Heterodyne-Detected Achiral and Chiral Vibrational Sum Frequency
Generation of Proteins at Air/Water Interface
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Abstract
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 α-chymotrypsin
were measured as model proteins. The obtained achiral Im[χ<sup>(2)</sup>] spectra gave us insights into the molecular orientation
of protein molecules and water at the interface. From the chiral Im[χ<sup>(2)</sup>] 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 α-chymotrypsin was also
performed by heterodyne-detected chiral VSFG. The signal intensity
of the chiral Im[χ<sup>(2)</sup>] 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 β-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