Studies on Spectra/Structure Correlations in Near-Infrared Spectra of Proteins and Polypeptides. Part I: A Marker Band for Hydrogen Bonds

FT-NIR spectra have been measured for various polypeptides and proteins with different secondary structures to find an NIR marker band for the structure of the proteins and polypeptides. Their FT-IR spectra have also been obtained to assist in the interpretation of the FT-NIR spectra. Comparison between the FT-NIR and FT-IR spectra shows that there is a clear correlation between the frequency of an NIR band near 4855 cm −1 , assignable to a combination of amide A and amide II, and that of an IR band near 3300 cm −1 due to amide A for the polypeptides investigated. Therefore, the NIR band (hereafter, we identify it as amide A/II) may be used as a practical indicator for the strength of hydrogen bonds in the amide groups, as in the case of amide A. The frequency of amide A/II changes little with the secondary structure for both the polypeptides and proteins, and thus it is rather difficult to use this band as a marker band of the secondary structure. For the globular proteins such as hemoglobin, albumin, and lysozyme, irrespective of their secondary structure, both amide A/II and amide A appear in similar positions. However, collagen (Type I, Type IV, and Type V) gives them much higher frequencies, suggesting that the hydrogen bonds in collagen are much weaker than those in typical globular proteins. The amide A/II band shows a significant upward shift upon the thermal denaturation of pepsin. This observation indicates that amide A/II can also be used for monitoring the destruction of the hydrogen bonds in the amides induced by the denaturation.