GAS-PHASE ACIDITIES OF THE ALPHA-AMINO-ACIDS

The gas phase acidities of 19 alpha amino acids have been determined using the kinetic method of Cooks and co-workers [R.G. Cooks and T.L. Kruger, J. Am. Chem. Soc., 99 (1977) 1279; S.A. McLuckey, D, Cameron and R.G. Cooks, J. Am. Chem. Soc., 103 (1981) 1313]. The range of gas phase acidities for this class of biomolecules spans from glycine, the least acidic (DELTAG(acid)-degrees = 1402.0 kJ mol-1), to histidine, the most acidic(DELTAG(acid)-degrees = 1356.0 kJ mol-1). For a few, simple amino acids (glycine, alanine, serine, and cysteine), ab initio theory at the HF/6-31 + G*//HF/3-21 (+)G* level was used to calculate gas phase acidities. There is good agreement between the theoretical and experimental acidities with the average deviation being +/- 8.5 kJ mol-1. Fully optimized structures are reported for these amino acids and their corresponding carboxylates.