aqueous peroxidase and polymer solutions. Whilst horse radish
peroxidase causes faster oxidation of ABTS, the biomimetic
porphyrin–polymer system is 70% as fast; in the absence of
either the authentic or artificial peroxidase, oxidation of ABTS
is not rapidly destroyed through auto-oxidation. In other work to
be reported elsewhere, it is shown that a catalytic system, in
which a porphyrin is covalently bonded between naphthalene
residues in an analogue of PSSS-VN polymers, also behaves
similarly and that even very simple sandwich molecules having
a porphyrin covalently bonded between aromatic residues
mimic the action of a peroxidase. It is concluded that porphyrins
held in PSSS-VN amphiphilic polymers mimic peroxidase
enzymic activity and do not behave like most other model
porphyrin enzyme mimics, which resemble P450 cyto-
chromes.
with H
horse radish peroxidase and the PSS-VN–porphyrin system as
catalysts in the H oxidation of guaiacol and mesidine
2 2
O is very slow. Similarly, the comparative effects of
2 2
O
showed that the authentic peroxidase and its mimic gave
comparable rates of reaction and both exhibited a greatly
increased rate compared with that for reaction with H O alone.
2 2
When the amphiphilic PSSS-VN polymer was replaced by the
more hydrophilic PSSS-VP, the normally water-insoluble
porphyrins dissolved as usual but the peroxidase-like activity
was greatly reduced. It is notable that, in all these oxidations, the
simple water-insoluble porphyrins such as manganese(iii)
The authors thank the Eschenmoser Trust (UK) and Procter
and Gamble (UK) for financial assistance (PAS).
Footnote and References
5
,10,15,20-tetrakis(4-methoxyphenyl)porphyrin, when solubi-
lised in water containing the amphiphilic polymer, were
destroyed only very slowly as compared with the rapidity of
auto-oxidation of similar water-soluble porphyrins in the
absence of polymer. This stability suggests that the water-
insoluble porphyrin molecules in the inner hydrophobic sphere
of the PSSS-VN polymer must be isolated from each other so as
to prevent mutual oxidation; the behaviour is characteristic of
natural peroxidases.
* E-mail: rj05@liv.ac.uk
1
See for example (a) R. A. Sheldon and J. K. Kochi, Metal-Catalysed
Oxidations of Organic Compounds, Academic Press, New York, 1981,
pp. 216–268; (b) J. Frew and P. Jones, Adv. Inorg. Bioinorg.
Mechanisms, 1984, 3, 3175.
2 For examples, see T. McMurry and J. T. Groves, Cytochrome P450,
Structure, Mechanism and Biochemistry, ed. P. Ortiz de Montellano,
Plenum, London, 1986.
The substrates to be oxidised (ABTS, guaiacol and mesidine)
are soluble in water. For horse radish peroxidase, their approach
to the metal centre in the porphyrin is prevented by the protein
coat. For the PSSS-VN–porphyrin system it is probable that the
porphyrin is not only held in the hydrophobic core of the PSSS-
VN polymer in water but is isolated by intercalation between the
many naphthalene residues in such a way that the oxidised metal
centre is not open to approach by the substrate. Aggregation of
3
For some examples, see D. Mansuy, M. Fontecave and J.-F. Bartoli,
J. Chem. Soc., Chem. Commun., 1983, 253; K. Suslick and B. R. Cook,
J. Chem. Soc., Chem. Commun., 1987, 200; J. R. Lindsay-Smith and
P. Sleath, J. Chem. Soc., Perkin Trans 2, 1983, 1991; A. M. d’A Rocha
Gonsalves, R. A. W. Johnstone, M. M. Pereira, J. Shaw and
A. J. F. N. Sobral, Tetrahedron Lett., 1991, 32, 1355.
4
S. Aibara, T. Kobayashi and Y. Morita, J. Biochem., 1981, 90, 489.
5 J. Putter and R. Becker, in Methods of Enzymatic Analysis, ed. H. U.
Bergmeyer, VCH, Weinheim, 1985, vol. 3, p. 286.
6 G. Labat and B. Meunier, J. Chem. Soc., Chem. Commun., 1990, 1414;
N. Colclough and J. R. Lindsay-Smith, J. Chem. Soc., Perkin Trans. 2,
polycyclic aromatic species through p–p interactions are well-
known through UV effects14 and it was found here that, in these
PSSS-VN polymer–porphyrin systems, the Soret band near 420
nm is significantly broadened and reduced in height as
1
994, 1139, and see ref. 1(a) therein.
