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

• We obtained proton affinity (PA) and ΔHacid (GA) for amino acids with cyclic side chains.
• We used the extended kinetic method and density functional theory calculations.
• PAs are highly-dependent on side-chain length, GAs are less so.
• Different conformers are preferred with respect to enthalpy and free energy.
• Calculated values are in excellent agreement with measured quantities.

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 (ΔHacid) 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.

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