Band-selective homonuclear dipolar recoupling in rotating solids

In this paper, we introduce approaches to band-selective homonuclear dipolar recoupling with the SPC-5 sequence described previously [Hohwy , J. Chem. Phys. 110, 7983 (1999)]. The technique, denoted SPC-5(3), restores the homonuclear dipolar coupling during magic angle spinning (MAS) and introduces a fifth-order residual offset term of controllable magnitude. The fifth order term truncates the dipolar coupling to spins that fall outside the operational bandwidth of the experiment. It is shown with high-order average Hamiltonian theory, multiple-spin simulations, and experiments, that polarization within a spin cluster can be propagated to destination spins with improved efficiency using this approach. Further, we show that a spin system subjected to band-selective recoupling obeys the equation of motion of the reduced spin cluster and that modeling of the polarization transfer process is simplified. Thus, in the important case of peptides and proteins, all of the backbone and side-chain carbonyl spins as well as aromatic spins can be neglected, leading to enhanced transfer efficiency among C-alpha, C-beta, C-gamma, etc. Experimental spectra of [U-C-13,N-15]-threonine and formyl-[U-C-13,N-15]-Met-Leu-Phe-OH, indicate that the backbone carbonyl and sidechain aromatic spins can be neglected in the polarization transfer equations. This leads to enhanced transfer efficiencies of up to a factor of similar to2. (C) 2002 American Institute of Physics.