Dynamic Structure of a Protein Hydrogel: A Solid-State NMR Study

13 C and 15 N solid-state NMR spectroscopy has been used to study the dynamic structure of a genetically engineered multidomain protein hydrogel that contains two leucine-zipper domains and a central polyelectrolyte domain. 13 C NMR spectra show that on the microsecond time scale the central domain is isotropically mobile while the leucine-zipper domains are rigid. This supports the hypothesis that the central domain acts as the flexible swelling agent of the gel network while the terminal domains form intermolecular aggregates. 13 C isotropic chemical shifts indicate that the terminal domains are helical, while the central domain has a random coil conformation. On the millisecond time scale, the leucine-zipper domains are highly dynamic, as determined from the 13 C-detected 15 N CODEX experiment. The motion is rigid-body in nature with a correlation time of about 80 ms at room temperature and has an average amplitude of about 50°. Several specific motional models are considered by comparing simulated and experimental exchange intensities as a function of the recoupling time for 15 N chemical shift anisotropy. The experimental data are consistent with two of the models considered: a random jump model and a uniaxial rotation model. The implications of this motion to strand exchange between helical bundles are discussed.