Gas-Phase Basicity of Glycine, Alanine, Proline, Serine, Lysine, Histidine and Some of Their Peptides by the Thermokinetic Method

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Abstract
The thermokinetic method is applied to a set of six amino acids (glycine, alanine, proline, serine, lysine, histidine) and 30 of their di-and tri-peptides for which experimental proton transfer rate constants were available. The comparison between the presently determined gas-phase basicities, GBs, of the amino acids with values obtained from equilibrium constant determination is generally good (a mean deviation of ∼3 kJ mol−1 is observed). Derived proton affinities values are discussed. The gas-phase basicities of peptides provided by the present study correct several previously estimated values thus offering a more firm basis for structural discussion. Composite reaction efficiency curves indicate the existence, for several peptides, of at least two non-interconverting populations of protonated forms.