title: Interaction between metal cation and unnatural peptide backbone mediated by polarized water molecules: study of infrared spectroscopy and computations.
abstract: In this work, the interaction between metal cation and a model β-peptide N-ethylpropionamide (NEPA) in aqueous solution is investigated using infrared absorption spectroscopy. Monovalent (Na(+)), divalent (Ca(2+), Mg(2+)), and trivalent (Al(3+)) metal cations added to NEPA/water solution at moderate concentrations split the amide-I frequency into a red- and blue-shifted component. Molecular dynamics simulations of NEPA in moderate cationic strength are conducted to gain insight into the structural details of the peptide-salt-water system, particularly in the vicinity of the amide group. Our results do not suggest a direct contact between cation and amide oxygen in the solution phase; otherwise, only a significant red shift in the amide-I frequency would occur due to the vibrational Stark effect, as evidenced by quantum chemistry computations. Instead, our results suggest it is the dynamical interaction between the formed cation/water/anion complexes and the amide group that causes the observed split in the amide-I peak, which indicates the presence of both salting-in (red-shifted) and salting-out (blue-shifted) NEPA species. The presence of dynamic and polarized water molecules between the amide oxygen and the cation complex is believed to be the key to the split amide-I peaks in the cation-rich environment. Our results can be useful to better understand the cationic Hofmeister series.
authors.affiliation: Beijing National Laboratory for Molecular Sciences; Molecular Reaction Dynamics Laboratory, Institute of Chemistry, Chinese Academy of Sciences , Beijing 100190, P. R. China.
authors.forename: Jipei
authors.initials: J
authors.lastname: Shi
authors.forename: Jianping
authors.initials: J
authors.lastname: Wang
mesh_terms: D000577:Amides / Q000737:chemistry; D002712:Chlorides / Q000737:chemistry; D008670:Metals / Q000737:chemistry*; D008968:Molecular Conformation; D056004:Molecular Dynamics Simulation; D010455:Peptides / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D014867:Water / Q000737:chemistry*
chemical_list: D000577:Amides; D002712:Chlorides; D008670:Metals; D010455:Peptides; D014867:Water