Publisher Summary The single cubane cluster ferredoxins, Fds, contain a [4Fe:4S] cluster typically coordinated by four cysteinyl residues. The Fds from three hyperthermophiles, the bacterium Thermotoga maritima , Tm , which thrives optimally at 80°, and the archaea Thermococcus litoralis , Tl , and Pyrococcus furiosus , Pf , which thrive optimally at 90° and 100°, respectively, have been investigated by nuclear magnetic resonance (NMR). In each case, the purified Fds have been found to be hyperthermostable, exhibiting undetectable denaturation after 24 hr at 95°. NMR can provide a detailed molecular model (structure) for the complete protein comparable to that obtained by X-ray crystallography and can serve as a unique probe of important aspects of the electronic structure and magnetic properties of the invariably paramagnetic cluster, 16-18 as well as allow the characterizafion of a variety of relevant dynamic properties of either the cluster or the protein as a whole. Hyperthermostable Fds do not pose any experimental problems to the profitable application of NMR to study molecular and electronic structure of dynamic properties. Rather, they provide significant advantages in that molecular and electronic structures such that it can be studied over an extremely wide temperature range and in the presence of organic solvents that generally denature Fds from mesophiles. This wide temperature range, moreover, broadens the range of rate processes that can be studied and facilitates more quantitative description of molecular conformational equilibria. The wide temperature range provides exceptional opportunities for determining if, and how, molecular structure differs at the temperature extremes.