Microscopic vibrational circular dichroism on the forewings of a European hornet: heterogenous sequences of protein domains with different secondary structures.

ABSTRACT

We developed a microscopic scanning for vibrational circular dichroism (VCD) spectroscopy in which a quantum cascade laser is equipped with a highly focused infrared light source to attain a spatial resolution of 100 µm. This system was applied to the forewing of a European hornet to reveal how the protein domains are organised. Two-dimensional patterns were obtained from the VCD signals with steps of 100 µm. We scanned the 1500-1740 cm-1 wavenumber range, which covers amide I and II absorptions. Zone sequenced α-helical and ß-sheet domains within an area of 200 µm2 in membranes close to where two veins cross. The sign of the VCD signal at 1650 cm-1 changed from positive to negative when probed along the zone axis, intermediated by the absence of VCD activity. The significance of this zone is discussed from the viewpoint of the mechanical properties required for flying motion. These features are unattainable using conventional FTIR (Fourier transform infrared) or FT-VCD methods with a spatial resolution of ∼10 mm2.