Since their discovery in the early fifties, scalar/coupling constants have been of great interest to the NMR spectroscopist. Their impact on structure determination by NMR spectroscopy is founded on the fact that the size of the coupling constant is directly related to molecular conformation. Today, for most chemical substances the parameters for the Karplus relationship, which relates the vicinial (3‐bond) coupling constant to the dihedral angle, have been determined. In addition to proton–proton distances, the application of coupling constants in modern conformational analysis is indispensable. In the study of larger molecules which are of current interest, more and more involved experiments are necessary in order to overcome signal overlap and increasing line widths. A large number of experimental techniques for the determination of coupling constants has been developed; however, for this reason the choice of the most appropriate experiment to use has become more difficult. This decision must be made carefully to maximize instrument usage and obtain the largest number of couplings with the greatest accuracy possible. Many of the computer programs used in structure calculations can directly apply coupling constant restraints, similar to proton–proton distances developed from NOEs. Therefore, not only is the quality of the structure improved, but the molecular motions (internal dynamics) are better described. In this article, we review the techniques that exist today with particular attention paid to helping the non‐expert to choose the appropriate experiment for the problem at hand. In addition, the use of coupling constants in computer simulations are discussed.

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Cyclosporin A (CsA), a potent immunosuppressant, is known to bind with high specificity to cyclophilin (CyP), a 17.7 kDa protein with peptidyl-prolyl isomerase activity. In order to investigate the three-dimensional structure of the CsA/CyP complex, we have applied a variety of multidimensional NMR methods in the study of uniformly 13C-labeled CsA bound to cyclophilin. The 1H and 13C NMR signals of cyclosporin A in the bound state have been assigned, and from a quantitative interpretation of the 3D NOE data, the bound conformation of CsA has been determined. Three-dimensional structures of CsA calculated from the NOE data by using a distance geometry/simulated appealing protocol were found to be very different from previously determined crystalline and solution conformations of uncomplexed CsA. In addition, from CsA/CyP NOEs, the portions of CsA that interact with cyclophilin were identified. For the most part, those CsA residues with NOEs to cyclophilin were the same residues important for cyclophilin binding and immunosuppressive activity as determined from structure/activity relationships. The structural information derived in this study together with the known structure/activity relationships for CsA analogues may prove useful in the design of improved immunosuppressants. Moreover, the approach that is described for obtaining the structural information is widely applicable to the study of small molecule/large molecule interactions.

A new bicyclic 19-peptide, BI-32169, has been isolated from the culture broth of Streptomyces sp. (DSM 14996). Its structure has been established by amino acid analysis, mass spectrometry, and 2D NMR analysis. BI-32169 consists exclusively of protein amino acids and is cyclized from the side chain of Asp(9) to the N-terminus of Gly(1). One disulfide bond between Cys(6) and Cys(19) forms a bicyclic structure. BI-32169 and its methyl ester derivative showed potent inhibitory activity against the human glucagon receptor (IC(50) 440 and 320 nM, respectively) in a functional cell-based assay.