Time-resolved resonance Raman, time-resolved UV-visible absorption and DFT calculation study on photo-oxidation of the reduced form of nicotinamide adenine dinucleotide

Photochemical reaction of the reduced form of nicotinamide adenine dinucleotide (NADH) was studied in aqueous solutions using nanosecond time-resolved resonance Raman and UV-visible absorption spectroscopies combined with DFT/B3LYP calculations. Four transients were detected. A transient exhibiting absorption bands at 370 and 560 nm was assigned to the cation radical NADH(+center dot). A transient having an absorption band at 394 nm was assigned to the deprotonated neutral radical NAD(center dot). A transient exhibiting an absorption band at 493 nm was considered to be attributable to the anion radical NADH(-center dot). The broad and strong band at about 700 nm was assigned to hydrated electrons. It was concluded that the photo-oxidation of NADH to NAD(+) proceeded stepwise via NADH(+center dot) and NAD(center dot). The Raman band at 1627 cm(-1) of NADH(+center dot) was assigned to the C=C symmetric stretch of the protonated pyridyl ring of the dihydronicotinamide moiety. It was shown that the C=C symmetric stretch of the protonated pyridyl ring exhibited a low-wavenumber shift from 1688 cm(-1) to 1627 cm(-1) on going from the ground state to NADH(+center dot). This implies that the protonated pyridyl ring is weakened considerably on ionization in good accord with the DFT calculations. Copyright (C) 2006 John Wiley & Sons, Ltd.