We recently described a triple resonance experiment, HN(C)N, for sequential correlation of H-N and N-15 atoms in (N-15, C-13) labeled proteins [J. Biomol. NMR. 20 (2001) 135]. Here, we describe an approach based on this experiment for estimation of one bond N-C-alpha J-couplings in medium size labeled proteins, which seem to show good correlations with psi torsion angles along the protein backbone. The approach uses the ratio of the intensities of the sequential and diagonal peaks in the F-2-F-3 planes of the HN(C)N spectrum. The reliability of the approach has been demonstrated using a short peptide wherein the coupling constants have been measured by the present method and also independently from peak splittings in HSQC spectra. The two results agree within 10%. The applicability of the procedure to proteins has been demonstrated using doubly labeled FK506 binding protein (FKBP, molecular mass similar to12 kDa). Coupling constant estimates have been obtained for 62 out of 100 non-proline residues and they show a correlation with psi torsion angles, as has been reported before. This semi-quantitative application of HN(C)N extends the significance of the experiment especially, in the context of structural genomics, since the single experiment, not only provides a great enhancement in the speed of resonance assignment, but also provides quantitative structural information. (C) 2003 Elsevier Inc. All rights reserved.