Absence of sum frequency generation in support of orthorhombic symmetry of α-chitin

The native polymorphic structures of chitin, namely α- and β-chitin, were studied with X-ray diffraction (XRD), infrared and Raman spectroscopies, and sum-frequency generation (SFG) vibrational spectroscopy. The currently proposed model of α-chitin crystal based on X-ray and electron diffraction study has an antiparallel chain arrangement imposed by the orthorhombic symmetry in contrast to parallel chain arrangements in β-chitin and native cellulose; however, the natural occurrence of antiparallel chain arrangement for α-chitin is controversial to widely accepted biosynthetic mechanisms for structural polysaccharides such as chitin and cellulose, where chains are successively polymerized and crystallized in a unidirectional way. We compared the SFG signals among samples with similar crystallinity, morphology, and textures but differing in allomorphs. The SFG signal from α-chitin was ∼40-fold weaker compared to β-chitin having a similar lateral dimension. The strong SFG signals arising from β-chitin can be explained by a net polar ordering of parallel packed chains. These results strongly supports the antiparallel chain orientation and high symmetry (orthorhombic) of α-chitin suggested by diffraction analysis.