Heterodyne-Detected Achiral and Chiral Vibrational Sum Frequency Generation of Proteins at Air/Water Interface

We present complex achiral and chiral vibrational sum frequency generation (VSFG) spectra at the air/water interface of protein solutions by using heterodyne-detected VSFG. Bovine serum albumin, pepsin, concanavalin A, and alpha-chymotrypsin were measured as model proteins. The obtained achiral Im[chi((2))] spectra gave us insights into the molecular orientation of protein molecules and water at the interface. From the chiral Im[chi((2))] spectra in the NH stretching and amide I regions, the secondary structures of the interfacial proteins were deduced. We attributed the chiral signals in the amide I and NH stretching regions to the interface on the basis of the phase of the signals. All the achiral and chiral spectra in each region showed the same sign despite different secondary-structure contents of the examined proteins. Real-time observation of the spectral change of alpha-chymotrypsin was also performed by heterodyne-detected chiral VSFG. The signal intensity of the chiral Im[chi((2))] spectra in the NH stretching and amide I regions decreased on the scale of 10 min, originating from the decrease of the portion of antiparallel beta-sheet conformation in the molecule. The conformational change occurred not in the bulk but at the interface. Heterodyne-detected achiral and chiral VSFG are capable of addressing the molecular orientation and conformation of proteins at air/water interfaces.