Comparative study of hydrogen-bonding interactions between cis-proline analogs and solvents

Only a small amount of peptide bonds exists in the cis conformation, however, they play an important role in the functioning of proteins. Herein, 2-pyrrolidinone (C4H7NO) was used as a cis-proline analog to study the hydrogen bonds between cis-proline and different solvents (CCl4, DMSO, and H2O). Interestingly, with the addition of solvents, the hydrogen-bonding interactions that involved N-H in the C4H7NO-CCl4 system are strengthened, while those in the C4H7NO-DMSO system are weakened, which is beyond our imagination. Combined with quantum chemical calculation and excess spectra, we find that it is due to the existence of the C4H7NO doubly hydrogen-bonded dimer, which is the most stable structure in the self-aggregation. During the dilution process, CCl4 hardly breaks double hydrogen bonds, DMSO only weakens the strong hydrogen bonds, and water thoroughly breaks the double hydrogen bonds. Several complexes, such as, doubly/singly hydrogen-bonded dimers, oligomers, monomers and their complexes with solvents, are identified in those three systems, which allow the dynamic changes in the solution structures of mixtures can be obtained. In addition, the band of singly hydrogen-bonded dimers is observed for the first time, thus reflecting the superiority of the excess spectra in improving the resolution. The findings of this work provide precious structural information on C4H7NO-solvents systems, reveal the effects of the solvents on cis-proline, and may inspire the understanding of the cis-proline and some biochemical processes. (C) 2019 Elsevier B.V. All rights reserved.