In-situ micro-FTIR spectroscopic observation on the hydration process of Poria cocos

In Poria cocos, triterpene compound and polysaccharides are the main compositions. The heteropolysaccharide was identified as a linear chain of β-(1 → 3)-d-glucan, which has the strong water absorbing capacity. In order to investigate the effect of water on the structure of Poria cocos, which belongs to a kind of Polyporaceae, the Micro-Fourier transform infrared spectroscopy (micro-FTIR) technique has been employed with the ambient relative humidity (RH) increasing. The gained IR spectra are measured and analyzed in detail. Because of strong overlaps between some bands, the differential spectra and band decompositions have been applied to analyze the structural change. IR spectra show the transformation of hydrogen bonds with the RH. The blue shift of the =CH₂ bending vibration from 1417 to 1424 cm^-1 with the increase in RH means that the hydrogen bonds are formed between =CH₂ groups and water molecules at lower RH and some transferred to weak hydrogen bonds. The further study suggests that the C1-OH, C2-OH, and C3-OH groups from polysaccharide bond formed C1-OH⋯H₂O, C2-OH⋯H₂O, and C3-OH⋯H₂O steady modes, respectively, with water molecules.