Large Molecules

222 Pages·1974·10.255 MB·English

S T R U C T U R E
A N D B O N D I N G
Volume 18
Editors: J. D. Dunitz, Ztirich
P. Hemmerich, Konstanz • R. H. Holm, Cambridge
J. A. Ibers, Evanston • .C K. Jorgensen, Gen~ve
J. B. Neilands, Berkeley • D. Reinen, Marburg
R. J. P. Williams, Oxford
With 43 Figures
/
Springer-Verlag
New York" Heidelberg" Berlin 1974
ISBN 0-387-06658-6 Springer-Verlag New York • Heidelberg • Berlin
ISBN 3-540-06658-6 Springer-Verlag Berlin • Heidelberg • New York
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stnetnoC
The Oxidation States and Reversible Redox Reactions of Metal-
loporphyrins. J.-H. Fuhrhop ............................... 1
Optical Activity of Conjugated Proteins. G. Blauer ........... 69
Some Aspects of the Heteropolymolybdates and Heteropolytung-
states. T. J. R. Weakley .................................. 131
Hydrogen Bonding in Solids. Correlation of Spectroscopic and
Cristallographic Data. A. Novak ........................... 177
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Professor Dr. J. D. Dunitz Laboratorium fiir Organische Chemie der Eid-
gen6ssischen Hochschule
CH-8006 Ztirich, elfartssti~tisrevinU 6/8
Professor Dr. P. Hemmerich ti~tisrevinU Konstanz, Fachbereich Biologie
D-7750 Konstanz, Postfach 337
Professor R. H. Holm Department of Chemistry, Massachusetts Institute
of Technology
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Professor J. A. Ibers Department of Chemistry, Northwestern University
Evanston, Illinois 60201/USA
Professor Dr. C. K. nesnegraJ ,15 Route de Frontenex,
CH-1207 Gen6ve
Professor J. B. sdnalieV1 University of California, Biochemistry Department
California Berkeley, 94720/USA
Professor Dr. D. Reinen Institut riR Anorganische Chemie der Universit/it
Marburg
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Professor R.J. P. Williams Wadham College, Inorganic Chemistry Laboratory
Oxford OX1 3QR/Great Britain
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The Oxidation States and Reversible Redox Reactions
of MetaUoporphyrins
.H-.J pohrhuF
Gesellschaft fi~r Molekularbiologisehe Forschung mbH, 3301 StSckheim iiber
Braunschweig, and Institut fi~r Organische Chemic der T.U. Braunschweig,
38 Braunschweig, Germany
Table of Contents
I. Introduction .................................................. 2
II. The Metal-Free Porphyrin Bases, their Protonated Dications and
I)eprotonated Anions ........................................... 3
.1 The Structure of the Porphyrin Ligands ........................ 3
2. The Ampholytic Character of the Porphyrin Ligands ............ 8
3. Electronic Properties of the Porphyrin Ligands ................. 9
4. The Redox Behavior of Free Porphyrin Bases, their N-Protonated
Dications and Deprotonated A~ions ........................... 10
III. Some General Remarks on the Oxidation States of Metalloporphyrins
(Electronic Spectra, esr Spectra, Electrochemical Regularities) ...... 11
IV. The Oxidation States of the Central Metal Ions in Metalloporphyrins 20
.1 Group-IIIa Porphyrins ..................................... 20
2. Group-IVa Porphyrins ...................................... 21
3. Group-Va Porphyrins ....................................... 22
4. Group-Via Porphyrins ...................................... 22
8. Group-VIIa Porphyrins ..................................... 28
6. Group-VIII Porphyrins ..................................... 27
7. Group-Ib Porphyrins ....................................... 39
8. Group-Ia, -IIa, -IIb, and -IIIb Porphyrins ................... 39
9. Group-IVb Porphyrins ...................................... 40
10. Group-Vb Porphyrins ...................................... 41
11, Summary ................................................. 41
V. The Oxidation States of the Porphyrin Ligands .................... 43
.1 General Description ......................................... 43
2. z-Cation Radicals and ~ Dications ............................. 48
3. z-Anion Radicals and ~ Dianions .............................. 52
4. Reversible Hydrogen Additions to Reduced Porphyrins .......... 53
5. Summary .................................................. 58
VI. Redox Reactions of Metalloporphyrins in Biological Systems ........ 5S
1. Cytochromes ............................................... SS
J.-H. Fuhrhop
2. Heine proteins which bind andor activate Molecular Oxygen ..... 57
3. Catalases and Peroxidases .................................... 89
4. Chlorophyll ~-Cation Radicals in Photosynthesis ................ 60
VII. Conclusion .................................................... 16
VIII. References .................................................... 62
"The apartments were so irregularly disposed that the vision embraced but
little more than one at a time. There was a sharp turn at every twenty or
thirty yards, and at each turn a novel egect ... The second chamber was
purple in its ornaments and tapestries, and here the panes were purple.
