Conformational flexibility of a microcrystalline globular protein: order parameters by solid-state NMR spectroscopy

The majority of protein structures are determined in the crystalline state, yet few methods exist for the characterization of dynamics for crystalline biomolecules. Solid-state NMR can be used to probe detailed dynamic information in crystalline biomolecules. Recent advances in high-resolution solid-state NMR have enabled the site-specific assignment of (13)C and (15)N nuclei in proteins. With the use of multidimensional separated-local-field experiments, we report the backbone and side chain conformational dynamics of ubiquitin, a globular microcrystalline protein. The measurements of molecular conformational order parameters are based on heteronuclear dipolar couplings, and they are correlated to assigned chemical shifts, to obtain a global perspective on the sub-microsecond dynamics in microcrystalline ubiquitin. A total of 38 Calpha, 35 Cbeta and multiple side chain unique order parameters are collected, and they reveal the high mobility of ubiquitin in the microcrystalline state. In general the side chains show elevated motion in comparison with the backbone sites. The data are compared to solution NMR order parameter measurements on ubiquitin. The SSNMR measurements are sensitive to motions on a broader time scale (low microsecond and faster) than solution NMR measurements (low nanosecond and faster), and the SSNMR order parameters are generally lower than the corresponding solution values. Unlike solution NMR relaxation-based order parameters, order parameters for (13)C(1)H(2) spin systems are readily measured from the powder line shape data. These results illustrate the potential for detailed, extensive, and site-specific dynamic studies of biopolymers by solid-state NMR.