The spectrophotometric evaluation of NAD(P) dehydrogenase enzymatic activity is very popular as the reduced form (NAD(P)H) absorbs at 340 nm, while the aromatic oxidised form does not. In this joint theoretical and experimental investigation, we identify the chromophoric unit of both the NAD(P)(+) and NAD(P)H forms. Rather than a modification of size or shape of the frontier orbitals, the sharp variation in the absorption upon reduction is mainly related to the stabilisation of the occupied orbitals due to the positive charge. In addition, TD-DFT nicely reproduces the experimental wavelengths. (C) 2007 Elsevier B.V. All rights reserved.