(H)NCAHA and (H)CANNH experiments for the determination of the vicinal coupling constants related to the phi-torsion angle.

A set of three-dimensional triple-resonance experiments is described which provide 3J(HNHalpha), 3J(HNCO), 3J(HNCbeta) and 3J(HalphaCO) coupling constants. The pulse sequences generate E.COSY-like multiplet patterns and comprise a magnetization transfer from the amide proton to the alpha-proton or vice versa via the directly bound heteronuclei. For residues with the 1H(alph) spin resonating close to the H2O signal, a modified HNCA experiment can be employed to measure the vicinal 1H(N),1H(alpha) couplings. Ambiguities associated with the conversion of 3J(HNHalpha) values into phi-angle constraints for protein structure determination can be resolved with the knowledge of the heteronuclear 3J-couplings. In favourable cases, stereospecific assignments of glycine alpha-protons can be obtained by employing the experiments described here in combination with NOE data. The methods are applied to flavodoxin from Desulfovibrio vulgaris.