Synthesis, structural characterisation and catalytic testing of bioinspired electron transfer catalysts István Pálinkó Department of Organic Chemistry, University of Szeged, Szeged, Hungary

Main issues of catalysis - activity (important) - selectivity (very important) - benign to the environment (very important)

A potentially successful way of addressing these issues is Bioinspired catalysis

Advantages - high activity - extremely high selectivity Disadvantages - temperature limit - solvent limitation - difficult to recover - extremely high selectivity

Possibilities of keeping the advantages and blocking the disadvantages - immobilising enzymes and immobilising the models on rigid or flexible supports - modelling the active sites of enzymes: functional modelling structural modelling (mimicking the active site) Pálinkó, I., Inorganic Biochemistry: Research Progress (Hughes, J.G., Robinson, A.J., eds.), Nova Science Publishers, Inc., Ch. 8, 2008.

Functional modelling Enzymes to be modelled - superoxide dismutase (SOD enzyme) - catalase - cathecolase The functional model - Cu,Zn complex resembling the active site of Cu,Zn-SOD enzyme Supports - silica gel (SG) - montmorillonite (Mont) Methods of immobilisation - hydrogen bond (adsorption) - ion exchange - covalent grafting

2 O H + = O 2 + H 2 O 2

The complex used as functional model

Catalytic activity

How good the model is? Testing the activity with the Beauchamp-Fridovich reaction – determining the IC 50 values the smaller the values the higher the SOD activity IC 50 (  M) Cu,Zn-SODCu,Zn-complexMont-i-Cu,ZnSG-h-Cu,ZnSG-c-Cu,Zn supports montmorillonite (Mont) silica gel (SG) methods of anchoring Mont - ion exchange (i) SG - covalent grafting (c) SG - hydrogen bonding (h)

The SOD-activities of the enzyme, the bare and the immobilised complexes

Why is SG-h-Cu,Zn complex peculiar?

In SG-h-Cu,Zn complex the geometry around the copper ion is the closest to that in the Cu,Zn-SOD enzyme, which is a distorted tetrahedron. Hernadi, K., Méhn, D. Labádi, I., Pálinkó, I., Bál Gy., Sitkei, E., Kiricsi, I., Stud. Surf. Sci. Catal. 142, 85 (2002). Szilágyi, I., Nagy, G., Hernadi, K., Labádi, I., Pálinkó, I., J. Mol. Struct. THEOCHEM 666/667, 451 (2003). Szilágyi, I., Labádi, I., Hernadi, K., Pálinkó, I., Kiss, T., J. Mol. Struct , 495 (2005). Szilágyi, I., Kele, Z., Labádi, I., Hernadi, K., Pálinkó, I., Kiss, T., Rapid Commun. Mass Spectrom. 19, 2878 (2005). Szilágyi, I., Labádi, I., Hernadi, K., Pálinkó, I., Nagy, N.V., Korecz, L., Rockenbauer, A., Kele, Z., Kiss, T., J. Inorg. Biochem. 99, 1619 (2005). Szilágyi, I., Labádi, I., Hernadi, K., Pálinkó, I., Fekete, I., Korecz, L., Rockenbauer, A., Kiss, T., New J. Chem. 29, 740 (2005).

Selectivity The Cu,Zn complex did not show catalase activity in any forms. The Cu,Zn and its supported forms displayed only moderate cathecolase activity. Szilágyi, I. Horváth, L., Labádi, I., Hernadi, K., Pálinkó, I., Kiss, T., Cent. Eur. J. Chem. 4, 118 (2006).

Structural modelling Enzyme to be modelled - superoxide dismutase (SOD enzyme) The structural model - Cu-amino acids and Cu-C- or N-protected amino acids Supports - rigid: chloropropylated silica gel (SG) - flexible: Merrifield’s resin (PS) Methods of immobilisation covalent grafting The amino acids - L-histidine, L-tyrosine and their C- or N-protected derivatives

L-H-His-OH L-H-His-OMe L-BOC-His-OH L-H-Tyr-OH L-H-Tyr-OMe L-BOC-Tyr-OH The amino acids and their derivatives

Merrifield’s resin

Methods of covalent grafting

Immobilised complexes with uniform ligands Jakab, N.I., Hernadi, K., Kiss, J.T., Pálinkó, I., J. Mol. Struct , 487 (2005). Berkesi, O., Szabó, T., Korbély, B., Hernadi, K., Pálinkó, I., ForumAcusticum 2005, Budapest (Hungary), 2005, pp Szilágyi, I., Berkesi, O., Sipiczki M., Korecz, L., Rockenbauer, A., Pálinkó, I., Catal. Lett. 127, 239 (2009).

