Dipolar Recoupling and Decoupling

In solid state nuclear magnetic resonance spectroscopy, the magic angle spinning technique is often used to remove anisotropic interactions and generate high resolution spectra. However, these interactions contain valuable information about molecular structure and dynamics. In particular, the dipole-dipole coupling strength between two spin-1/2 nuclei is a direct reflection of their interatomic distance. Two ways of recoupling dipolar interactions into spinning experiments are explored here. In the first, longitudinal dipolar exchange experiments employing spin echoes are examined in homonuclear spin systems. The accurate measurement of internuclear distances is demonstrated for spin pairs. Two-dimensional exchange spectroscopy using n pulses is applied to the retinal conformations in dark-adapted bacteriorhodopsin and the relationship between the retinal chromophore and the neighboring aspartic acid residues. The distance of the retinal-14 position in light-adapted bacteriorhodopsin to the aspartic acid-212 sidechain is also established within less than 5.5 A using spin diffusion experiments. A second dipolar recoupling experiment involves the frequency-selective reintroduction of heteronuclear interactions among spin-1/2 nuclei. This approach is designed to examine weak dipolar interactions in a multiple spin environment, where it is desirable to isolate the influences of individual couplings. Simulations including losses of spin coherence within an exponential model are introduced for both the homonuclear and heteronuclear techniques. A theoretical framework for understanding compensation for pulse imperfections in multiple pulse echo sequences is presented, and the effect of insufficient proton decoupling on dilute spins is also discussed. Finally, a simple phase modulation scheme is introduced for heteronuclear spin decoupling in rotating solids, yielding improved linewidths over those obtained with continuous-wave decoupling under