The thermodynamics and dynamics of the Cys21-Cys48 disulfide “S” S “R” conformationalisomerism in the three-iron, single cubane cluster ferredoxin (Fd) from the hyperthermophilic archaeonPyrococcus furiosus (Pf) have been characterized by 1H NMR spectroscopy in both water and water/methanol mixed solvents. The mean interconversion rate at 25 °C is 3 103 s-1 and ¢G298 ) -0.2kcal/mol [¢H ) 4.0 kcal/mol; ¢S ) 14 cal/(molâK)], with the S orientation as the more stable form atlow temperature (100 °C, where the organism thrives.The distinct pattern of ligated Cys â-proton contact shifts for the resolved signals and their characteristictemperature behavior for the forms of the 3Fe Fd with alternate disulfide orientations have been analyzedto determine the influences of disulfide orientation and methanol cosolvent on the topology of the interironspin coupling in the 3Fe cluster. The Cys21-Cys48 disulfide orientation influences primarily thespin couplings involving the iron ligated to Cys17, whose carbonyl oxygen is a hydrogen bond acceptorto the Cys21 peptide proton. Comparison of the Cys â-proton contact shift pattern for the alternate disulfideorientations with the pattern exhibited upon cleaving the disulfide bridge confirms an earlier [Wang,P.-L., Calzolai, L., Bren, K. L., Teng, Q., Jenney, F. E., Jr., Brereton, P. S., Howard, J. B., Adams, M.W. W., and La Mar, G. N. (1999) Biochemistry 38, 8167-8178] proposal that the structure of the sameFd with the R disulfide orientation resembles that of the Fd upon cleaving the disulfide bond.