22 INORGANIC CHEMISTRY ESSENTIALS
do not indicate Fe – Fe bonds. It is known that for the Fe 4 S 4 cubane found in
biological systems, oxidations are accompanied by increasing distortion of the
cubane frame. Nevertheless,^57 Fe M ö ssbauer spectra indicate that the four iron
atoms remain equivalent, suggesting delocalization within the Fe – S frame-
work. Most biological iron – sulfur clusters deviate substantially from the
electron - counting rules for iron – sulfur clusters discussed here.
In this text, iron – sulfur clusters are discussed because they appear in pro-
teins and enzymes: (1) cytochrome b(6)f, Rieske [2Fe – 2S] cluster (Section 7.5
and Figure 7.26 ); (2) cytochrome bc 1 , Rieske [2Fe – 2S] cluster (Section 7.6 and
Figure 7.30 ); and (3) aconitase, [4Fe – 4S] cluster (Section 7.9.2.1 , and Figure
7.50 ). The iron – sulfur protein (ISP) component of the cytochrome b(6)f and
cytochrome bc 1 complexes, now called the “ Rieske ” ISP, was fi rst discovered
and isolated by John S. Rieske and co - workers in 1964 (in the cytochrome bc 1
complex). More information about the RISP is found in Section 7.5.1. Section
7.9.2 briefl y discusses other proteins with iron – sulfur clusters — rubredoxins,
ferrodoxins, and the enzyme nitrogenase. The nitrogenase enzyme was the
subject of Chapter 6 in the fi rst edition of this text — see especially the fi rst
edition ’ s Section 6.3 for a discussion of iron – sulfur clusters. In this second
edition, information on iron – sulfur clusters in nitrogenase is found in Section
3.6.4. See Table 3.2 and the descriptive examples discussed in Section 3.6.4.
Many systems in organometallic chemistry involve the activation of hard -
to - oxidize alkanes or other organic moieties. In this regard, several metallo-
enzymes discussed in this text are effi cient alkane oxidizers. Cytochrome P450
(Section 7.4 ) enzymes are monooxygenases that insert one atom of a dioxygen
molecule into a wide variety of organic substrates. The other oxygen atom of
the dioxygen molecule is converted to water during the P450 catalytic cycle.
Methane monooxygenase (Section 7.9.3.1) catalyzes the conversion of the
hardest - of - all - to - oxidize alkanes, methane, CH 4 , to CH 3 OH. Biomimetic bioin-
organic researchers model the active centers of these metalloenzymes as small
molecules, to learn more about the metalloenzyme ’ s catalytic cycle, or to
design effi cient organometallic catalysts for industrial processes.
Researchers studying the metalloenzyme hydrogenase would like to design
small compounds that mimic this enzyme ’ s ability to reversibly reduce protons
to H 2 and H 2 to 2H + , using an active center that contains iron and nickel.
Cobalamins (vitamin B 12 and its derivatives) contain an easily activated
Co – C bond that has a number of biological functions, one of which is as a
methyl transferase, 5 - methyltetrahydrofolate - homocysteine methyltransferase
(MTR). This enzyme converts homocysteine (an amino acid that has one more
CH 2 group in its alkyl side chain than cysteine; see Figure 2.2 ) to methionine
as methylcobalamin is converted to cobalamin.
1.8 Electron Transfer,
Many reactions catalyzed by metalloenzymes involve electron transfer. On the
simplest level, one can consider electron transfer reactions to be complemen-