Role of methylation on the thermochemistry of alkali metal cation complexes of amino acids: N-methyl proline

► Energy-resolved CID of alkali cation complexes of N-methyl proline are studied using guided ion beam tandem mass spectrometry. ► Threshold CID data are analyzed to yield absolute bond energies for M+(N-methyl proline) where M = Li, Na, K, and Rb. ► Quantum chemical calculations explore the possible conformations of N-methyl proline and its alkali metal cation complexes. ► Agreement between experimental and theoretical bond energies show that M+(N-methyl proline) have zwitterionic structures.