The third was green throughout, and so were the casements."
E. A. Poe, The Masque o the Red Death
I. Introduction )*
Redox reactions of metalloporphyrins usually can be followed visually:
an electroneutral metalloporphyrin gives purple solutions, reductions or
oxidation almost invariably change this color to green or brownish-
green. The physical, chemical and biological applications of these re-
actions are very numerous and many natural scientists have had a part
in the task of illuminating the purple and green chambers of nature as
well as the many corridors connecting them from various viewpoints.
The classification of the redox reactions for metalloporphyrins
includes four main types, according to the occurrence of:
.1 Reversible changes in the formal oxidation number of the metal
("inorganic" redox reactions)
2. Reversible changes of the oxidation state of the porphyrin
("organic" reversible redox reactions)
3. Reversible or irreversible redox reactions of the central metal ion
with axial ligands, in particular 02.
)1 Conventions used in this article: Nomenclature of porphyrins is explained in
Section II1. Redox potentials are always given versus aq. SCE; electronic spectra
are given by the wavelength of the peaks in nm and their extinction coefficients,
if available. Generally, only data which are not in Falk's book (56) are given.
Solvents and electrolytes used in spectroscopy and electrochemistry are mention-
ed only occasionally, Metalloporphyrin spectra are usually run in chloroform
solution, oxidation potentials are obtained in butyronitrile, and reduction po-
tentials in DMSO. Only the first author's name is given in the references in the
text.
ehT Oxidation States and Reversible Redox Reactions of sniryhpropollateM
4. Reversible or irreversible redox reactions of the porphyrin ligand,
in particular addition of 02 or H2.
The biological activities of metalloporphyrins can be classified sim-
ilarly:
.1 Electron transport is by virtue of a reversible valency change of
the "inorganic" heine iron the main biological function of the cyto-
chromes in various biological redox reactions (e.g. respiration, photo-
synthesis). The porphyrin ligand can always participate in the se-
quence of reactions.
2. The chlorophylls contain redox inert magnesium as the central ion
and form "organic" a-cation radicals on oxidation. Such radicals play a
prominent role in photosynthesis and thebiosynthesis of protochlorophyll.
3. Some hemoproteins function in the transport and activation of
molecular oxygen as well as in the decomposition of hydrogen peroxide
(e.g. hemoglobin, tryptophan dioxygenase, cytochrome P450, catalases).
4. The metabolism of naturally occurring tetrapyrrole metal com-
plexes includes irreversible oxygenation and hydrogenation reactions
(bile pigment formation, protochlorophyll hydrogenation).
This review deals mainly with the chemical and physical aspects of
the thermodynamically reversible redox reactions of porphyrins.
The relevant biochemical work will be referred to only briefly,
although it should be kept in mind that many very detailed physical
and chemical studies have been triggered by curious phenomena observed
in biological systems containing hemoproteins or chlorophylls. It was
the common interest of physicists, chemists, biologists, and physicians
in the properties of metalloporphyrins, that led to the emergence of their
redox chemistry as it is known today, and it is one of the purposes of
this article to catalog the different approaches and techniques which
have been used to illuminate these red and green chambers of the
natural sciences within which so many scientific disciplines have some
laboratory space. Early work on the redox chemistry of iron porphyrins
has been covered admirably in at least three books Clark (33), Falk (56),
Lemberg, Legge (121)1 and will be mentioned only briefly.
II. The Metal-Free Porphyrin Bases, their N-Protonated
Dications and Deprotonated Anions
.1 The Structure of the Porphyrin Ligands
The best known porphyrin ligand is certainly protoporphyrin, which
in the form of one of its derivatives (Ia--d) is frequently used for redox
experiments with metalloporphyrins.
