NiFe oxidation state

Figure 5.6 Activities and states of the [NiFe] centre and iron-sulfur clusters in the [NiFe] hydrogenase of D. gigas. Higher oxidation states are at the top, lower at the bottom.

Nickel-iron hydrogenases [NiFe] (Figure 8.2) are present in several bacteria. Their structure is known [22, 23] to be a heterodimeric protein formed by four subunits, three of which are small [Fe] and one contains the bimetallic active center consisting of a dimeric cluster formed by a six coordinated Fe linked to a pentacoordinated Ni (III) through two cysteine-S and a third ligand whose nature changes with the oxidation state of the metals in the reduced state it is a hydride, H, whereas in the oxidized state it may be either an oxo, 0, or a sulfide,... 

As ascribed, the EPR spectrum with g = 2.10 can be low-spin Fec(III). When the isolated enzyme is reductively titrated this signal disappears at a potential Emj -0.3 V [65]. This would seem to indicate that the putative Fec(III) form is not relevant, at least not to hydrogen-production activity. The cubane is a one-electron acceptor as it can shuttle between the 2+ and 1 + oxidation states. Therefore, if the active center were to take up a total of two electrons, then the oxidation state of the Fec would, as least formally, shuttle between II and I. Recently, a redox transition in Fe hydrogenase with an Em below the H2/H+ potential has been observed in direct electrochemistry [89]. This superreduced state has not been studied by spectroscopy. It might well correspond to the formal Fec(I) state. For NiFe hydrogenases Fec(I) has recently been proposed as a key intermediate in the catalytic cycle [90] (cf. Chapter 9). 

Fig. 2. Molecular hydrogen complex (structure 2) in [NiFe] hydrogenase. The oxidation states are Ni(II) and Fe(II). Distances are given in A. Atoms marked with were kept frozen from the X-ray structure during the optimization.

Volbeda A, Martin L, Cavazza C, et al. Structural difference between the ready and unready oxidized states of [NiFe] hydrogenases. J Biol Inorg Chem. 2005 10(5) 239 49. 

The nickel site has been proposed to be redox active and changes between Ni(III) and Ni(II), while the iron site remains as Fe(II) in all spectrally defined redox states of the enzyme. " The EXAFS/EPR studies indicate that the formal oxidation state of the Ni center is paramagnetic Ni(III) in Ni-A, Ni B and Ni-C states. Actually, the active form Ni-C (the paramagnetic Ni-C intermediate) of [NiFe] H2ase was proposed to exist as the [(Scys-H)Ni -H -Fe] intermediates after an active state Ni Sla (silent-active [(Scys-H)Ni (Scys)3]) is passed. Ni-C is beheved to be an intermediate in the catalytic cycle. Upon illumination. 

The metallocyanates are a large class of compounds, the most widely studied of which are probably Prussian Blue (Na[FeFe(CN)fi]) in its various oxidation states and nickel hexacyanoferrate (Na2[NiFe(CN)6]). These materials can be prepared chemically or electrochemically as thin films on surfaces [200]. They differ from redox polymers in that, because of their lattice structure, there is significant interaction between the redox sites within the film and there is strong size-charge selectivity towards incorporated counter-ions. This arises because the counter-ions have to be accommodated within the lattice structure and this... 

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