Characterization and supramolecular architecture of the cellulose-protein fibrils in the tunic of the sea peach (Halocynthia papillosa,Ascidiacea, Urochordata)

Summary— The cellulose-protein fibrils, which constitute by far the bulk of the fibrous fraction of the sea peach tunic (Halocynthia papillosa), were structurally and chemically characterized, either in situ or after extraction procedures, with the use of classical electron microscoy combined with diffraction contrast imaging and electron diffraction, histochemistry, affinity cytochemistry and chemical analysis. These fibrils exhibit a cross-sectional shape close to a parallelogram. The cyrstallites forming their core, with lateral dimensions ranging from roughly 5 to 20 nm, are composed of native cellulose of higher crystallinity than that of plant cellulose. They are associated with acid mucopolysaccharidés (amps) and proteins which form a coating material appearing as a continuous sheath enveloping the axial crystallite in the cuticular layer or as patches more-or-less periodically distributed around and along the fibre axis in the fundamental layer. Tunicin, the alkali-insoluble fibrous fraction, is not pure cellulose, yielding only 22–60% of its dry weight as glucose equivalents, depending on the tunical layer. It is suggested that in addition to the high degree of crystallinity of the tunical cellulose, the presence of a significant amount of coating material composed of amino acids and proteoglycans firmly linked to cellulose molecules contributes to tunicin's high resistance to hydrolysis.