J.-H. Fuhrhop
Analogous properties are possessed by the synthetic porphyrins, like
octaethylporphyrin (If, (Ig, ETI0);
OEP), and etioporphyrin their
properties help to avoid complications due to the reactive vinyl groups
of PROTO and its derivatives or the formation of isomer mixtures in the
case of reactions on these porphyrin macrocyclic ligands. All these
Iigands will be treated together as ETIO-type porphyrins in this review.
The chemical properties of the unsubstituted porphin ligand (Ie, POR)
are not known in detail. The methine bridges are also called mesoposi-
tions, and the peripheral carbons that bear the substituents are
named fl-pyrrolic positions.
,~,~R ~ 3R
R7 7 6R
I
Table .1 Important porphyrins of the ETIO-type (/~-pyrrolic alkyl substituents,
no mesosubstituents)
Name of Abbreviation Substituents in the various fl-pyrrolic
compound positions
Protoporphyrin- 1 2 3 4 5 6 7 8
-dimethylester Ia; PROTO Me V Me V Me P P Me
Deuteroporphy-
rin-dimethyl-
ester Ib; DEUTERO Me H Me H Me P P Me
Mesoporphyrin-
-dimethylester Ic; MESO Me Et Me Et Me P P Me
Haematoporphy- CHOH CHOH
rin-dimethyl- I d; HAEMATO Me Me Me P P Me
ester CH3 CH~
Porphin Ie; POR H H H H H H H H
Octaethyl-
porphyrin If; OEP Et Et Et Et Et Et Et Et
Etioporphyrin I Ig; ETIO*) Me Et Me Et Me Et Me Et
)* Only the ETIO I structure is given; the three other isomers presumably have
identical redox chemistry.
P = propionic acid methylester
V = vinyl
ehT Oxidation States dna elbisreveR Redox snoitcaeR fo sniryhpropollateM
R
R R
R
II
Another class of synthetic porphyrins comprises the meso-tetra-
phenylporphyrins, which can be easily prepared in lane amounts, and
which have some unusual properties, not found in natural porphyrins.
0nly the parent compound tetraphenylporphyrin (II R=H, TPP)
with unsubstituted benzene rings is discussed here.
~/ 0
eMOOC
III
H H
VI
The second major class of metalloporphyrins besides the hemes are
the chlorophylls, with chlorophyll a (Chla) and bacteriochlorophyll
B( Chl) as its main representatives. These compounds contain magnesium
J.-H. pohrhuF
as the central ion, one meso-substituent in the form of the isocyclic
ring and, as a further complication, one or two reduced pyrrole units.
Porphyrins with one hydrogenated pyrrole unit are called chlorins
(e.g. octaethylchlorin, OEC), and those with two opposite hydrogenated
rings bacteriochlorins (e.g. Tetraphenylbacteriochlorin, TPBChl).
Ligand III is called methylphaeophorbide a (Pheoa). Its magnesium
complex is usually chosen for in-vitro redox experiments and differs
from the porphyrin ligand of Chla only by the esterifying alcohol on one
proponic acid side chain (methanol instead of phytol). The phlorins (IV)
constitute another important type of dihydroporphyrins. Here one
hydrogen atom is added to a methine bridge of a porphyrin and another
hydrogen atom is added to a nitrogen.
v
Finally some remarks will be included on the redox properties of
metal phthalocyanines V( Pht) because a comparison with the metallo-
porphyrins yields some information as to how the different metal oxi-
dation states are stabilized in the porphyrin cavity. Closely related
porphyrins are the tetrabenzoporphyrins and the ms-tetraaza-porphy-
rins, which produce absorption spectra similar to those of phthalocyani-
nes.
All of the above ligands (H2P) contain on the central nitrogens
two extremely weakly acidic protons, which can be substituted by two
monovalent metal ions (M2I--P) or one divalent metal ion (MII--P). If
the central ion has an oxidation state of +3, an extra monovalent
anion )X( is added to it in a plane perpendicular to the porphyrin plane
(MIIIX--P), if the oxidation state is +4, two extra monovalent anions
join the metal on both sides of the porphyrin plane (MIVX2--P), or there
is one divalent anion, usually oxygen, e.g. (MolVO--P). Very often the
coordination sites 5 and 6 of bivalent central ions are occupied by rather
firmly bound neutral extra ligands. Familiar examples are ,OC H202
and 02 in iron, 02 in cobalt, pyridine and water in almost all metal
complexes of the porphyrins.
Some iron porphyrins have trivial names, defined for brevity as
follows: heroes (or haems) are iron II porphyrins (Fell--P) ; hemochromes