Electronic isomerism in oxidized high-potential iron-sulfur proteins revisited

The pair-of-pairs model that successfully explained the Mossbauer and NMR data on oxidized High Potential Iron-sulfur Proteins (HiPIP) in terms of equilibria between two electronic isomers is reviewed. A bridging of the high temperature NMR data with the low temperature EPR data is attempted through extrapolation to low temperature of the species distribution detected at room temperature by NMR. This extrapolation is validated by recent H-1 ENDOR data. With one exception, the more abundant isomer is the one with the mixed-valence iron pair localized either on irons II-III or on irons III-IV. It is suggested that one or two more isomers besides these two, such as I-III or II-IV, may be present in smaller amount.