(H)NCAHA AND (H)CANNH EXPERIMENTS FOR THE DETERMINATION OF VICINAL COUPLING-CONSTANTS RELATED TO THE PHI-TORSION ANGLE

A set of three-dimensional triple-resonance experiments is described w hich provide (3)J(HNH alpha), (3)J(HNCO), (3)J(HNC beta) and (3)J(H al pha CO) coupling constants. The pulse sequences generate E.COSY-like m ultiplet patterns and comprise a magnetization transfer from the amide proton to the alpha-proton or vice versa via the directly bound heter onuclei. For residues with the H-1(alpha) spin resonating close to the H2O signal, a modified HNCA experiment can be employed to measure the vicinal H-1(N),H-1(alpha) couplings. Ambiguities associated with the conversion of (3)J(HNH alpha) values into phi-angle constraints for pr otein structure determination can be resolved with the knowledge of th e heteronuclear (3)J-couplings. In favourable cases, stereospecific as signments of glycine alpha-protons can be obtained by employing the ex periments described here in combination with NOE data. The methods are applied to flavodoxin from Desulfovibrio vulgaris.