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Stabilization and destabilization of crystals of biopolymers initiated by the environment
Crystalline structure of biopolymers is shown to be greatly affected by the environment. The ubiquitous presence of moisture is found to disturb the crystalline structure of a biosynthesized model polypeptide. We propose a step-by-step mechanism to describe the process of hydration-induced structural changes. On the other hand, in the study of silk fibroin crystallization behavior, it is found that the presence of an oriented substrate can enhance the crystallization of silk II structure even from a dilute aqueous solution with or without salt. The mechanism of substrate-induced crystallization is discussed. The model polypeptide used in the hydration study has a lamellar structure with uniform lamellar thickness, a folding surface decorated with functional groups and a crystalline core with $\beta$-sheet chain conformation. Its structure is changed by moisture sorption in a sequential way. The folding surface functionality is hydrated most quickly followed by the expansion of crystalline core in the intersheet distance. The crystalline chain conformation is the last to be changed. The expansion is proposed to be due to unfavorable surface energetics induced by the incorporation of water molecules on the folding surface. Comparison of hydration-induced structural changes between two polypeptides with different lamellar thickness supports this explanation. The change in chain conformation is ascribed to the direct interaction of water and peptide units. Silk fibroin known to be unable to crystallize from dilute aqueous solution with or without salt is found to crystallize in the silk II form on the poly(tetrafluoroethylene) oriented substrate. The orientation and chain conformation of silk fibroin crystals thus formed are characterized with external reflection infrared spectroscopy. Both lattice matching and surface topography are possible nucleation mechanisms. The same oriented substrate is used in the study of crystallization of nylons from formic acid solution induced by nonsolvents. Different nonsolvents are found to induce different degree of orientation in the nylon crystals. Nylon 6 and nylon 66 exhibit different molecular arrangements on the oriented substrate. This difference in molecular arrangement is supposed to be due to the different phase separation behavior between these two polymers.
Chen, Chao-Cheng, "Stabilization and destabilization of crystals of biopolymers initiated by the environment" (1996). Doctoral Dissertations Available from Proquest. AAI9619379.