The School of Molecular and Cellular Biology at the University of Illinois at Urbana-Champaign

Research in the Rienstra group aims to establish solid state nuclear magnetic resonance (SSNMR) as a preferred method for routine, atomic resolution structural and dynamic analysis of biological macromolecules. The field of SSNMR is in the midst of a revolution not unlike the advances in X-ray crystallography that occurred during the 1970's, and solution NMR in the 1980's. Those methods have been responsible for the vast majority of all protein structures known to date. However, neither method has been applied in a general fashion to membrane proteins and protein-lipid complexes, which have profound importance to biochemistry, yet remain vastly underrepresented in the database of known protein structures. SSNMR has two distinct advantages relevant to the study of biological membranes. First, unlike in solution NMR, where global correlation (tumbling) times impose fundamental restrictions of the particle size that may be studied, in the solid state, spectral intensities and homogeneous line widths of individual NMR signals do not depend upon molecular weight. Second, unlike in crystallography, in our SSNMR experiments long-range order is not required, because the inhomogeneous line widths are determined by the degree of order in the local (5 to 10 Å) environment; therefore heterogeneous sample environments (e.g., asymmetric oligomeric assemblies, protein-lipid complexes, etc.) are inherently no less qualified for structural analysis by SSNMR than single crystals. These two characteristics make SSNMR the best (and often the only) method for studying atomic resolution structure in biological membranes, precipitated peptide aggregates (e.g., β-amyloid peptides), glasses, frozen solutions and lyophilized powders (e.g., trapped enzyme-substrate intermediates). Therefore intense interest has been focused on the problem of global structure determination by SSNMR.

Multi-dimensional SSNMR methods are in a relatively early stage of development, due to a variety of technical limitations that have been overcome in recent years, and a number of ongoing challenges that we intend to address. Historically, solid state NMR studies were performed at low field (200 to 400 MHz), using samples labeled at only a few positions with spin-1/2 (¹³C, ¹⁵N) nuclei, in order to acquire 1D spectra with inherently low resolution. At Illinois, we are upgrading the School of Chemical Sciences 500 MHz wide bore and 750 MHz NMR spectrometers to perform state-of-the-art, high resolution, multi-dimensional, triple and quadruple resonance magic-angle spinning experiments. Only a handful of laboratories worldwide currently have access to such high field magnets for SSNMR. Furthermore, through both collaborations with major spectrometer vendors and independent efforts in Urbana, we are building several new types of magic-angle spinning SSNMR probes for novel experiments. We intend to leverage these capabilities in combination with 2D, 3D, and 4D correlation methods to resolve all of the hundreds to thousands of NMR signals in proteins that have been uniformly enriched with ¹⁵N and ¹³C isotopes, thereby increasing throughput by 2 to 3 orders of magnitude compared to traditional SSNMR methods. With resolved and assigned signals, a variety of quantum mechanical observables (such as dipolar and quadrupolar couplings, isotropic and anisotropic chemical shifts) can be uniquely measured and mapped to each individual site, to reveal structural and dynamic properties (internuclear distances, torsion angles, and relaxation rates).