A spectroscopic study of changes in the secondary structure of proteins of biological fluids of the oral cavity by synchrotron infrared microscopy

On the basis of the data of infrared spectroscopy with synchrotron radiation, the secondary structure of proteins of the dentinal and gingival fluids during the development of cariosity in deep dentin tissues is studied. It is shown that the change in the shape of the profile of the amide I band in the region of 1700–1605 cm$^{-1}$ is associated both with a change in the ratio of the integrated absorption intensities of the $\alpha$-helix and $\beta$-sheet secondary structures and with the position of the $\beta$-coil and $\beta$-sheet components in the spectrum. It is established that the $\alpha$-helix/$\beta$-sheet ratio for both dentinal and gingival fluids is below the threshold level, at which significant changes in the secondary structure of proteins of biological fluids are observed, unequivocally indicating the development of pathology in hard dental tissues. The features that we discovered in the profile of the amide I band of biological fluids of the oral cavity, together with the spectral markers of the development of cariosity in dentin, are reliable spectroscopic signatures of the pathology and can be detected using the gingival fluid.