Flavin Adenine Dinucleotide Photophysics in Ice

Steady-state, time-resolved emission and pump−probe techniques were employed to study the nonradiative process of flavin adenine dinucleotide (FAD) in methanol-doped ice and in a deuterated solvent mixture of methanol-d−D2O in a large range of temperatures of 79−268 K. We found that the nonradiative process depends on the temperature, i.e., the lower the ice temperature the smaller the nonradiative rate. The time-resolved emission is nonexponential, and a large portion of the decay curve could be reasonably fitted to a stretched exponent with α = 0.55 in a large temperature range. We found a relatively large and unexpected isotope effect, KIE = 1.8, on the nonradiative rate. The large nonradiative rate in FAD (compared to flavin mononucleotide (FMN)) possibly arises from a coupled electron−proton transfer from adenine to flavin in a “closed” conformation existing predominantly in an ice crystal.