Making the surface-anchored complex Solvent - ligand-excess conditions - toluene - water - isopropanol

Catalytic activity

How good the models are? Testing the activity with the Beauchamp-Fridovich reaction – determining the IC 50 values the smaller the values the higher the SOD activity

BOC-His-OH–Cu(II)–BOC-His-OCH2Phe-PS PS-PheCH2-His-OMe–Cu(II)–H-His-OMe Why do the surface-grafted Cu(II)-protected amino acid complexes work better?

In the surface-anchored Cu(II)-protected histidine complexes the geometries around the copper ion is the closest to that in the Cu,Zn-SOD enzyme, which is a distorted tetrahedron. surface-grafted Cu(II)-C-protected histidine surface-grafted Cu(II)-N-protected histidine

Making the surface-anchored complex Solvent - ligand-poor conditions - ligand-excess conditions - water - toluene - isopropanol The amino acid derivatives covalently grafted to SG Support - silica gel (SG) - L-H-Tyr-OMe - L-BOC-Tyr-OH

How good the models are? Testing the activity with the Beauchamp-Fridovich reaction – determining the IC 50 values the smaller the values the higher the SOD activity

Selectivity Pálinkó, I., Ordasi, A., Kiss, J.T., Labádi, I., AIP Conference Proceedings ICOPVS 2008 (International Conference on Perspectives in Vibrational Spectroscopy), Vol. 1075, pp , American Institute of Physics, Woodbury, New York, Jakab, I.N., Szabó, É., Hernadi, K., Pálinkó, I., Sampling Catalyis Research in the Pannonian Region, Proc. 8th Pannonian Int. Symp. Catal., Szeged (Hungary), 2006 pp

The Cu(II)-tyrosine derivative complexes covalently anchored to silica gel did not display either substantial SOD or catalase activities, but they had appreciable tyrosinase activities.

Immobilised complexes with mixed ligands Csendes, Z., Bugris, V., Lackó, L., Labádi, I., Kiss, J.T., Pálinkó, I., Anal. Bioanal. Chem. in press.

Making the surface-anchored complex Solvent - ligand-poor conditions - ligand-excess conditions - isopropanol The amino acid derivatives covalently grafted to SG Support - silica gel (SG) - 1:1 mixture of L-H-Tyr-OMe and L-H-His-OMe - 1:1 mixture of L-BOC-Tyr-OH and L-BOC-His-OH Structural characterisation - FT-IR spectroscopy, AAS, titrimetry

Suggested coordination modes C-protected amino acid mixture ligand-poor conditions N imidHissurf O carbonylHissurf O carbonylTyrsurf O phenolateTyrsurf ligand-excess conditions N imidHissurf O phenolateTyrsurf N imidHisadded O carbonylTyradded N-protected amino acid mixture ligand-poor conditions N imidHissurf O BOCcarbonylHissurf O BOCcarbonylTyrsurf O phenolateTyrsurf ligand-excess conditions N imidHissurf O phenolateTyrsurf N imidHisadded O BOCcarbonylTyradded

Catalytic activity

Heterogenised catalysts are very often more benign to the environment than the homogeneous counterparts, simply, because they can be recovered easily. Therefore, they may be of better choice even if they are often less active, but, of course, only if there is no dramatic drop in selectivity. Environmental issues

Main conclusion

Co-workers István Szilágyi Noémi I. Jakab János T. Kiss Imre Labádi Adrienn Ordasi Mónika Sipiczki Zoltán Kele Ottó Berkesi Nóra Nagy László Horváth Klára Hernádi Tamás Kiss Gábor Nagy István Fekete Antal Rockenbauer László Korecz Zita Csendes Valéria Bugris Linda Lackó Funding National Science Fund of Hungary (OTKA) and my own money

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