The solution structure of oxidized Saccharomycescerevisiae Cys102Ser iso-1-cytochromechas been determined using 1361 meaningful NOEs (of 1676 total) after extending the published proton assignment [Gao, Y., et al. (1990) Biochemistry 29, 6994-7003] to 77% of all proton resonances. The NOE patterns indicate that secondary structure elements are maintained upon oxidation in solution with respect to the solid state and solution structures of the reduced species. Constraints derived from the pseudocontact shifts [diamagnetic reference shift values are those of the reduced protein [Baistrocchi, P., et al. (1996) Biochemistry 35, 13788-13796]] were used in the final stages of structure calculations. After restrained energy minimization with constraints from NOEs and pseudocontact shifts, a family of 20 structures with rmsd values of 0.58 +/- 0.08 and 1.05 +/- 0.10 A (relative to the average structure) for the backbone and all heavy atoms, respectively, was obtained. The solution structure is compared with the crystal structure and the structures of related systems. Twenty-six amide protons were detected in the NMR spectrum 6 days after the oxidized lyophilized protein was dissolved in D2O (pH 7.0 and 303 K); in an analogous experiment, 47 protons were observed in the spectrum of the reduced protein. The decrease in the number of nonexchanging amide protons, which mainly are found in the loop regions 14-26 and 75-82, confirms the greater flexibility of the structure of oxidized cytochrome c in solution. Our finding of increased solvent accessibility in these loop regions is consistent with proposals that an early step in unfolding the oxidized protein is the opening of the 70-85 loop coupled with dissociation of the Met80-iron bond.