The reactivity of modified and unmodified amino acids, nucleobases and nucleosides towards sodium was studied. Sodium cationic affinities (SCA) were determined first by mass spectrometry with ESI/ITMS, then by ab initio calculations. The experimental method allowed us to determine the SCA values for amino acids, nucleobases and nucleosides by Cooks kinetic method by decomposing the studied dimers in gas phase. Ab initio calculations allowed us to determine the SCA and the structure of the cationized molecules formed in gas phase. After determination of the best calculation system (DFT), the very good correlation between experimental and theoretical results allowed us to determine the cationisation sites and so the structures of modified and unmodified amino acids, nucleobases and nucleosides.
