Silk from various species such as insects and arachnids along with structures such as termite mounds and beaver dams constitutes an extended phenotype [1]. Silk fibers, which are an aggregate of a single or multiple types of proteins (called fibroins), present remarkable evidence of evolutionary convergence and divergence among the various species. Key to the usage of various extended phenotypes by organisms is their adaptive variability to changing environmental conditions. Yet our understanding of the underlying causes and adaptive value of the extended phenotypes is ill defined. Silks and spider webs are ideal models to study various aspects of extended phenotypes, with relative ease of measuring key functional properties, and the remarkable utility of accessing detailed insight into the protein aggregate structure via biomolecular NMR spectroscopy. Here, we present a structural comparison of silk fibres from various organisms covering a range of insect and arachnid species. While isotopic enrichment is not always readily afforded, and obtaining sufficient sample size can be challenging, none the less, significant insight into the structure of the silk aggregates is gained by solid state NMR spectroscopy.
For the silkworm moths (insect species) Bombyx mori (the only domesticated insect species in the world after honeybees), Antheraea assamensis and Samia cynthia ricini (both undomesticated silkworms) NMR reveals distinct structural motiefs in silk fiber structure with variation in the inter-residue hydrogen bond strength well correlated to overall tensile properties of silk fibers [2]. However intra-fiber variation in properties which was also noted, was found to be due to nanovoid formation in the wild-type silks.
In the ultra-high strength dragline silk from Latrodectus hasselti, Argiope keyserlingi and Nephila plumipes, structural variations are also evident, with motifs in N. Plumipes showing remarkable similarity to that of B. Mori, a species separated by more than 500 million years of evolution [3]. Here, the ~1 micron fiber diameter of spider silks make then extremely amenable to DNP-NMR with ~ 60 fold signal enhancement achieved. We also present recent work on nano-silks a form of dry adhesive prey capture silk formed by the cribellate spider Badumna longinqua[4].