13C-13C dipolar recoupling under very fast magic angle spinning in solid-state nuclear magnetic resonance:: Applications to distance measurements, spectral assignments, and high-throughput secondary-structure determination

A technique is presented to recouple homonuclear dipolar couplings between dilute spin pairs such as C-13-C-13 systems under very fast magic angle spinning (MAS) in solid-state nuclear magnetic resonance (NMR) spectroscopy. The presented technique, finite pulse rf driven recoupling (fpRFDR), restores homonuclear dipolar interactions based on constructive usage of finite pulse-width effects in a phase- and symmetry-cycled pi -pulse train in which a rotor-synchronous pi pulse is applied every rotation period. The restored effective dipolar interaction has the form of a zero-quantum dipolar Hamiltonian for static solids, whose symmetry in spin space is different from that obtained by conventional rf driven recoupling (RFDR) techniques. It is demonstrated that the efficiency of recoupling by fpRFDR is not strongly dependent on chemical shift differences or resonance offsets in contrast to previous recoupling methods under very fast MAS. To realize distance measurements without effects of spin relaxation, a constant-time version of fpRFDR (CT-fpRFDR) is introduced, in which the effective evolution period is varied by refocusing dipolar evolution with a rotor-synchronized solid echo while the total recoupling period is kept constant. From CT-fpRFDR experiments at a spinning speed of 30.3 kHz in a field of 17.6 T, the C-13-C-13 distance of [1-C-13]Ala-[1-C-13]Gly-Gly was determined to be 3.27 A, which agrees well with the value of 3.20 Angstrom obtained by x-ray diffraction. Also, two-dimensional (2D) C-13/C-13 chemical-shift correlation NMR spectrum in a field of 9.4 T was obtained with fpRFDR for fibrils of the segmentally C-13- and N-15-labeled Alzheimer's beta -Amyloid fragments, A beta (16-22) (residues 16-22 taken from the 40-residue A beta peptide) in which Leu-17 through Ala-21 are uniformly C-13- and N-15-labeled. Most C-13 resonances for the main chain as well as for the side chains are assigned based on 2D C-13/C-13 chemical-shift correlation patterns specific to amino-acid types. Examination of the obtained C-13 chemical shifts revealed the formation of beta -strand across the entire molecule of A beta (16-22). Possibility of high throughput determination of global main-chain structures based on C-13 shifts obtained from 2D C-13/C-13 chemical-shift correlation under very fast MAS is also discussed for uniformly/segmentally C-13-labeled protein/peptide samples.