since the cysteine is protonated in the ground state, the S-H stretching vibration appears as a negative band in the FTIR difference spectrum. In fact, a negative band at 2563 cm21 is observed and, thus, assigned to C254 of the ground state of aureochrome 1. The frequency of the S-H stretching vibration of C254 is downshifted by 7 cm21 in comparison to the corresponding vibrations in YtvA, LOV1 and LOV2 of phototropin at 2570 cm21, The position of this band rather fits to a shoulder at 2562 cm21 that was observed in the spectra of LOV1 domain. The appearance of this shoulder was interpreted as the band of the second rotamer configuration of the side chain of cysteine. One rotamer is closer to FMN and in a more polar environment as the other rotamer, which is in close vicinity to the methyl group of a nearby leucine Clonazepam residue. This interpretation is in line with the frequency shift of the S-H stretching vibration to lower wavenumbers when organic thiols are dissolved in polar solvents. Therefore, we infer from our IR study that C254 of aureochrome 1 seems to prefer the rotamer configuration that is closer to N5 of the isoalloxazine ring of FMN. The photoreaction of full-length aureochrome 1 from Vaucheria frigida was studied by molecular spectroscopy. The visible absorption spectrum shows only one broad band in the UVA region, which indicates that the LOV domain of aureochrome1 resembles the LOV2 domain of phototropin and LOV domain of YtvA. This is in line with the fact that T222 and N229 in aureochrome 1, which strongly influence the spectral features in the UVA range are conserved in LOV2 domains.