Orientation of a Key Glutamine Residue in the BLUF Domain from AppA Revealed by Mutagenesis, Spectroscopy, and Quantum Chemical Calculations.

The article characterizes the effects of Gln63 → Leu (Q63L) mutation, which removes a putative hydrogen bond between the flavin ring and the Gln63 side chain, on Raman and ultraviolet-visible absorption spectra in the BLUF domain of AppA. The results demonstrate the absence of a hydrogen bond between Gln63 and the flavin C4++O in the dark state AppA in solution. The observed spectral changes are theoretically evaluated using quantum chemical calculations based on density functional theory.