Direct Anionic Effect on Water Structure and Indirect Anionic Effect on Peptide Backbone Hydration State Revealed by Thin-Layer Infrared Spectroscopy.

Abstract

In this work, the anionic effect on water structure and on the peptide backbone and water interaction was investigated directly in aqueous solution using thin-layer transmission infrared spectroscopy. The chaotropic anions were found to weaken the water hydrogen-bonding strength and red shift the HOH bending frequency, while the kosmotropic anions were found to strengthen the water hydrogen-bonding network and blue shift the HOH bending frequency. The kosmotropes, especially F-, blue shift the vibrational frequencies of both amide II and amide III bands of N-methylacetamide (NMA), indicating NMA is in the "salting-in" state; while the chaotropes (Cl-, NO3-, Br-, I-, and SCN-) red shift the frequencies of the two normal modes, indicating NMA is in the "salting-out" state. Furthermore, the changes of the vibrational frequencies of the HOH bending, amide II and III bands were found to generally follow the Hofmeister anionic series. Our results suggest that hydrated anion influences the peptide backbone mainly through the N-H group, but a weak and indirect effect through the amide C═O group also contributes. Thus, these amide modes can be used as vibrational measures of anionic influences on peptide backbone's hydration state. Our work also suggests that deuteration of the amide unit decreases the sensitivity of the amide II and III vibrational modes in this regard.