Abstract
Solid-state NMR (SSNMR) is established as a powerful tool for extracting structural parameters from biological and chemical systems.1-3 However, the inherent low sensitivity of NMR often limits the scope of structural studies. With dynamic nuclear polarization (DNP) it is possible to address this issue since signal intensities can be enhanced by 2 to 3 orders of magnitude. This gain in sensitivity is accomplished by transferring polarization from electrons in exogenous paramagnetic molecules to nuclear spins via microwave irradiation at or near the EPR Larmor frequency.4 The size of the DNP signal enhancement is dependent on a number of factors including the nuclear spin lattice relaxation time, T1, which is maximized by performing experiments at low temperatures. Systems with an abundance of methyl groups, such as virus particles and membrane proteins, often have short T1's even at low temperatures and could be particularly challenging candidates for DNP experiments. However, these are also some of the most interesting cases for SSNMR structural studies since they often cannot be examined with either solution NMR or X-ray diffraction. In this communication we demonstrate that it is possible to efficiently polarize archetypal examples of each of the systems mentioned above-namely the viral particle fd and the purple membrane containing bacteriorhodopsin (bR) and its accompanying lipids. Furthermore, by comparing the DNP signal enhancements in the 15N and 31P spectra of fd bacteriophage, we show that 1H spin diffusion evenly distributes the enhanced polarization throughout a large macromolecular assembly. These results suggest that DNP may be a generally applicable approach for sensitivity enhancement in SSNMR experiments.