Characteristic terahertz absorption spectra of paramylon and paramylon-ester compounds

Paramylon is a long-chain polysaccharide, composed of glucose units connected via β-(1,3) glycosidic bonds, that spontaneously forms a three-strand helical bundle. Paramylon-esters can be made by partially or fully replacing saccharide chain hydroxide groups with carboxylic functional groups, such as stearoyl (CH₃(CH₂)₁₆CO) and palmitoyl (CH₃(CH₂)₂₄CO). The paramylon-ester with carboxylic acids has superior characteristics, including high thermal resistance, stability and transparency under visible light, which are necessary for thermoplastic applications. In this study, the absorption coefficient α(ν) and absorbance spectra of paramylons and paramylon-esters were measured in the 0.3-8.0 THz range and compared with the corresponding spectra of glucose and cellulose. Paramylon and paramylon-ester molecules were found to exhibit unique, so-called fingerprint, α(ν)peaks at 4.0, 6.0 and 8.0 THz, and 2.5 and 5.0 THz, respectively. We speculate that the spectral features observed are owing to intermolecular interaction modes of the weakly coupled polysaccharide chains. The paramylons with different molecular weights show very similar absorption features in the low-frequency side, both in spectral shapes and intensities, indicating that absorption is independent of molecular size. The paramylon-esters with varying degrees of substitution (DS) are similar spectral shapes but different intensities. A linear correlation between α(ν) peak intensity and the DS of paramylon-esters was established with the R² value above 0.99. This behavior can be used for the detection and identification of novel paramylon-ester molecules.