Effect of electrostatic interaction on phase separation behaviour of chitin crystallite suspensions.

Optical and electron microscopy were used to observe the dynamics of the phase separation in aqueous chitin suspensions prepared by HCl hydrolysis of crab chitin. Freeze-fracture transmission electron microscopy reveals that chitin crystallites are partially aggregated in the suspension and have an average length of 200 nm and an average width of 8 nm. They exhibit a positive surface charge of approximately 0.5 e/nm2 when fully protonated. The liquid crystal-forming aqueous suspensions of such crystallites are investigated through phase diagrams and Zeta potential measurements for different ionic strength. Exposure of the suspension to a low concentration of univalent electrolyte has a negligible effect on phase separation because of the contribution of the charged crystallites themselves to the ionic strength. The thickness of the effective repulsive layer is estimated both from the phase diagrams according to Onsager's theory and from the computed interaction energy derived from the Poisson-Boltzmann equation using the experimental Zeta potential as surface potential. When the contribution of crystallites to the ionic strength is taken into account and an hypothetical linear charge density close to the Manning limit is assumed, there is good agreement with the Debye length.

In vitro chiral nematic ordering of chitin crystallites.

Microfibrillar fragments of purified crab and shrimp chitin were prepared by hydrolysis in 3 M HCl at its boiling point (104 degrees C). After removal of the acid by centrifugal washing and dialysis, an ultrasound treatment converts the residual product to a colloidal suspension stabilized by NH3+ charges. When dewatered to a critical concentration, spontaneous formation of a two-phase equilibrium system system occurs. The upper phase (lower concentration) is isotropic and the lower phase is anisotropic. The latter displays chiral nematic order and dries to a solid film which mimics the helicoid organization characteristic of the chitin microfibrils in the cuticle of arthropods.

Helicoidal self-ordering of cellulose microfibrils in aqueous suspension.

In many skeletal support systems of plants and animals, cellulose, chitin, and collagen occur in the form of microfibrils ordered in a chiral nematic fashion (helicoids). However, these structures remain poorly understood due to the many constituents present in biological tissues. Here we report an in vitro system that attracts by its simplicity. Only one chemical component, cellulose, is present in the form of fibrillar fragments dispersed in water. Above a critical concentration the colloidal dispersion separates spontaneously into a chiral nematic liquid crystalline phase. On drying this phase solidifies into regularly twisted fibrillar layers that mimic the structural organization of helicoids in nature.

Lattice resolution in alpha-chitin.

The lattice images of the alpha-chitin microfibrils from lobster tendon were recorded with a transmission electron microscope operated at 120 keV. It was concluded that a close resemblance exists between alpha-chitin microfibrils and cellulose microfibrils. In both cases, the microfibrils are elongated single crystals (crystallites) of high perfection, with the chains aligned and probably fully extended along the microfibril axis.

Distribution and conformation of crystalline nigeran in hyphal walls of Aspergillus niger and Aspergillus awamori.

Hyphal walls of Aspergillus awamori containing increased amount of the alpha-glucan, nigeran, became correspondingly more opaque when viewed in the electron microscope as shadowed preparations. However, increased polymer deposition was not accompanied by any significant change in wall thickness. The nigeran of both A. awamori and Aspergillus niger occurred in situ in a crystalline conformation identical to that of single crystals prepared with pure polysaccharide. Furthermore, this polymer was the dominant crystalline material in the hyphae whether or not they were enriched in nigeran. Enzymic digestion of nigeran in A. niger and A. awamori revealed that the bulk of the polymer was exposed to the cell's exterior. However, a certain fraction was accessible to enzymic attack only after the wall was treated with boiling water. A third portion, detectable only by x-ray diffraction, was associated with other components and could not be extracted, even with prolonged boiling. It was removed by hot, dilute alkali and was associated in the wall with another glucan fraction. Dry heating of A. niger walls altered their susceptibility to enzymic digestion of nigeran in situ. It is proposed that this treatment introduces interstices in the crystal surface that facilitate attack.