Attachment of alkali cations on beta-D-glucopyranose: matrix-assisted laser desorption/ionization time-of-flight studies and ab initio calculations.

In matrix-assisted laser desorption/ionization mass spectrometry, carbohydrates ionize by attachment of an alkali cation, and the ion yield varies with the nature of the cation. In an attempt to contribute to the understanding of the mechanisms involved, we have conducted matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) experiments on the simple glucose molecule with the alkali cations Li+, Na+ and K+, and have also performed ab initio calculations. The calculations show that, for the most stable carbohydrate-cation geometry, the carbohydrate ring is twisted and the cation is coordinated to four oxygen atoms. Calculations also show that in these complexes the positions of the three cations are very similar, and the smaller the cation, the closer it is to the oxygen atoms. Finally, the calculated formation enthalpies of the different complexes agree with the experimental results obtained for the order of stability of these complexes.