Direct Anionic Effect on Water Structure and Indirect Anionic Effect on Peptide Backbone Hydration State Revealed by...

Area of Science:

• Physical Chemistry
• Biophysical Chemistry
• Spectroscopy

Background:

• Anions significantly influence the properties of aqueous solutions.
• Understanding these effects is crucial for biological systems, particularly peptide hydration.
• The Hofmeister series describes the effects of ions on protein solubility and stability.

Purpose of the Study:

• To directly investigate the influence of anions on water structure and peptide backbone hydration in aqueous solution.
• To elucidate the mechanisms by which different anions affect peptide-water interactions.
• To establish vibrational spectroscopic methods for quantifying anionic effects on peptide hydration states.

Main Methods:

• Thin-layer transmission infrared spectroscopy was employed to study aqueous solutions.
• N-methylacetamide (NMA) was used as a model peptide.
• Vibrational frequencies of water (HOH bending) and NMA (amide II and III bands) were analyzed.

Main Results:

• Chaotropic anions weakened water hydrogen bonding and red-shifted HOH bending frequencies.
• Kosmotropic anions strengthened water hydrogen bonding and blue-shifted HOH bending frequencies.
• Kosmotropes induced a "salting-in" effect on NMA, while chaotropes induced a "salting-out" effect, consistent with the Hofmeister series.

Conclusions:

• Hydrated anions influence peptide backbones primarily via the N-H group, with a minor indirect effect through the C═O group.
• Amide vibrational modes serve as effective probes for anionic influences on peptide hydration.
• Deuteration of the amide unit reduces the sensitivity of amide II and III modes to these anionic effects.