Gas‐phase basicity of methionine

Proton affinity and protonation entropy of methionine (Met) were determined by the extended kinetic method from ESI‐Q‐TOF tandem mass spectrometry experiments. The values, PA (Met) = 937.5 ± 2.9 kJ mol −1 and Δ p S °(Met) = − 22 ± 5 J mol −1 K −1 , lead to gas‐phase basicity GB (Met) = 898.2 ± 3.2 kJ.mol −1 . Quantum chemical calculations using density functional theory confirm that the proton affinity of Met is indeed in the 940 kJ mol −1 range and that a significant entropy loss, of at least − 25 J mol −1 K −1 , occurs upon protonation. This last point is evidenced here for the first time and suggests revision of the tabulated protonation thermochemistry of Met. A comparison with previous experimental data allows us to propose the following evaluated thermochemical values: PA (Met) = 943 ± 4 kJ mol −1 and Δ p S °(Met) = − 35 ± 15 J mol −1 K −1 and GB (Met) = 900 ± 2 kJ mol −1 . Copyright © 2007 John Wiley & Sons, Ltd.