Accessibility of Hydroxyl Groups of Brown-Rot Degraded Spruce Wood to Heavy Water

The accessibility of alcoholic and phenolic hydroxyl (O–H) groups in non-degraded and brown-rot degraded spruce wood (Piceas abies L. Karst.) was investigated with Fourier transform near infrared (FT-NIR) transmission spectroscopy in combination with deuterium exchange with heavy water (D₂O). In the presence of excess D₂O, accessible O–H groups were converted into O–D groups and characteristic O–H first overtone bands disappeared in the FT-NIR spectra. Hydroxyl groups of cellulose I_α were more accessible than those of cellulose I_β. O–H groups of wood degraded by brown-rot fungi showed much lower overall accessibility, and free O–H groups from amorphous matrix polysaccharides, preferentially those located in primary and outer secondary cell walls and middle lamellae were least accessible, whereas H-bonded O–H groups associated with cellulose microfibrils showed residual accessibility in brown-rotted wood of incipient, early and pronounced degradation stages.