Ultrafast fluorescence dynamics of NADH in aprotic solvents: Quasi-static self-quenching unmasked.

We have recently provided some experimental evidence that there are ultrafast quenched conformation(s) ("dark states") of NADH. In this paper, the ultrafast fluorescence dynamics of NADH free in aprotic solvents (DMSO/DMF) has been investigated, using both a femtosecond up-conversion spectrophotofluorometer and a picosecond time-correlated single-photon counting (TCSPC) apparatus. The fluorescence kinetics in three solvents were recovered by constructing DAS (decay associated spectra) and TRES (time-resolved emission spectra). With the decrease of the ability of solvent to provide hydrogen bond interaction, apparent slow solvent relaxation (SSR) amplitudes were reduced, and the signal of "pure" quasi-static self-quenching (QSSQ) emerges. This fully positive DAS (positive at all emission wavelengths) appeared only in DMF, and this component accounts for about 20% of the total DAS. This confirms a model in which the ultrafast quenching portion of DAS is masked at times by the negative-going signal from slow solvent relaxation (SSR). Further, we have shown that the ratio of any slow solvent relaxation and QSSQ terms should be accounted for when quantifying NADH via fluorescence lifetime imaging microscopy (FLIM). Eventually, the other properties of the solvent (not only hydrogen bonding, but also polarity, viscosity, etc.) incorporated in QM-MM simulation must be fully considered to predict this ultrafast quenching vs. SSR mixture more accurately. For now, we exploit the greatly reduced H-bonding to decrypt the QSSQ in a mixture.