Ab Initio Studies of NMNH(2-) Conformers in Water-Methanol Solutions: Comparative Analysis of the Biexponential Fluorescence Signals for NMNH(2-) and NADH.

Abstract

Ab initio studies of the structure of reduced β-nicotinamide d-ribonucleotide (NMNH(2-)) conformations in water and methanol solutions have been carried out for clarifying the role of the phosphate groups in fluorescence parameters of the NMNH(2-) molecule and the reduced β-nicotinamide adenine dinucleotide (NADH) molecule. Relaxed potential energy surfaces as a function of the dihedral rotation angle of the amide group in the NMNH(2-) molecule were calculated in the ground electronic state and the first excited electronic state to better understand the effect of phosphate groups on the nonradiative decay rates in the nicotinamide chromophore groups. The differences in the weighting coefficients in the biexponential fluorescence signals for NMNH(2-) and NADH molecules in solution were explained. A strong hydrogen bonding between the amide hydrogen atom and the nearest oxygen O- atom from the phosphate group was detected by ab initio calculations for the folded NMNH(2-) conformations in the ground electronic state at trans configurations of the nicotinamide ring. This hydrogen bonding turned out to be much weaker for the first excited electronic state. These calculated data show that, after optical excitation of the NMNH(2-) molecule, a rapid change in the geometry of the molecule is possible. The strong interaction of the phosphate group with the amide group in NMNH(2-) molecules in aqueous solution leads to the predominance of the folded NMNH(2-) conformations and trans configurations of the nicotinamide ring. This explains the reason for the dominance of one fluorescence decay time of NMNH(2-) in the aqueous solution. Based on these data, an important conclusion can be drawn that the contribution of the exponent with the short decay time τ ≈ 0.28 ns to the fluorescence signal of NMNH(2-), NADH, and NADPH molecules is related to the trans configuration of the nicotinamide ring.