Dynamic nuclear polarization-enhanced 13C NMR spectroscopy of static biological solids
Available online 27 February 2013
Publication year: 2013
Source:Journal of Magnetic Resonance
We explore the possibility of using dynamic nuclear polarization (DNP) to enhance signals in structural studies of biological solids by solid state NMR without sample spinning. Specifically, we use 2D 13C-13C exchange spectroscopy to probe the peptide backbone torsion angles (?,?) in a series of selectively 13C-labeled 40-residue ?-amyloid (A ?1-40) samples, in both fibrillar and non-fibrillar states. Experiments are carried out at 9.39 T and 8 K, using a static double-resonance NMR probe and low-power microwave irradiation at 264 GHz. In frozen solutions of A ?1-40 fibrils doped with DOTOPA-TEMPO, we observe DNP signal enhancement factors of 16-21. We show that the orientation- and frequency-dependent spin polarization exchange between sequential backbone carbonyl 13C labels can be simulated accurately using a simple expression for the exchange rate, after experimentally determined homogeneous 13C lineshapes are incorporated in the simulations. The experimental 2D 13C-13C exchange spectra place constraints on the ? and ? angles between the two carbonyl labels. Although the data are not sufficient to determine ? and ? uniquely, the data do provide non-trivial constraints that could be included in structure calculations. With DNP at low temperatures, 2D 13C-13C exchange spectra can be obtained from a 3.5 mg sample of A ?1-40 fibrils in 4 hr or less, despite the broad 13C chemical shift anisotropy line shapes that are observed in static samples.
Graphical abstract
Highlights
? DNP allows for rapid accumulation of 2D exchange spectra. ? Samples with varying labeling and structure show distinct 2D patterns. ? 2D exchange orientation dependence can be included into simulation. ? Comparison with simulations reveals allowed ranges of backbone torsion angles.
Source:
Journal of Magnetic Resonance