Interaction of MALDI matrix molecules with Na+ in the gas phase

The sodium complexes of six molecules commonly used as matrices for matrix-assisted laser desorption ionization (MALDI) have been investigated by ab initio calculations. Several isomers have been considered in each case, including salt bridges. In one case, the sodium complex of nicotinic acid (NA), the lowest energy isomer is a salt bridge. The adequate computational level has been established through calibration on the sodium complex of 2,5-dihydroxybenzoic acid (DHB). Accurate sodium ion binding enthalpies have been derived for DHB (34.5 kcal/mol), anthranillic acid (37.0 kcal/mol), NA (37.5 kcal/mol), sinapinic acid (39.0 kcal/mol), picolinic acid (42.9 kcal/mol), and 4-OH-alpha-cyanocinnamic acid (44.3 kcal/mol). Some of these values can be compared to recent experimental measurements. Their relevance to the interpretation of secondary MALDI processes in the plume is discussed.