In Situ Misfolding of Human Islet Amyloid Polypeptide at Interfaces Probed by Vibrational Sum Frequency Generation

摘要

Kinetic analysis of conformational changes of proteins at interfaces is crucial for understandingnmany biological processes at membrane surfaces. In this study, we demonstrate that surface-selectivensum frequency generation (SFG) spectroscopy can be used to investigate kinetics of conformational changesnof proteins at interfaces. We focus on an intrinsically disordered protein, human islet amyloid polypeptiden(hIAPP) that is known to misfold into the u0001-sheet structure upon interaction with membranes. Using thenssp polarization setting (s-polarized SFG, s-polarized visible, and p-polarized infrared), we observe changesnin the amide I spectra of hIAPP at the air/water interface after addition of dipalmitoylphosphoglycerol (DPPG)nthat correspond to the lipid-induced changes in secondary structures. We also used the chiral-sensitivenpsp polarization setting to obtain amide I spectra and observed a gradual buildup of the chiral structuresnthat display the vibrational characteristics of parallel u0001-sheets. We speculate that the second-order chiralopticalnresponse at the antisymmetric stretch frequency of parallel u0001-sheet at 1622 cm-1 could be a highlyncharacteristic optical property of the u0001-sheet aggregates not only for hIAPP, but possibly also for othernamyloid proteins. Analyzing the achiral and chiral amide I spectra, we conclude that DPPG induces thenmisfolding of hIAPP from R-helical and random-coil structures to the parallel u0001-sheet structure at the airwater interface. We propose that SFG could complement existing techniques in obtaining kinetic andnstructural information for probing structures and functions of proteins at interfaces.