Self-Organization (Assembly) in Biosynthesis of Silk Fibers - A Hierarchical Problem
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SELF-ORGANIZATION (ASSEMBLY) IN BIOSYNTHESIS OF SILK FIBERS - A HIERARCHICAL PROBLEM DAVID L. KAPLAN*, STEPHEN FOSSEY*, CHRISTOPHER VINEY**, WAYNE MULLER* "*Biotechnology Division, U.S. Army Natick Research Center, Natick, Massachusetts, 01760, USA **Center for Bioengineering, University of Washington, Seattle, Washington, USA ABSTRACT In natural systems, structural macromolecules undergo prescribed recognition and assembly steps during synthesis and processing. These associations lead to more complex assemblies that exhibit useful multifunctional properties. Many of these processes are not well understood. Some aspects of these processes are presented using the fibrous protein polymer silk as an example. Issues such as polymer chain biosynthesis, chain interactions, processing into fibrils, and complex engineering into supra-assemblies are addressed and biochemical, spectroscopic and modeling studies are reviewed. Genetic level controls of chain composition, crystalline/amorphous domain distribution, chain aggregation, chain registry, silk I-silk II phase transitions, nematic liquid crystalline phase, loss of water, global molecular alignment, and solution spinning are some of the characteristics of this biological system that are addressed. Although some information is available at the molecular and macro-scale levels, a key issue is the paucity of information at the meso-scale level to fully understand the role of structural hierarchy in the silk fiber assembly process. INTRODUCTION Efficient assembly processes inherent in biological systems are required for the formation of complex multifunctional materials. There are an impressive number of structures and systems that could be examined for insights into these processes, from complex membrane channel proteins and studies of microtubule assembly to flagellar motors. We will focus on assembly processes for biological fibers. Although fiber assembly would appear to be a simpler process when compared with some of the more complex systems mentioned above, very little detail is still understood. The possible role of hierarchy in this process will be described, where "hierarchy" [1] refers to structures with: (1)different scales of organization (e.g. molecular, nano-, micro-, meso-, macro-), (2) characterized by specific interactions between the different components, and (3) characterized by complex architecture to achieve desired functional properties. Many different biological fibers could be examined for insight into these processes, including structural proteins (e.g., keratin, collagen, silk, troponin, elastin, actin) and structural polysaccharides (e.g., cellulose, chitin). We will focus on silks because of the structural hierarchy exhibited from the genetic level to the web or cocoon level, the unusual mechanical properties (Table I), the correlation between structure and function evident even by amino acid composition analysis of different silks from one species of spider (Table II), and the fact that this is the most common spun protein fiber in biological sy
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