Mossbauer Study of Cys56Ser Mutant 2Fe Ferredoxin from Clostridium Pasteurianum: Evidence f6r Double Exchange in an [Fe_2S_2]~+ Cluster

摘要

Ferredoxins containing Fe_2S_2(cys)_4 clusters have been studied intensively for the past 30 years. The iron--sulfur cluster of these election transport agents undergoes redbx transitions between an Fe~(3+)Fe~(3+) form and a mixed-valence Fe~(2+)Fe~(3+) state. In the latter state, designated [Fe_2S_2]~+, the local spins (S_1 = 2, S_2 =5/2) are coupled by strong antifenomagnetic Heisenberg--Dirac-van Vleck (HDvV) exchange (> 150 cm~(-1) S_1 centre dot S_2) to yield a cluster ground state with S = 1/2.Mossbauer spectroscopy has revealed two distinct, valence-trapped Fe~(2+) and Fe~(3+) sites. The parameters listed in Table 1 are typical for such sites. In contrast, [FesSJ0 and [Fe_4S_4]~(+,2+,3+) clusters contain delocatized Fe~(2.5+)Fe~(2.5+) pairs. In addition to HDvV exchange, the sites of the delocalized pairs are coupled by double exchange (spindependent resonance delocalization).