Gas-phase acid-base properties of 1-aminocycloalkane-1-carboxylic acids from the extended kinetic method

The gas-phase proton affinities (PA) and gas-phase acidities (GA) for the non-protein amino acids 1-aminocyclopropane-1 -carboxylic acid (1), 1-amino-1-cyclobutane-carboxylic acid (2), cycloleucine (3) and 1-amino-1-cyclohexane (4) have been determined using the extended kinetic method in ESI-tandem mass spectrometers. These non-protein amino acids are found in a variety of foods and can compete with other aliphatic amino acids in a variety of biochemical processes. We find a positive trend in proton affinity with an increasing ring size for the four amino acids. Experimental proton affinities of 896 +/- 8.0, 913 +/- 8.0, 931 +/- 8.0, and 933 +/- 8.0 were determined for 1-4, respectively. Hybrid density functional theory calculations at the B3LYP/6-311 + + G(d,p)//B3LYP/6-31 + G(d) level of theory give predictions for the proton affinities of 1-4 that are in excellent agreement with the measured values and support the positive trend. In contrast, we find that the gas-phase acidities (AHacid) for 1-4 are the same within error. Acidities of 1425 +/- 8 kJ/mol, 1424 +/- 8 kJ/mol, 1428 +/- 10 kJ/mol, and 1423 +/- 8 kJ/mol were determined for 1-4. Theoretical predictions for the acidities for 1-4 are in excellent agreement with the measured acidities and support the conclusion that there is no trend with changing ring size. (C) 2014 Elsevier B.V. All rights reserved.