Structure and dynamics of aromatic residues in spider silk: 2D carbon correlation NMR of dragline fibers

The structure of aromatic residues in spider silk has remained relatively uncharacterized. This is primarily due to the relatively low abundance of aromatic residues in dragline spider silk. NMR characterization has been further hindered by the inability to ¹³C-isotopically enrich these amino acids in the silk effectively. In the present contribution, it is shown that feeding spiders an aqueous solution of [U-¹³C/¹⁵N]-L- phenylalanine ¹³C-enriches Tyr to a level of 20-30% in dragline spider silk. This enables the collection of 2D through-bond ¹³C double quantum/single quantum (DQ/SQ) correlation spectra with the refocused INADEQUATE solid-state NMR pulse sequence. These 2D spectra provide the complete unambiguous assignment of the Tyr resonances in dragline silks from two spider species: N. clavipes and A. aurantia. Additionally, weak resonances are detected for Phe in A. aurantia dragline silk that is present at levels <1%. The conformation dependence of the Cα, Cβ, and carbonyl chemical shifts show that Tyr and Phe present in the GGX and GPGXX motifs in the spider silk proteins MaSp1 and MaSp2 are in disordered helical structures and not incorporated in the β-sheet domains.