Water structural changes in the activation process of the LOV2 domain of Adiantum phytochrome3

In a plants' blue-light receptor phototropin, LOV domains contain a flavin mononucleotide (FMN) as the chromophore. Illumination of LOV domains produces an adduct formation between C(4a) of FMN and an S-H group of surrounding cysteine in an active S390 state. In this paper, we studied water structural changes during photoactivation processes of the LOV2 domain of Adiantum phytochrome3 (phy3-LOV2) using low-temperature FTIR spectroscopy. Water bands were observed only in the 2700-2500 cm−1 region for phy3-LOV2, which correspond to weak hydrogen bonds. The mutation study suggested that these water bands originate from water25 and 45 that are present near the cysteine (Cys966). While the secondary structure of peptide backbone exhibits temperature-dependent changes, water bands were temperature-independent at low temperatures. We concluded that water bands are sensitive to the microenvironment of the adduct formation between Cys966 and the FMN chromophore.