Fe-S clusters

Biological Fe-S clusters come in many sizes and flavours:

2Fe-2S clusters ligated by four cysteines

2Fe-2S clusters ligated by three cysteines and one aspartate

2Fe-2S clusters ligated by cysteines and histidines (the so-called Rieske clusters)

3Fe-4S clusters ligated by three cysteines

4Fe-4S clusters ligated by four cysteines

4Fe-4S clusters ligated by three cysteines and one aspartate

the hideously complex cluster present in hybrid cluster protein (also known as fuscoredoxin or "prismane protein")

the P-cluster in nitrogenase

etc., etc., etc.
The large number of electrons in Fe and the complexity of the possible couplings between spin states make the theoretical analysis of the electronic structures in Fe-S clusters quite difficult.
Takano et al. have recently published a paper on the differences between a Cys₃Asp ligated 4Fe-4S cluster and the "regular" (all Cys) 4Fe-4S cluster. The authors nicely analyze the influence of the Asp (and other) ligands on the electronic structure of the 4Fe-4S cluster, observe a -0.10 V difference in redox potential (vs. normal 4Fe-4S) in high dielectric constants, and offer this observation as the reason for the low potential of this cluster.
I do not accept this last conclusion for two reasons:

redox potentials of Cys-ligated 4Fe-4S clusters may differ by >0.4 V from each other, which shows that the influence of the charge distribution of the protein is much more important than the small difference observed by the authors

the 0.1 V difference found amounts to ca. 2.3 kcal/mol, which is well within the error range of the computational methods used.