High‐resolution neutron structure of nicotinamide adenine dinucleotide

The structure of the free‐acid form of the coenzyme NAD + was determined at 100 K from a single‐crystal neutron experiment. NAD + is the oxidized form of the coenzyme redox pair NAD + /NADH and plays an important role in the catalysis of biological processes. The molecule crystallizes in space group P 1 with one NAD + and four water molecules per unit cell. The structure is compared with the previous X‐ray models of NAD + [Reddy et al. (1981), J. Am. Chem. Soc. 103 , 907–914; Parthasarathy & Fridey (1984 b ), Science , 226 , 969–971; Guillot et al. (2000), Acta Cryst. C 56 , 726–728]. The crystal packing and the hydrogen‐bond pattern are discussed as well as four short C—HO contacts involving the pyridine and adenine rings. The structure displays stereochemical distortions owing to the hydrogen bonding and crystal‐packing constraints, reflecting the adaptability of the NAD + molecule in various chemical environments.