In this joint experimental and theoretical study, the solution-state conformation of [6-bromo-1-(4-fluorophenylmethyl)4(1H)-quinolinon-3-yl)]-4-hydroxy-2-oxo -3-butenic acid (QDKA), a potential HIV-1 integrase inhibitor, is investigated by using UV-visible spectroscopy and Time-Dependent Density Functional Theory. The neutral, mono-anionic and di-anionic species have been identified and their spectral characteristics rationalized. The possibility of forming enol tautomers and keto structures is assessed. (C) 2009 Elsevier B. V. All rights reserved.