Gas-phase basicity of methionine

Area of Science:

• Physical Chemistry
• Biochemistry
• Mass Spectrometry

Background:

• Accurate thermochemical data for amino acids are crucial for understanding biochemical processes.
• Previous studies have provided estimates for methionine's protonation thermodynamics, but with potential inaccuracies.

Purpose of the Study:

• To accurately determine the proton affinity (PA) and protonation entropy (ΔpS°) of methionine (Met).
• To provide revised, high-precision thermochemical values for Met protonation.
• To validate experimental findings with quantum chemical calculations.

Main Methods:

• Utilized the extended kinetic method with Electrospray Ionization Quadrupole Time-of-Flight (ESI-Q-TOF) tandem mass spectrometry.
• Performed density functional theory (DFT) quantum chemical calculations for theoretical validation.

Main Results:

• Experimental determination yielded PA(Met) = 937.5 ± 2.9 kJ mol⁻¹ and ΔpS°(Met) = -22 ± 5 J mol⁻¹ K⁻¹.
• DFT calculations confirmed the proton affinity range and significant entropy loss upon protonation.
• Proposed revised evaluated thermochemical values: PA(Met) = 943 ± 4 kJ mol⁻¹, ΔpS°(Met) = -35 ± 15 J mol⁻¹ K⁻¹, and GB(Met) = 900 ± 2 kJ mol⁻¹.

Conclusions:

• The study provides the first experimental evidence for significant entropy loss upon methionine protonation.
• The results necessitate a revision of tabulated protonation thermochemistry for methionine.
• The refined thermochemical data enhance the understanding of methionine's behavior in the gas phase.