Raman Spectroscopic Evidence for Side-Chain Unfolding in Spider Dragline Silk under Tensile Deformation
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Raman Spectroscopic Evidence for Side-Chain Unfolding in Spider Dragline Silk under Tensile Deformation Xiaojun He1, Michael S. Ellison1* and Jacqueline M. Palmer2 1 School of Materials Science & Engineering, 161 Sirrine Hall; 2 Department of Genetics, Biochemistry & Life Science Studies; Clemson University, Clemson, South Carolina 296340971 ABSTRACT In-situ Raman spectra were collected on the N. clavipes spider dragline silk under a tensile deformation rate of 15mm/min. The most prominent features on the spectra were due to those bands near 1100 cm-1, which present as a sensitive probe to structural changes associated with side-chains of silk peptide. A downshift of Raman bands at 1095 cm-1 and 1089 cm-1 was detected with increasing strain. Furthermore, an increase in the intensity of the Raman band at 1062 cm-1 due to the vibration of trans structure without lateral coupling was prominent at certain strain levels. This was interpreted in terms of a morphology transition from the random configuration to the trans conformation modulated by the reorganization of the hydrogen bonding among the side-chain. INTRODUCTION Spiders can produce as many as six kinds of silk, each with distinctive mechanical properties owing to differences in the underlying protein composition and associated higher order structures. Among these silks, the dragline silk has been the material of choice for most studies because of its impressive combination of strength and toughness. However, the limited commercial productivity of spiders restricts natural dragline silk in this wealth of potential applications. Researchers have therefore turned to genetically engineered silk attempting to make it perform close to the dragline silk. Clearly, a better understanding of the design strategy based on the relationship between the protein sequence and the structure/function of dragline silk will permit the production of silk-mimetic engineered proteins. The physics of Raman spectroscopy renders it a non-invasive, nondestructive method to investigate the structural features of the silk morphologies. With the introduction of fiber optics to Raman instrumentation, in-situ measurements becomes an invaluable tool for understanding the response of spider silk under different environments. The responses of the microstructure of spider silk to tensile deformation under low extension rates for Nephila type spiders have been investigated recently via Raman spectroscopy, in which Raman bands at 1095 cm-1, 1085 cm-1 and 1230 cm-1 were chosen to examine the molecular deformation [1, 2]. In our present study a portable Raman probe was utilized to monitor the conformation of spider dragline silk under a relatively high tensile deformation rate in order to provide new insights into the structure – property correlation. EXPERIMENTAL Obtaining dragline silk The dragline silks used in this study were from the species Nephila clavipes, raised in the laboratory and fed crickets. Before being silked, the spider was anesthetized by being placed in a refrigerator at 4 °
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