| To design synthetic fibers for extreme applications, many research groups are trying to identify how the structure of natural fibers (such as spider silk) leads to desirable properties such as high tensile strength.;Spider silk is a biological fiber having mechanical properties that exceed those of the best man made fibers in terms of high tensile strength and large extensibility, this combination providing the silk with a large work of rupture. While the bulk structure, composition and properties have been intensively studied elsewhere, this study focuses on elucidating the Nephila clavipes spider dragline silk surface structure and composition, as well as examining the possibility of a pattern to the arrangement of the amino acids on the surface. These spiders are traditionally studied since they are orb weaving spiders with relatively high silk toughness values. While these spiders fabricate several types of silk, all with different biological uses, this study focuses only on dragline silk.;The silk surface morphology was observed initially using contact mode atomic force microscopy in 0.01 M phosphate buffered saline (PBS) solution. In general, the roughness of dragline silk, which has a nominal diameter of five micrometers, was found between 20 nm and 100 nm. We studied the roughness as a function of the collection speed and of age of the silk (time since collection). It was found that surface roughness is independent of collection speed: values are included in the same range of values (40 nm to 60 nm) and no trend is demonstrated. Roughness study as a function of time since collection also showed that there is no change in roughness as the fiber ages. This demonstrates the surface stability of the dragline silk over time in terms of roughness. There were surface features that may have been cracking in a worm-like fashion which may have been formed by stretching during sample preparation.;To determine the arrangement of amino acids along the dragline silk, functionalized gold nanoparticles were used to "mark" the charged amino acid locations. The gold nanoparticles functionalized with COOH groups (respectively, NH2 groups) are used to find positively charged (respectively, negatively charged) amino acids. The density of negatively charged amino acids (glutamic acid, aspartic acid) is higher than that of the positively charged ones (lysine, asparagine, and histidine). This correlates with the relative amounts of amino acids found by amino acid analysis by other researchers [2.1, 2.2, 2.3]. A pattern might have been found in the arrangement of negatively charged amino acids, in that they might be spaced with a certain frequency at some locations, but additional work is needed to confirm this. On the other hand, positively charged amino acids were randomly arranged on the surface. |
