Abstract

Esters of glycine, alanine and valine are investigated by FTIR and Raman spectroscopy in supersonic jets as gas phase model systems for the neutral peptide N-terminus. The NH-stretching vibrations exhibit very large temperature- and substitution-dependent intensity anomalies which are related to weak, bifurcated intramolecular hydrogen bonds to the carbonyl group. Comparison to theory is only satisfactory at low temperature. Spectral NH aggregation shifts are small or even negligible and the associated IR intensity is remarkably low. In the case of valine, chirality recognition effects are nevertheless detected and rationalized. Comparison to quantum-chemical calculations for dimers shows that dispersion interactions are essential. It also rules out cooperative hydrogen bond topologies and points at deficiencies in standard harmonic treatments with the linear dipole approximation.

Highlights

  • Tetramethylsilane was used as reference and the signals were locked on the deuterated dissolvent as internal standard

  • The mixture was stirred at 0◦ C for five minutes and brought to room temperature

  • After another five minutes of stirring the solution was extracted with dichloromethane (3×100 mL), dried over Na2 SO4, and the solvent was evaporated under reduced pressure

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Summary

Syntheses and NMR data

General information: NMR spectra were recorded on a Varian Unity 300 or a BRUKER-DPX 200 spectrometer. Tetramethylsilane was used as reference and the signals were locked on the deuterated dissolvent as internal standard. Chemical shifts (δ) are reported in ppm, coupling constans (J ) in Hz. The multiplicities of the signals were abbreviated as follows: s singlet, d doublet, t triplet, q quadruplet, dq doublet of quadruplet, d sept doublet of septuplet and br broad signal. All commercially available compounds were used without further purification

Glycine ethyl ester
MG s
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