Synthesis and characterization of a new family of iron porphyrins

Article abstract of DOI:10.2533/000942905777676759

A significant tool for better understanding the complex nature of the cofactor of heme thiolate proteins such as Cytochromes P450 is the investigation of model compounds. In this context a new family of iron porphyrins has been synthesized by replacing th

Full text of DOI:10.2533/000942905777676759

LAUREATES: AWARDS AND HONORS SCS FALL MEETING 2004
85
CHIMIA 2005, 59, No. 3
Chimia 59 (2005) 85–87
© Schweizerische Chemische Gesellschaft
ISSN 0009–4293
Synthesis and Characterization of a New
Family of Iron Porphyrins
Dominik N. Meyer§ and Wolf-D. Woggon*
§Mettler Toledo Award Winner (Oral Presentation)
Abstract: A significant tool for better understanding the complex nature of the cofactor of heme thiolate proteins
such as Cytochromes P450 is the investigation of model compounds. In this context a new family of iron porphyrins
has been synthesized by replacing the native thiolate ligand for a SO3– group coordinating the heme iron.
Keywords: Cytochrome P450 · Enzyme models · Iron porphyrins · Monooxygenases · Redox potential
1. Introduction
a set of new enzyme models was conceived converted to 9 to increase the yield (F).
carrying a SO3– group as the fifth ligand.
Final iron insertion gave model compound
10. A 2,6-dichlorophenyl-meso-substituted
model 11 was synthesized in a similar fash-
ion starting from 2,6-dichlorobenzaldehyde
(12) instead of mesitylaldehyde (1).
Cytochromes P450 are heme-thiolate pro-
teins abundant in nature. These mono-oxy-
genases catalyze diverse reactions signifi-
cant to the metabolism of xenobiotics as
well as to the biosynthesis of important
biomolecules [1].
Earlier investigations on iron porphyrin
active site analogues carrying a thiolate as
the fifth ligand (Fe(III)…S–) [2] revealed a
rather negative Eo < –600 mV (vs. SCE) in
contrast to e.g. P450cam, one of the best-
known P450s, displaying Eo = –280 mV for
the resting state. From more recent X-ray
studies of P450cam it could be deduced that
this difference is due to H-bonding of the
thiolate ligand to amino acid residues of the
protein backbone. Taken into account this
obviously reduced charge density at sulfur
2. Strategy
–
DFT calculations on SO3 coordinated
iron porphyrins [3] supported our idea that
one of the oxygens of the SO3– indeed co-
ordinates to iron donating a charge of 0.3
instead of 1.0 for S–. Energy-profile calcu-
lations further assigned the reactivity of the
SO3– system to be very similar to the Fe…
S–- coordination. To improve the stability
of the model compounds aromatic substitu-
ents were introduced at the oxygen-sensi-
tive meso-positions to prevent μ-oxo dimer
formation through steric congestion.
4. Characterization
The X-ray structure of 10 (Fig. 1)
validates the synthetic procedures and
the assumption of one of the oxygens of
–
the SO3 group coordinating to iron. The
analysis further shows a slightly strained
system with the iron out of plane towards
the fifth ligand in agreement with the EPR
spectrum displaying g-values characteristic
of a high-spin FeIII system (toluene, 94 K,
g-factor: 5.7).
3. Synthesis
The UV-Vis spectrum of 10 exhibits
typical iron porphyrin absorptions (CH2Cl2
λmax : 415 nm (100, Soret) and 511 nm (12),
580 nm (3) , 691 nm (2) (Q-bands)).
The synthetic pathway is outlined in the
Scheme. From mesitylaldehyde (1) on reac-
tion with pyrrol (2) a light sensitive dipyrro-
methane 3 was obtained, which underwent
cyclization with 2-methoxy-benzaldehyde
(4) to form the properly substituted por-
phyrin ring structure 5 following standard
procedures [4]. The latter was deprotected
to obtain the free phenol 6 as a mixture of
atropisomers (α,α-6 and α,β-6) that inter-
convert at room temperature. Condensa-
tion under diluted conditions with the S-
protected ‘bridge’ 7, which had been pre-
pared according to our own protocol, gave
product 8. On treatment with a strong base
under oxygen-saturated conditions 8 was
converted to 9. Intermediates that were not
Cyclovoltammetry (Fig. 2) reveals re-
dox potentials of 10 and 11 similar to the
resting state of P450 enzymes (Table). This
underlines their value as model compounds
in this field. A further advantage of iron
–
complexes with SO3 is the stability rela-
tive to S–-coordination under aerobic con-
ditions, which makes their handling much
more convenient.
*Correspondence: Prof. Dr. W.-D. Woggon
University of Basel
Department of Chemistry
St. Johanns-Ring 19
CH–4056 Basel
Tel.: +41 61 267 1102
Fax: + 41 61 267 1109
E-Mail: wolf-d.woggon@unibas.ch
www.chemie.unibas.ch/~woggon/
5. Reactivity
From spectroscopic data [1] the domi-
nant reactive oxidant in the natural system
is claimed to be a FeIV-porphyrin radical
cation (Cpd I) which is formed from the
–
oxidized completely to SO3 under these
conditions were collected and separately

