Gas-phase basicity of methionine

✍ Scribed by Sylvain Desaphy; Christian Malosse; Guy Bouchoux

• Book ID: 102376086
• Publisher: John Wiley and Sons
• Year: 2007
• Tongue: English
• Weight: 448 KB
• Volume: 43
• Category: Article
• ISSN: 1076-5174
• DOI: 10.1002/jms.1289

✦ Synopsis

Abstract

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.