MULTIDIMENSIONAL SOLID-STATE NUCLEAR-MAGNETIC-RESONANCE FOR DETERMINING THE DIHEDRAL ANGLE FROM THE CORRELATION OF C-13-H-1 AND C-13-C-13 DIPOLAR INTERACTIONS UNDER MAGIC-ANGLE-SPINNING CONDITIONS

Multidimensional solid-state nuclear magnetic resonance (NMR) under ma gic-angle spinning (MAS) conditions has been developed to determine th e dihedral angle for a H-1(alpha)-C-13(alpha)-C-13(beta)-H-1(beta) moi ety in powdered states. The pulse sequence for this experiment include s (CH)-C-13-H-1 dipolar evolution periods for C-alpha and C-beta, whic h are correlated through a coherent (CCbeta)-C-13-C-alpha 13 dipolar m ixing period. Theoretical analysis based on the symmetry of the spin s ystem indicates that the dipolar correlation spectrum only due to the (CHalpha)-H-alpha and (CHbeta)-H-beta dipolar couplings is strongly de pendent on the dihedral angle chi about the (CCbeta)-C-alpha bond axis , but two chi angles give the same spectrum in the chi range from 0 de grees to about 140 degrees, where chi=0 degrees corresponds to the cis conformation. Inclusion of the (CCbeta)-C-alpha dipolar coupling toge ther with the weak (CHbeta)-H-alpha and (CHalpha)-H-beta dipolar coupl ings, however, breaks the symmetry of the system with respect to chi i n the range from 0 degrees to 180 degrees. These properties are confir med by the spectra calculated for the pulse sequence as a function of chi and the root-mean-square deviation between them. The bond lengths, bond angles, and dihedral angle also alter the dipolar correlation sp ectrum differently. This enables us the experimental determination of all the structural parameters, which improves the accuracy of the dihe dral angle determination. The high resolution due to C-13 isotropic ch emical shifts under MAS conditions in this multidimensional NMR permit s its application to molecules having a number of C-13-labeled sites. Experimental results are presented for powdered L-valine uniformly lab eled with C-13 and N-15 nuclei. Effects of the structural parameters a nd noise on the dihedral angle determination are evaluated numerically . The accuracies of the determined structural parameters are discussed , (C) 1998 American Institute of Physics. [S0021-9606(98)01030-7].