Crystal analysis and high-resolution imaging of microfibrillar α-chitin from Phaeocystis

The ultrastructure of α-chitin microfibril produced by marine alga Phaeocystis was investigated by FT-IR spectroscopy, X-ray diffraction and electron microscopy. The average size of the microfibril was 17.1±1.8 μm in length and 39.8±8.8 nm in width. The FT-IR spectrum shows typical α-chitin pattern, and each band was sharper than crustacean chitin's, indicating higher crystallinity of the Phaeocystis chitin. The X-ray diffraction gave crystallite size more than twice of crustacean tendon's. The fiber diffraction pattern is consistent with previous studies with two-chains orthorhombic unit cell (Minke and Blackwell, 1978; Sikorski et al., 2009), and refined unit cell dimensions are a=4.742 Å, b=18.871 Å, c=10.338Å. High-resolution electron microscopy of ultrathin sections gave the cross-sectional shape of microfibril as hexagon. The lattice images of (020) plane (d=0.94 nm) were frequently observed extending the entire cross-section of microfibril, indicating its single crystalline nature. These results allowed construction of a molecular packing model for α-chitin crystal.