Determination of the glycosidic torsion angles in uniformly 13C-labeled nucleic acids from vicinal coupling constants 3JC2/4-H1′ and 3JC6/8-H1′

A two-dimensional, quantitative J-correlation NMR experiment for precise measurements of the proton-carbon vicinal coupling constants (3)J(C2/4-H1') and (3)J(C6/8-H1') in uniformly C-13-labeled nucleic acids is presented. To reduce loss of signal due to H-1 -C-13 dipole-dipole relaxation, a multiple-quantum constant time experiment with appropriately incorporated band selective H-1 and C-13 pulses was applied. The experiment is used to obtain the (3JC2)/(4-H1') and (3)J(C6/8-H1') coupling constants in a uniformly C-13, N-15-labeled [d(G(4)T(4)G(4))](2) quadruplex. The measured values and glycosidic torsion angles in the G-quadruplex, obtained by restrained molecular dynamics with explicit solvent using the previously published restraints, along with selected data from the literature are used to check and modify existing parameters of the Karplus equations. The parameterizations obtained using glycosidic torsion angles derived from the original solution and recently determined X-ray structures are also compared.