Design of a two-stage oven source to determine the enthalpies of interconversion of the lowest energy conformers of N-acetyl-glycine-methylamide (NAGMA)
Shirley, Caitlyne Celeste
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Modeling the gas-phase energetics of biomolecules is challenging because of their low vapor pressure and the presence of many low energy conformations. However, helium nanodroplet isolation (HENDI) techniques can trap and cool vibrationally hot model biomolecules in an environment with negligible solvent effects. Sufficiently high-resolution vibrational spectra obtained for biomolecules in this environment allows for the separation of bands due to different conformations. Enthalpies of interconversion between conformers can therefore be determined from a van’t Hoff analysis of the temperature dependence of vibrational bands in the infrared spectrum. In this study, a two-stage oven source was designed to introduce a constant vapor pressure of the model biomolecule N-acetyl-glycine-methylamide (NAGMA) into the gas phase. Its infrared spectrum was probed at various temperatures in order to determine the temperature dependence of the equilibrium constant associated with the interconversion of two low energy conformers. The first oven was kept at a fixed temperature to ensure that a constant number density of NAGMA was introduced into a second variable temperature oven source. From this data, the equilibrium constant was determined as a function of temperature and a van’t Hoff plot was generated to determine the enthalpy of conformer interconversion. The ∆H°(C5↔C7)=-4.52±0.12 kJ/mol.