Chiral Water Superstructures around Antiparallel ß-Sheets Observed by Chiral Vibrational Sum Frequency Generation Spectroscopy.

Hydration modulates every aspect of protein structure and function. However, studying water structures in hydration shells remains challenging mostly due to overwhelming background from bulk water. We used vibrational sum frequency generation (SFG) spectroscopy to characterize hydrated films of an antiparallel ß-sheet peptide (LK7ß) adsorbed on glass slides. The hydrated films give chiral SFG response from water only when the peptide self-assembles into antiparallel ß-sheets. Experiments of isotopic labeling, isotopic dilution of water, and H2O-D2O exchange kinetics corroborate the assignments of the chiral SFG response to water stretching modes. Because individual water molecules are achiral, the chiral SFG response indicates formation of chiral superstructures of water around the antiparallel ß-sheet, implying that a protein secondary structure can imprint its chirality onto the surrounding water. This result demonstrates chiral SFG spectroscopy as a promising tool for probing water structures in protein hydration and addressing fundamental questions of protein structure-function.