Tracking Transitions in Spider Wrapping Silk Conformation and Dynamics by (19)F Nuclear Magnetic Resonance Spectroscopy.

Aciniform silk protein (AcSp1) is the primary component of wrapping silk, the toughest of the spider silks because of a combination of high tensile strength and extensibility. Argiope trifasciata AcSp1 contains a core repetitive domain with at least 14 homogeneous 200-amino acid units ("W" units). Upon fibrillogenesis, AcSp1 converts from an α-helix-rich soluble state to a mixed α-helical/β-sheet conformation. Solution-state nuclear magnetic resonance (NMR) spectroscopy allowed demonstration of variable local stability within the W unit, but comprehensive characterization was confounded by spectral overlap, which was exacerbated by decreased chemical shift dispersion upon denaturation. Here, (19)F NMR spectroscopy, in the context of a single W unit (W1), is applied to track changes in structure and dynamics. Four strategic positions in the W unit were mutated to tryptophan and biosynthetically labeled with 5-fluorotryptophan (5F-Trp). Simulated annealing-based structure calculations implied that these substitutions should be tolerated, while circular dichroism (CD) spectroscopy and (1)H-(15)N chemical shift displacements indicated minimal structural perturbation in W1 mutants. Fiber formation by W2 concatemers containing 5F-Trp substitutions in both W units demonstrated retention of functionality, a somewhat surprising finding in light of sequence conservation between species. Each 5F-Trp-labeled W1 exhibited a unique (19)F chemical shift, line width, longitudinal relaxation time constant (T1), and solvent isotope shift. Perturbation to (19)F chemical shift and nuclear spin relaxation parameters reflected changes in the conformation and dynamics at each 5F-Trp site upon addition of urea and dodecylphosphocholine (DPC). (19)F NMR spectroscopy allowed unambiguous localized tracking throughout titration with each perturbant, demonstrating distinct behavior for each perturbant not previously revealed by heteronuclear NMR experiments.