Vibrational Spectroscopy And Gas-phase Thermochemistry Of The Model Dipeptide N-acetyl Glycine Methyl Amide

Abstract

The structure-function relationship in proteins is widely recognized, motivating numerous investigations of isolated neutral and ionic polypeptides that generally employ conformation specific, multidimensional UV and IR spectroscopies. This data taken in conjunction with computed harmonic frequencies has provided a snACShot of the underlying molecular physics at play in many polypeptides, but few experiments have been able to probe the energetics of these systems. In this study, we use vibrational spectroscopy to measure the gas-phase enthalpy change for isomerization between two conformations of the dipeptide N-acetyl glycine methyl amide (NAGMA). A two-stage oven source is implemented producing a gas-phase equilibrium distribution of NAGMA molecules that is flash frozen upon pickup by He nanodroplets. Using polarization spectroscopy, the IR spectrum is assigned to a mixture of two conformers having intramolecular hydrogen bonds made up of either five- or seven-membered rings, C$_{5}$ and C$_{7}$, respectively. The interconversion enthalpy, obtained from the van’t Hoff relation, is 4.52${\pm}$0.12 kJ/mol for isomerization from the C$_{7}$ to the C$_{5}$-conformer. This experimental measurement is compared to computations employing a broad range of theoretical methods.