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

Quantitative thermodynamic information is obtained from the study of the gasphase interactions of the alkali metal cation complexes of N-methyl proline (NMP) with Xe using a guided ion beam tandem mass spectrometer (GIBMS). Absolute bond dissociation energies (BDEs) of M+ = Li+, Na+, K+, and Rb+ to NMP are determined experimentally from threshold collision-induced dissociation (TCID) measurements of the M+(NMP) complexes. Analysis of their kinetic energy cross sections provide the 0 K bond enthalpies after accounting for unimolecular decay rates, internal energy of reactant ions, and multiple ion-molecule collisions. Quantum chemical calculations of the M+(NMP) BDEs are found to be in good agreement with the experimental values, establishing that the zwitterionic form is the lowest energy structure for all the metal ion complexes. Compared to M+(Pro) BDEs, the metal binding in these zwitterions is slightly enhanced by the CH3 group on the ring nitrogen, presumably a result of an inductive effect and its higher polarizability. More profound consequences of the methyl group emerge in the charge-solvated conformers calculated for M+(NMP) where it directs multiple conformations of the pyrrolidine ring. This is unlike the ring puckering phenomenon seen in M+(Pro) complexes, where fewer conformations are found, apparently because inversion at the nitrogen center is more facile.