82 INSTRUMENTAL METHODS
and Cu – S shell is necessary to obtain an adequate fi t to the EXAFS data.
Therefore it was concluded that a Cu – S bond is present in the compound.
Chapter 7 discusses the ubiquitous cytochrome proteins. Many of these
proteins and their model compounds have been studied using EXAFS. For
instance, the cytochrome P450 enzyme, one of a large group of enzymes that
utilize atomospheric oxygen to functionalize biological molecules, exhibits a
very short Fe – O bond at one point during its catalytic cycle. This has been
assigned as a ferryl(oxo), Fe(IV) = O, bond using EXAFS spectroscopy. EXAFS
data indicated an Fe = O bond length of 1.65 ± 0.05 Å , similar to that found for
the so - called “ compound I ” intermediates in many iron - containing enzymes
such as hemoglobin, myoglobin, catalases, horseradish peroxidase, cytochrome
c peroxidase, and chloroperoxidases.^6 Many researchers have sought to iden-
tify, characterize, and synthesize model compounds corresponding to the oxi-
dizing species called compounds I and II that occur in iron - containing
metalloenzymes. The compounds I and II intermediates are known to corre-
spond to formal iron - oxidation states two and one above the resting iron(III)
state. One sought - after model compound for cytochrome P450 would feature
a Fe(IV) = O state with a sulfur thiolate ligand attached directly opposite the
oxo ligand. In this regard, Eckard M ü nck and Larry Que have recently pub-
lished a report on the synthesis and characterization of [Fe IV (O)(TMCS)] + ,
where TMCS is a pendadentate ligand, the monoanion of 1 - mercaptoethyl -
4,8,11 - trimethyl - 1,4,8,11 - tetraaza cyclotetradecane.^7 Figure 3 of reference 7
shows the EXAFS spectrum for the model compound discussed. The ligand
provides a square pyramidal, non - heme, (N 4 SR) apical ligand set that is similar
to that in cytochrome P450 (see Figure 7.23 ). Extended X - ray absorption fi ne
structure (EXAFS) indicates one oxygen atom at 1.70(2) Å , three nitrogen/
oxygen atoms at 2.09(2) Å , one sulfur atom at 2.33(2) Å , and four carbons
at 2.95(2) Å. The supposed Fe = O bond distance is slightly longer than that
found for other characterized Fe = O species — usually found at 1.65 – 1.67 Å. A
detailed discussion of cytochrome P450 and model compounds synthesized to
mimic its structure or function is found in Section 7.4.
Cytochrome c and ubiquinol oxidases are part of an enzyme superfamily
coupling oxidation of ferrocytochrome c (in eukaryotes) and ubiquinol (in
prokaryotes) to the 4 e − /4 H + reduction of molecular oxygen. At points in the
cytochrome c oxidase catalytic cycle, dioxygen (O 2 ), superoxide ( O 2 −) or
peroxide ( O 22 −) ions coordinate to Fe(II/III) and Cu(I/II) metal ions within
the metalloenzyme. Kenneth Karlin, E. I. Solomon, and co - workers have
studied cytochrome c oxidase biomimetic compounds — model compounds
that mimic the structure or function of the parent protein or enzyme. In one
example, the group studied the geometric and electronic structure of
( 82 ) III−−(^2 −) II( ) F TPP Fe. This complex is prepared by the O Cu TMPA ClO
reaction of a [(TMPA)Cu(I)(RCN)] + complex with a [(F 8 TPP)Fe(II)] complex.
The TMPA ligand is tris (2 - pyridylmethyl)amine, RCN = MeCN = acetonitrile,
and F 8 TPP = tetrakis (2,6 - difl uorophenyl)porphyrinate ( − 2 charge). The model
complex was characterized using Cu and Fe K - edge EXAFS spectroscopy and