The solution structure of reduced Bacillus pasteurii cytochrome c, which has only 71 amino acids, has been determined by NMR to an RMSD of 0.46 +/- 0.08 A for all backbone atoms and 0.79 +/- 0.08 A for all heavy atoms and refined through restrained energy minimization. The target function out of 1645 constraints is 0.52 +/- 0.11 A(2), and the penalty function is 66 +/- 12 kJ mol(-)(1). The structure appears very similar to that in the oxidized state, only Trp87 and the propionates showing significant differences. The mobility was investigated through (15)N R(1) and R(2) relaxation rates, (15)N-(1)H NOE, and (1)H/(2)H exchange. It is found that the oxidized form is generally more mobile than the reduced one. By comparing the redox-state dependence of the structural/dynamic properties of Fe-S proteins, cytochrome c, and blue copper proteins, hints are provided for a better comprehension of the electron transfer processes.