7
R. A. W. Johnstone, M. L. P. G. Nunes, M. M. Pereira, A. M. d’A Rocha
Gonsalves and A. C. Serra, Heterocyclics, 1996, 43, 1423; A. M. d’A
Rocha Gonsalves, R. A. W. Johnstone, M. M. Pereira, A. M. P. de
SantAna, A. J. F. N. Sobral and P. A. Stocks, Heterocycles, 1996, 43,
8
compared with the free porphyrin. Also, M o¨ ssbauer spectros-
copy of the solid isolated by evaporation of water from a PSSS-
VN polymer–iron porphyrin system revealed a small shift in the
iron peak, suggesting electronic interaction of the porphyrin
8
29.
8
with surrounding groups. Further, the substrates to be oxidised
8 P. A. Stocks, PhD Thesis, University of Liverpool, 1995.
9 R. A. W. Johnstone, P. A. Stocks, F. E. Hardy, J. G. L. Pluyter and
A. J. Simpson, PCT Int. Appl. WO 9 525 267, 8th March, 1994; Chem.
Abstr., 1996, 46, 599.
are water soluble and would not be expected to appear in the
hydrophobic core of the polymer. Thus, for reaction to occur, an
edge or edges of the porphyrin must be available for electron
transfer reactions at the junction of the outer hydrophilic and
inner hydrophobic surfaces of the polymer. If this is the case
then the oxidation reactions catalysed by the PSSS-VN–
porphyrin system would be closely similar to those typical of
natural peroxidases and might be expected to have similar
activity.
1
0 E. Sustar, M. Nowakowska and J. E. Guillet, J. Photochem. Photobiol.,
1
990, 53, 233.
1 For examples, see O. Almarsson and T. C. Bruice, J. Am. Chem. Soc.,
995, 117, 4533; P. Hoffmann, G. Labat, A. Robert and B. Meunier,
1
1
Tetrahedron Lett., 1990, 31, 1991; S. Banfi, F. Montanari, S. Quici,
S. V. Barkanova, O. L. Kaliya, V. N. Kopranenkov and E. A. Luk’ya-
nets, Tetrahedron Lett., 1995, 36, 2317; T.-C. Zheng and D. E. Richard-
son, Tetrahedron Lett., 1995, 36, 837.
One difference between the natural enzymes and the PSSS-
VN system lies in the absence of a formal apical ligand to the
metal centre in the porphyrin in the latter. In horse radish
peroxidase, this function is served by an imidazolyl nitrogen
12 For some examples, see T. C. Bruice, Aldrichim. Acta, 1988, 21, 87;
A. M. d’A Rocha Gonsalves, R. A. W. Johnstone, M. M. Pereira,
J. Shaw and A. J. F. N. Sobral, Tetrahedron Lett., 1991, 32, 1355.
1
3 For examples, see J. Putter and R. Becker, Methods of Enzymatic
Analysis, ed. H. U. Bergmeyer, VCH, Weinheim, 1985, vol. 3,
p. 286.
atom.1b In the PSSS-VN system at pH 8, the only apical ligand
2
is likely to be H
2
O or OH . As catalytic activity in
metalloporphyrins is influenced by the nature of such apical
ligands, the activities of the horse radish peroxidase and the
PSSS-VN–porphyrin mimic might not be expected to be
identical. However, it has been demonstrated for oxidising
systems with simple porphyrins as catalysts that the easily
1
4 For references, see F. Ribo, J. Crustas, J. A. Farrera and M. Valero,
J. Chem. Soc., Chem. Commun., 1994, 681; R. F. Pasternack,
L. Francesconi, D. Raff and E. Spiro, Inorg. Chem., 1973, 12, 2606.
15 (a) A. M. d’A Rocha Gonsalves, M. M. Pereira, R. A. W. Johnstone and
J. Shaw, J. Chem. Soc., Perkin Trans. 1, 1991, 645; (b) A. Thellend,
P. Battioni and D. Mansuy, J. Chem. Soc., Chem. Commun., 1994,
23.
oxidised imidazole ligand can be profitably replaced by other
ligands,1
5a,b
including non-oxidisable inorganic varieties.15a It
is encouraging that the rates of oxidation in the two systems are
closely similar and that the porphyrin in the PSSS-VN polymer
Received in Cambridge, UK, 13th August 1997; 7/05934E
2278
Chem. Commun., 1997