GAS-PHASE BASICITIES AND SITES OF PROTONATION OF GLYCINE OLIGOMERS (GLYN, N = 1-5)

The gas-phase basicities (GB) of glycine and its oligomers (GLY-GLY, GLY-GLY-GLY, GLY-GLY-GLY-GLY, GLY-GLY-GLY-GLY-GLY) are determined by bracketing measurements using an external source quadrupole Fourier transform mass spectrometry instrument. The GB of glycine is found to be 206.4 +/- 2 kcal/mol. The larger oligomers have similar basicities corresponding to 213.5 +/- 2 kcal/mol for diglycine, 215.0 for tri- and tetraglycine, and 217.1 for pentaglycine. Various base sites are examined on diglycine using the semiempirical AM1 molecular orbital calculation method. It is proposed that the most basic sites for glycine and diglycine are similar and correspond to the protonation of the terminal amine with hydrogen bonding to the adjacent carbonyl oxygen. The increase in GB between the two oligomers is due to the increased hydrogen-bonding interaction when the carboxylic acid is converted to an amide.