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

Abstract

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 ∆E(DFT)<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 ∆E(MP)<10 kJ.mol(-1). A conformation containing the three intramolecular H-bonds, i.e. C=O(sc)…HN(bb), C=O(bb)…HN(sc) and OH(bb)…N(bb) 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).