STUDIES ON SPECTRA-STRUCTURE CORRELATIONS IN NEAR-INFRARED SPECTRA OFPROTEINS AND POLYPEPTIDES .1. A MARKER BAND FOR HYDROGEN-BONDS

FT-NIR spectra have been measured for various polypeptides and protein s with different secondary structures to find an NIR marker band for t he structure of the proteins and polypeptides. Their FT-IR spectra hav e also been obtained to assist in the interpretation of the FT-NIR spe ctra. 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 in vestigated. Therefore, the NIR band (hereafter, we identify it as amid e A/II) may be used as a practical indicator for the strength of hydro gen bonds in the amide groups, as in the case of amide A. The frequenc y of amide A/II changes little with the secondary structure for both t he polypeptides and proteins, and thus it is rather difficult to use t his band as a marker band of the secondary structure. For the globular proteins such as hemoglobin, albumin, and lysozyme, irrespective of t heir secondary structure, both amide A/II and amide A appear in simila r positions. However, collagen (Type I, Type IV, and Type V) gives the m much higher frequencies, suggesting that the hydrogen bonds in colla gen are much weaker than those in typical globular proteins. The amide A/II band shows a significant upward shift upon the thermal denaturat ion of pepsin. This observation indicates that amide A/II can also be used for monitoring the destruction of the hydrogen bonds in the amide s induced by the denaturation.