Jacco D. van Beek

The design principles of spider dragline silk, nature's high-performance fiber, are still largely unknown, in particular for the noncrystalline glycine-rich domains, which form the bulk of the material. Here we apply two-dimensional solid-state NMR to determine the distribution of the backbone torsion angles (phi,psi) as well as the orientation of the polypeptide backbone toward the fiber at both the glycine and alanine residues. Instead of an "amorphous matrix," suggested earlier for the glycine-rich domains, these new data indicate that all domains in dragline silk have a preferred secondary structure and are strongly oriented, with the chains predominantly parallel to the fiber. As proposed previously, the alanine residues are predominantly found in a beta sheet conformation. The glycine residues are partly incorporated into the beta sheets and otherwise form helical structures with an approximate 3-fold symmetry.

The local structure of dragline silk from the spider Nephila madagascariensis is investigated by solid-state nuclear magnetic resonance. Two-dimensional (2D) spin-diffusion experiments show that the alanine-rich domains of the protein form β-sheet structures in agreement with one-dimensional NMR results from a different species of the genus Nephila (Simons, A.; Ray, E.; Jelinski, L. W. Macromolecules 1994, 27, 5235) but at variance with diffraction results. The microstructure of the glycine-rich domains is found to be ordered. The simplest model that explains the experimental findings is a 31-helical structure. Random coils, planar β-sheets, and α-helical conformations are not found in significant amounts in the glycine-rich domains. This observation may help to explain the extraordinary mechanical properties of this silk, because 31-helices can form interhelix hydrogen bonds.

The thickness of dendronized polymers can be tuned by varying their generation g and the dendron functionality X. Systematic studies of this effect require (i) synthetic ability to produce large samples of high quality polymers with systematic variation of g, X and of the backbone polymerization degree N, (ii) a theoretical model relating the solvent swollen polymer diameter, r, and persistence length, lambda, to g and X. This article presents an optimized synthetic method and a simple theoretical model. Our theory approach, based on the Boris-Rubinstein model of dendrimers predicts r approximately n(1/4)g(1/2) and lambda approximately n(2) where n = [(X - 1)(g) - 1]/(X - 2) is the number of monomers in a dendron. The average monomer concentration in the branched side chains of a dendronized polymer increases with g in qualitative contrast to bottle brushes whose side chains are linear. The stepwise, attach-to, synthesis of X = 3 dendronized polymers yielded gram amounts of g = 1-4 polymers with N approximately = 1000 and N approximately = 7000 as compared to earlier maxima of 0.1 g amounts and of N approximately = 1000. The method can be modified to dendrons of different X. The conversion fraction at each attach-to step, as quantified by converting unreacted groups with UV labels, was 99.3% to 99.8%. Atomic force microscopy on mixed polymer samples allows to distinguish between chains of different g and suggests an apparent height difference of 0.85 nm per generation as well as an increase of persistence length with g. We suggest synthetic directions to allow confrontation with theory.