Near-Infrared and Mid-Infrared Fourier Transform Vibrational Circular Dichroism of Proteins in Aqueous Solution

Vibrational circular dichroism (VCD) of a series of proteins in 1120 solution with differing secondary structure are reported for the first time in the near-infrared (NIR) region as well as the NH-stretching region. The Fourier transform (FT) near-infrared (NIR) measurements were carried out between 6000 to 4000 cm(-1). FT-VCD measurements were simultaneously carried out for the mid-infrared (mid-IR) region from 2000 to 800 cm I for direct comparison to VCD in the NIR region. The NIR VCD spectra of proteins show distinct spectral features for different protein structural motifs, indicating a valuable new method to study protein conformations. The principal VCD transitions in the NIR region are two combination hands, the amide A-II and B-II hands, of the amide A and B fundamentals with the amide II fundamental, and the second overtone of the amide II, referred to as the amide 3 x II band. VCD in the amide A and B band region consisting primarily of NH stretching motions were successfully obtained in H2O for the first time for an insulin fibril sample. Similar to the enhanced VCD signal observed in amide I and II regions, the amide A and It VCD of insulin fibril shows strong intensity, enhancements, providing an additional valuable probe of protein fibril growth and development in solution. The relative sensitivities of the mid-IR, N-H stretching, and NIR regions are discussed.