Barstar, a polypeptide inhibitor of ribonucleases, has been studied by 2D and 3D NMR techniques using uniformly 15N-labeled protein. Backbone (15NH-CαH-CβH) resonances were assigned for all but 5 of the 89 residues. Dihedral angle and deuterium exchange studies were used in conjunction with medium range inter-proton NOEs to characterize the secondary structure of barstar. The protein is composed of four α-helices and three short stretches of extended strand. By further analysis of the NOE data three of the helices were found to be parallel to each other with the single disulphide bond linking the second and fourth helices at their C-terminal ends.
