Relative interaction strengths between cations (X = Li+, Na+, K+, NH4+) and anionic
carboxylate groups of acetate and glycine in aqueous solution are determined. These
model systems mimic ion pairing of biologically relevant cations with negatively charged
groups at protein surfaces. With oxygen 1s X-ray absorption spectroscopy we can
distinguish between spectral contributions from H2O and carboxylate, which allows to
probe the electronic structure changes of the atomic site of the carboxylic group being
closest to the counter cation. From the intensity variations of the COOaq- O 1s X-ray
absorption peak, which quantitatively correlate with the change in the local partial
density of states from the carboxylic site, interactions are found to decrease in the
sequence Na+ >Li+ > K+ > NH4+. This ordering, as well as the observed bidental nature of
the COO-aq and X+aq interaction is supported by combined ab initio and molecular
dynamics calculations.