Femtosecond Fluorescence Spectra of NADH in Solution: Ultrafast Solvation Dynamics

The ultrafast solvation dynamics of reduced nicotinamide adenine dinucleotide (NADH) free in solution has been investigated, using both a femtosecond upconversion spectrophotofluorometer and a picosecond time-correlated single-photon counting (TCSPC) apparatus. The familiar time constant of solvent relaxation originating in "bulk water" was found to be ∼1.4 ps, revealing ultrafast solvent reorientation upon excitation. We also found a slower spectral relaxation process with an apparent time of 27 ps, suggesting there could either be dissociable "biological water" hydration sites on the surface of NADH or internal dielectric rearrangements of the flexible solvated molecule on that timescale. In contrast, the femtosecond fluorescence anisotropy measurement revealed that rotational diffusion happened on two different timescales (3.6 ps (local) and 141 ps (tumbling)); thus, any dielectric rearrangement scenario for the 27 ps relaxation must occur without significant chromophore oscillator rotation. The coexistence of quasi-static self quenching (QSSQ) with the slower relaxation is also discussed.