Determination of molecular geometry by high-order multiple-quantum evolution in solid-state NMR

The principles of molecular geometry determination by high-quantum heteronu clear local field spectroscopy in solid-state NMR are discussed. The extrem e multiple-quantum coherences in a cluster of nuclear spins are allowed to evolve in the presence of heteronuclear through-space couplings to two spin s of a different type, The multiple-quantum dephasing curve is independent of the homonuclear spin-spin couplings and may be described in terms of geo metric parameters. The triple-quantum version of the experiment is demonstr ated by determining the psi torsion angle in a [N-15(2), C-13(3)]-labeled s ample of the peptide ala-ala-gly. Two regions of torsion angle space fit th e experimental data, one in the neighborhood of -152 degrees and one in the neighborhood of +161 degrees. The latter determination is in excellent agr eement with the X-ray estimate of +160.5 degrees. (C) 2000 Academic Press.