DIPOLAR CORRELATION NMR-SPECTROSCOPY OF A MEMBRANE-PROTEIN

We demonstrate the application of a new NMR experiment-RF-driven recoupling (RFDR)-for establishing spatial connectivities and measuring internuclear distances in spinning solids. RFDR employs rotor-synchronized pi-pulses in a longitudinal mixing scheme to reintroduce dipolar couplings into magic angle spinning (MAS) NMR experiments. We have utilized this technique to measure a 0.25 nm distance in polycrystalline D,L-alanine and to determine relative distances in the integral membrane protein bacteriorhodopsin (bR). In the latter case, we have focused on an internuclear distance that defines the configuration about the retinal-protein Schiff base linkage in the two conformers comprising dark-adapted bR. Our results demonstrate that RFDR is a valid and practical technique for structural investigations of solids including biological molecules with molecular weights as great as 85 kDa.