The cytochromes c' are paramagnetic heme proteins generally consisting of two identical 14 kDa subunits. The 1H and 15N resonances of the ferricytochrome c' from the purple phototrophic bacterium Rhodobacter capsulatus have been extensively assigned by the TOCSY-HSQC, NOESY-HSQC, HSQC-NOESY-HSQC, and HNHA 3D heteronuclear experiments performed on an 8 mM sample labeled with 15N. In addition, the 13C alpha and 13CO resonances were assigned by the HNCA and multiple-quantum HNCOCA 3D experiments performed on a 0.5 mM sample labeled with 13C and 15N. The assignment of the backbone 13C resonances was used to confirm the 1H and 15N assignments and to better define secondary structure. On the basis of medium-range NOEs, 3JHN alpha coupling constants, and backbone 13C chemical shifts, the secondary structure consists of four helices: helix-1 (3-29), helix-2 (33-49), helix-3 (78-97), and helix-4 (103-117). On the basis of long-range NOE contacts, the Rb. capsulatus ferricytochrome c' is a four-helix bundle protein in which consecutive helices are antiparallel with respect to one another.