Dynamic structure of a protein hydrogel: A solid-state NMR study

C-13 and N-15 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. C-13 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. C-13 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 C-13-detected N-15 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 degrees. Several specific motional models are considered by comparing simulated and experimental exchange intensities as a function of the recoupling time for N-15 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.