GAS-PHASE BASICITIES OF SERINE AND DIPEPTIDES OF SERINE AND GLYCINE

The gas-phase basicities of serine and dipeptides containing amino aci d residues of serine and glycine were determined by proton transfer re actions in a Fourier transform ion cyclotron resonance mass spectromet er. The gas-phase basicity (GB) of L-serine was found to be 205.9 kcal /mol, with addition of a hydroxymethyl group (-CH2OH) increasing the b asicity by 4.5 kcal/mol relative to the simplest amino acid glycine (G B = 201.4 kcal/mol). This is attributed to a combination of intramolec ular hydrogen bonding, induction, and symmetry effects. For the dipept ides, addition of a hydroxymethyl group does not result in a large inc rease in basicity relative to the basicity of glycylglycine (GB = 208. 0 kcal/mol). The gas-phase basicities determined for glycyl-L-serine, L-serylglycine, and L-seryl-L-serine are 209.3, 210.6, and 210.9 kcal/ mol, respectively. In comparison to glycylglycine, addition of the hyd roxymethyl group at the N terminus has a greater impact on basicity th an its placement at the C terminus. These data suggest that the proton ation site for these dipeptides is the N-terminal amino nitrogen.