Ultrafast Solvation Dynamics of Flavin Mononucleotide in the Reductase Component of p-Hydroxyphenylacetate Hydroxylase

The reductase unit of p-hydroxyphenylacetate hydroxylase contains flavin mononucleotide (FMN) as a cofactor. Fluorescence decay curves measured by fluorescence up-conversion method were remarkably dependent on monitored emission wavelength. The fluorescence lifetime was shorter at the shorter emission wavelengths and longer at the longer wavelengths. Spectral shift correlation function of p-coumaric acid in water and FMN in C 1 protein in buffer solution were expressed by two-exponential functions. Correlation times, φ 1 and φ 2 , of p-coumaric acid were 0.053 and 0.650 ps, respectively, which was similar to previous works. φ 1 and φ 2 of C 1 were 0.455 and 250 ps, respectively. The Stokes shift from t = 0 to t = ∞ was 2200 cm ―1 , while it is 500 cm ―1 in the static Stokes shift obtained by the solvent effect of the fluorescence spectrum under static excitation. This suggests that the isoalloxazine ring of FMN in C 1 is exposed in hydrophilic environment. Such large Stokes shift was unusual among flavoproteins. The biphasic decay of the spectral correlation function in C 1 was discussed and compared to the biphasic decay of tryptophan in proteins.