Complexation of carboxylate anions with the arginine gas-phase amino acid: Effects of chain length on the geometry of extended ion binding

Complexation of deprotonated carboxylic acids with arginine was investigated using collision-induced dissociation to probe the nature of isolated carboxylate–amino acid interactions as a function of anion size. Monocarboxylic CH3(CH2)nCOO−·Arg (n = 3–7, 9, 10) and dicarboxylic acid COOH(CH2)nCOO−·Arg (n = 3–5, 7–10) complexes were investigated. For the dicarboxylic acid clusters, chain length has a significant effect on the %fragmentation energies with the n = 9, 10 systems fragmenting at significantly lower energies than the corresponding shorter chain systems. Molecular mechanics calculations suggest that this fragmentation energy shift is associated with the longer-chain dicarboxylic acid–Arg clusters switching to ring structures.