Comparative analysis of IR and Raman spectra of lyophilized human and bovine collagens types I and III

Collagen is the primary structural protein of connective tissues. While all collagens share a similar triple-helical structure, different types of collagens perform different functions, and collagen composition is an important health indicator. Vibrational spectroscopy techniques are believed to be promising tools for non-invasive distinguishing between collagen types. However, many studies lack a rigorous spectroscopic foundation for interpretation of the collagen spectrum, including peak assignment and considering side effects not related to collagen type. To address this gap, we analyzed together polarized Raman and IR spectra. Spectra were compared with results of density functional theory simulations on a set of tripeptides and tetrapeptides with the most common amino sequences in collagen triple helix. We verified the interpretation of the most popular collagen peaks and suggested assignments to the peaks that were previously ignored. Comparative analysis of collagens type I and III combined with analysis of amino acid content in these proteins indicates the most transparent spectral marker to distinguish collagens type I and III is the intensity ratio between phenylalanine peak 1004 cm^-1 which we recommend normalizing by the peak intensity at 816 cm^-1. This indicator is weakly sensitive to changes in conformational states and impurities that are not associated with other proteins.