The Unexpected Functional Diversity of Photoexcited NAD

Despite the vital role of nicotinamide adenine dinucleotide (NAD) as a cofactor in all living organisms, the diversity of its functions is poorly understood. Particularly in interaction with ultraviolet (UV) light, a variety of photorelaxation channels can be accessed, which current models lack to explain. In this work, for the first time, we used picosecond UV pump, mid-infrared (mIR) probe spectroscopy and accurate quantum-chemical calculations to elucidate the ultrafast photodynamics of NAD + and NADH to unify contradictory mechanisms from the past decades in the big picture. We found direct evidence for a long-lived (∼900 ps) charge-separated state in NADH, which has been unobserved previously and results in the parallel population of a fluorescent state. The photochemical pathways demonstrated here open up functions of NAD in chemistry and molecular biology, such as an electron donor, as a FRET agent or as a redox pair switch, which have not been considered previously. TOC GRAPHICS