title: Uncovering the Sensitivity of Amide-II Vibration to Peptide-Ion Interactions.
abstract: In this work, linear infrared spectroscopy was used to examine the effect of salt on the amide-II mode in a model β-peptide (N-ethylpropionamide, NEPA) in its deuterated form, to reveal the sensitivity of this mode in reporting peptide-ion interactions. In comparison to the case of NEPA in water, the amide-II spectra mainly showed a red-shifted component in four typical saline solutions (NaCl, CaCl2, MgCl2, and AlCl3) examined in this work. Our results suggest that highly populated hydrated ion complexes under high salt concentration conditions destroy the hydration layer of the model peptide and result in mostly a salting-out state of the peptide. Molecular dynamics simulations suggest that the hydrated cation mainly interacts with the peptide backbone on the amide C═O side, whereas the hydrated anion interacts on the amide N-H side. As the amide-II mode is mainly a combination of the C-N stretching and N-H in-plane-bending vibrations, this mode is advantageous in being responsive to ionic interaction from both the C═O and N-H sides. Such a dual sensitivity should be very useful in probing the breaking and/or formation of the interamide hydrogen bond between the C═O and N-H groups, which is a very important interaction involved in the solvation and stabilization, as well as folding/unfolding of proteins.
authors.affiliation: Beijing National Laboratory for Molecular Sciences and Molecular Reaction Dynamics Laboratory, Institute of Chemistry, Chinese Academy of Sciences , Beijing 100190, P. R. China.
authors.forename: Juan
authors.initials: J
authors.lastname: Zhao
authors.affiliation: Beijing National Laboratory for Molecular Sciences and Molecular Reaction Dynamics Laboratory, Institute of Chemistry, Chinese Academy of Sciences , Beijing 100190, P. R. China.|University of Chinese Academy of Sciences , Beijing 100049, P. R. China.
authors.forename: Jianping
authors.initials: J
authors.lastname: Wang
mesh_terms: D000577:Amides / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D056004:Molecular Dynamics Simulation; D010455:Peptides / Q000737:chemistry*; D011789:Quantum Theory; D013055:Spectrophotometry, Infrared; D014732:Vibration
chemical_list: D000577:Amides; D007477:Ions; D010455:Peptides