Protein Structures and Folding at Interfaces Probed by Chiral Sum Frequency Generation Vibrational Spectroscopy

Characterization of protein secondary structures at interfaces in situ and in real time is important to understand biological processes associated with cell membranes and develop biomaterials and biosensors. However, such characterization remains challenging because it requires techniques that are both selective to interfaces and protein secondary structures. Here, we demonstrate that chiral sum frequency generation spectroscopy (cSFG) can provide peptide amide I and N-H stretch vibrational signals that are free of water background and characteristic to parallel beta-sheet, anti-parallel beta-sheet, alpha-helix, 3-10 helix and random coil. This enables cSFG to distinguish protein secondary structures at interfaces, similar to circular dichroism spectroscopy for protein characterization in solution. Using cSFG, we followed the kinetics of misfolding of an amyloid protein, human islet amyloid polypeptide (IAPP) that is associated with type II diabetes. We observed the misfolding of IAPP from disordered structures to alpha-helices and then to beta-sheets upon interactions with a lipid surface. These results demonstrate the selectivity of cSFG to interfaces and secondary structures, implying its potential in addressing a wide range of fundamental and engineering problems related to protein at interfaces.