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CHIMIA 2005, 59, No. 3
Fig. 1. ORTEP representation of model compound 10
Fig. 2. Cyclovoltammogram of 10 (0.6 mM) in 0.1M LiClO4 soln. in DMF with
ferrocene as an internal standard. Scan rate: 100 mVs–1
Table. Redox potentials of two model compounds measured by
cyclovoltammetry. The given values (vs. SCE) are calculated from the
relative potential to ferrocene used as an internal standard.
Scheme. Key: A: 0.3 equiv. BF3OEt2, 1 h, RT, 30%; B: 1.8 equiv. TFA,
CH2Cl2, 0.5 h, RT, then 2 equiv. DDQ, 1 h, reflux, 27%; C: 32 equiv. BBr3,
CH2Cl2, 16 h, RT, 79%; D: 30 equiv. Cs2CO3, DMF, 0.5 h, 80 ºC then 1.5
equiv. 7, 4 h, 80 ºC, 75%; E: 60 equiv. KOMe, dioxane, O2, 16 h, reflux; F:
2 equiv. nBu4NHSO5, CH2Cl2, 2d, RT, E&F: 55%; G: 10 equiv. FeBr2 and
2,6-lutidine, toluene, 1 h, reflux, 86%
1st ox
1st red
2nd red
3rd red
10
11
920 mV
1010 mV
–340 mV
–280 mV
–1480 mV
–1420 mV
–1970 mV
–1900 mV
6. Outlook
on enzymatic reactions such as epoxida-
tion [3], oxidation of non-activated posi-
resting state after substrate binding, reduc-
tion, oxygen binding and reductive oxygen
cleavage [5].
The O=Fe(IV) porph.+ species (Cpd I)
can be obtained from 10 or 11 on reaction
with oxidants such as mCPBA, PhIO, H2O2
or certain N-oxides [6]. In that way we ob-
tained UV-Vis spectra (Fig. 3) in agreement
with published data for simpler porphyrin
systems in the Cpd I – state [6][7].
The synthesis and characterization of tions and carbon–carbon bond cleavage.
a new family of iron porphyrins has been Preliminary results indicate the capability
accomplished and assigns them promising of our model compounds to cleave vicinal
capacity as P450 enzyme models. These diols to the corresponding aldehydes. This
results are a prerequisite to employ our reaction represents the last step of the C–C
model compounds for further enzyme-mi- bond cleavage in the biotransformation of
metic studies which are currently under cholesterol to pregnenolone by P450scc
investigation. Therein our interest focuses (CYP 11A1) in the mammalian steroid hor-

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CHIMIA 2005, 59, No. 3
Wavelength [nm]
Fig. 3. UV-Vis change after addition of 1.5 equiv. of mCPBA (10 µM, CH2Cl2, –50 ºC), dotted line:
spectrum 30 sec after addition, full line: 25 min after addition
[3] S. Kozuch, T. Leifels, D. Meyer, L. Sba-
ragli, S. Shaik, W.-D. Woggon, Synlett
2005, in press.
[4] B. Stäubli, H. Fretz, U. Piantini, W.-D.
Woggon, Helv. Chim. Acta 1987, 70,
1173.
mone biosynthesis [8]. The same reaction
sequence is also claimed to be part of other
biotransformation processes e.g. in the bio-
tin biosynthesis in bacillus subtilis [9].
Acknowledgements
[5] W.-D. Woggon, Top. Curr. Chem. 1997,
184, 39.
[6] R. Weiss, A. Gold, A.X. Trautwein, J. Ter-
ner in ‘the Porphyrin Handbook’, Volume
4, Eds. K. M. Kadish, K. M. Smith, R.
Guilard, Academic Press, 2000.
The authors thank M. Neuburger for crystal
structure analysis of 10, L. Sbaragli for synthesis
of 11 and the Swiss National Science Foundation
for financial support.
Received: December 13, 2004
[7] J.T. Groves, Y. Watanabe, J. Am. Chem.
Soc. 1988, 110, 8443.
[8] S. Lieberman, Y. Lin, J. Steroid Biochem.
Mol. Biol. 2001, 78, 1.
[9] J. Stok, J. De Voss, Arch. Biochem. Bio-
phys. 2000, 384, 351.
[1] P.R. Ortiz de Montellano, in ‘Cytochrome
P450: Structure, Mechanism and Bioche-
mistry’, 2nd ed., Ed. P.R. Ortiz de Montel-
lano, Plenum Press, New York, 1995.
[2] W.-D. Woggon, Chimia 2001, 55, 366.