We are currently working on two MAS solid-state NMR methods aimed at determining of three-dimensional molecular structure and demonstrating them in the context of secondary structure determination in peptides. The experiments can be used in complementary fashion to determine the backbone torsion angles ~ and y. The first experiment measures the dipole coupling between two """"""""C carbonyl spin labels, which is a function of the ~ angle, affords the highest sensitivity at 1 20' 1 !~ 0 !~ 1 140' 1. The resolution of the experiment is enhanced by (i) using a y encoded pulse sequence of the C7-type to restore the 13C_13 C homonuclear dipole coupling and (ii) allowing the spin system to evolve under the dipolar interaction for relatively long times (ca. 40ms). In the second experiment, a double quantum state between two carbonyl """"""""C nuclei is simultaneously evolved for a variable time t, under both chemical shielding (CSA) interactions. The evolution profile depends on the relative orientation of the two """"""""C CSA tensors, which is primarily a function of the 0 and V torsion angles. The experiment can be performed at relatively high spinning rates (co/27c - 10 kHz), using with double quantum filtration to suppress the natural abundance """"""""C signals. Numerical simulations indicate good sensitivity and resolution for 0 and xV in many of the biologically interesting regions (experiment has a particularly large dynamic range for torsion angles commonly observed in helical peptides and proteins). Both experiments can be carried out on a peptide specifically 13C labeled at the carbonyl positions of two adjacent'amino acid residues. In our studies, we employed tripeptide L-Ala-L-Gly-L-Gly with labels at Ala- 1 and Gly-2.
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