Gas-phase lithium cation affinity of glycine

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Biophysical Chemistry

Background:

• Understanding the gas-phase thermochemistry of amino acids is crucial for various chemical and biological processes.
• Lithium cation binding affinities provide insights into molecular interactions and solvation.

Purpose of the Study:

• To determine the gas-phase lithium cation binding thermochemistry of glycine.
• To compare theoretical calculations with experimental data for glycine-lithium cation interactions.

Main Methods:

• Quantum chemical calculations using the G4 level of theory.
• Extended kinetic method employing electrospray ionization quadrupole time-of-flight tandem mass spectrometry.

Main Results:

• Theoretical lithium cation affinity of glycine (∆(Li)H°(298)(GLY)) calculated at 241.4–242.3 kJ.mol⁻¹.
• Experimental ∆(Li)H°(298)(GLY) values ranged from 228.7±0.9 to 235.4±1.0 kJ.mol⁻¹, depending on the reference scale.
• An excess entropy gain of ~15 J.mol⁻¹K⁻¹ was observed, attributed to bidentate Li⁺ interaction with glycine.

Conclusions:

• The study provides a comprehensive understanding of glycine's lithium cation binding thermochemistry.
• Discrepancies between theoretical and experimental values highlight potential issues in experimental affinity scales.
• The bidentate interaction of Li⁺ with glycine is confirmed, contributing to the observed entropic gain.