The conformational behavior and H-bond structure of asparagine: A theoretical and experimental matrix-isolation FT-IR study

Due to the high importance of the structural properties of peptides, the conformational behavior of one of their elementary building blocks, asparagine, has been investigated in this work. Matrix-isolation FT-IR spectroscopy is a suitable technique to investigate the intrinsic properties of small molecules. Asparagine has been subjected to matrix-isolation FT-IR spectroscopy supported with DFT and MP2 calculations. DFT optimization of asparagine resulted in 10 stable conformations with ∆EDFT < 10 kJ.mol− 1. Compared to a previous study, one new conformation has been revealed. Further optimization at the MP2/6-31++G** level resulted in seven conformations with ∆EMP < 10 kJ.mol− 1. A conformation containing the three intramolecular H-bonds, i.e. C = Osc…HNbb, C = Obb…HNsc and OHbb…Nbb appeared to be the most stable one at both levels despite the large negative entropy contribution due to these 3 H-bonds. At the sublimation temperature of 353 K, the DFT method predicts four and the MP2 method six conformations to be present in the experimental matrix-isolation spectrum. These conformations have different intramolecular H-bonds, which has allowed to identify at least 4 low energy conformations in the FT-IR spectrum. Detailed comparison between theory and experiment resulted in a mean frequency deviation of 7.6 cm− 1.

Highlights► Ab initio exploration of the conformational landscape of asparagine. ► Three intramolecular H-bonds in the most stable form. ► A strong H-bond interaction between the side chain and the backbone. ► Experimental identification by matrix-isolation FT-IR spectroscopy. ► A mean frequency deviation of 7.6 cm− 1.