The field of heteropolymetallic systems with magnetic metal centers occupies a crossing point between biology and physics. For instance the Cu(II)-Fe(III) interaction in cytochrome oxidase is of the same nature as the Cu(II)-Mn(II) interaction in a novel system which could be the first molecular ferromagnet. The mechanism of the interaction is discussed, both from a phenomenological view point using a spin Hamiltonian, and from an orbital view point. An orbital model for the isotropic interaction is presented. It is based on the concept of natural magnetic orbitals. The mechanism of the anisotropic and antisymmetric interactions is more briefly treated. The role of the Zeeman perturbation is then considered in relation with the magnetic and EPR properties of the heterobimetallic complexes. Several examples are presented to emphasize that the nature, ferro- or antiferromagnetic of the isotropic interaction is controlled by the symmetry of the magnetic orbitals. The concept of overlap density is introduced. It permits an estimation of the magnitude of the ferromagnetic stabilization in the case of orthogonality of the magnetic orbitals. The Cu(II)-Fe(III) interaction, in relation to the situation encountered in cytochrome oxidase, the Cu(II)-Ni(II) interaction and a few additional selected examples are discussed. A section deals with the case where one of the interacting ions has an orbital degeneracy. Afterwards, the heterotrinuclear complexes are studied. The important concept of regular and irregular spin state structure is developped and the Mn(II)Cu(II)Mn(II) triad is presented as a spectacular example of irregular spin state structure. A section is devoted to the ordered bimetallic chains. The theory is presented, both at a qualitative and quantitative levels and the already reported compounds of this kind are discussed. One of them may be considered as one of the first molecular ferromagnets. The last but one section concerns the systems with even more subtle spin orders. In conclusion, the vast perspectives of this area are outlined.

Magnetism of the heteropolymetallic systems

https://www.jbc.org/content/268/28/21120.full.pdf

Macrophage nitric oxide synthase subunits. Purification, characterization, and role of prosthetic groups and substrate in regulating their association into a dimeric enzyme.

A challenge in chemistry is an artificial enzyme that mimics the functions of the natural system but is simpler than proteins. The intensive development of artificial enzymes that use a variety of matrices, the rapid progress in supramolecular gels, and the apparent “superactivity” exhibited by supramolecular hydrogel-immobilized enzymes, prompted us to evaluate whether supramolecular hydrogels will improve the activity of artificial enzymes for catalyzing reactions in water or in organic media. To demonstrate the concept, we used hemin as the prosthetic group to mimic peroxidase, a ubiquitous enzyme that catalyzes the oxidation of a broad range of organic and inorganic substrates by hydrogen peroxide or organic peroxides. The structures of the active site as well as the reaction mechanism of peroxidases are well studied. Much effort has been focused on incorporating metalloporphyrins into protein-like scaffolds in the quest towards peroxidase mimetics, which do not show satisfactory activity and selectivity mainly owing to the lack of the peptidic microenvironment that exists in the native peroxidase. The shape of the protein pocket and the amino acid residues or functional groups that surround the active site bring about the special inclusion behavior between the enzyme and the substrate. As a result, b-cyclodextrins (bCDs), which are a frequently used model system, act as an excellent enzyme model owing to their appropriate size and their fairly rigid and hydrophobic cavities that provide favorable binding of the prosthetic group as well as substrates. Experimentally, b-CD-modified hemins have showed higher activity relative to free hemin, suggesting that supramolecular hydrogels may act as an alternative matrix to encapsulate hemin for the mimetic of peroxidase. Supramolecular hydrogels, formed by the self-assembly of nanofibers of amphiphilic oligopeptides or small molecules, have served as scaffolds for tissue engineering, a medium for screening inhibitors of enzymes, 14] a matrix for biomineralization, and as biomaterials for wound healing. The application of supramolecular hydrogels as the skeletons of artificial enzymes has yet to be explored. Similar to peptide chains that form active sites in enzymes, the selfassembled nanofibers of amino acids in the supramolecular hydrogels could act as the matrices of artificial enzymes. Thus, the supramolecular-hydrogel systems serve two functions: 1) as the skeletons of the artificial enzyme to aid the function of the active site (e.g., hemin) and, 2) as the immobilization carriers to facilitate the recovery of the catalysts in practical applications. Herein, we mixed hemin chloride (3) into the hydrogel formed by the self-assembly of two simple derivatives of amino acids (1 and 2). The activity of this new type of artificial enzyme is higher than the activity of free hemin, hemin in bCD, or hemin in polymeric hydrogels. This artificial enzyme shows the highest activity in toluene for an oxidation reaction, reaching about 60% of the nascent activity of horseradish peroxidase (HRP). This result is particularly interesting because it implies that the control of the structure of hydrogelators could tailor the nanofibers as an adjustable microenvironment around active centers and thereby affect the performance of artificial enzymes. Moreover, the supramolecular hydrogel acts as an effective carrier to minimize the dimerization and oxidative degradation of free hemin in the peroxidization reaction. Overall, the supramolecular-hydrogel-based artificial enzyme offers a new opportunity to achieve catalysis with high operational stability and reusability, which ultimately would benefit industrial biotransformation. Scheme 1 illustrates the simple procedure of using supramolecular hydrogels to encapsulate hemin. Equal molar equivalents of 1 and 2 and two equivalents of Na2CO3 in water formed a suspension, which turned into a clear solution at about 60 8C. Then, 3 was added and dissolved immediately in the solution. The subsequent cooling of the solution to room temperature afforded a supramolecular hydrogel containing hemin molecules (Gel II). Without the addition of 3, the same procedure gave the control (Gel I). As a point of reference, we made the artificial peroxidases by using b-CD or a polyacrylamide hydrogel to encapsulate hemin according to the literatures. As shown in the TEM and AFM images in Figure 1, Gel I and Gel II have different morphologies. The networks of Gel I have 50–500-nm pores formed by the nanofibers (roughly 20 nm in diameter) of the self-assembled 1 and 2. Besides the relatively large pores, the TEM image of the nanofibers in Gel II, however, shows two distinct regions: the dark part (fibers of approximately 20 nm in diameter) and the gray part (surface layer of approximately 6 nm thickness). [*] Dr. Q. Wang, Dr. Z. Yang, Prof. C. K. Chang, Prof. B. Xu Department of Chemistry The Hong Kong University of Science and Technology Clear Water Bay, Hong Kong (P.R. China) Fax: (+852)2358-1594 E-mail: chang@ust.hk chbingxu@ust.hk

A Supramolecular-Hydrogel-Encapsulated Hemin as an Artificial Enzyme to Mimic Peroxidase**

Using synthetic systems to mimic natural enzymes with high catalytic activity and distinct substrate selectivity has been a challenge for the last several decades. Hemin, the catalytic center for many protein families including cytochromes, peroxidases, myoglobins and hemoglobins, can catalyze a variety of oxidation reactions like peroxidase enzymes.[1] However, direct application of hemin as an oxidation catalyst is of significant challenge because of its molecular aggregation in aqueous solution to form catalytic inactive dimers and oxidative self-destruction in the oxidizing media, which causes passivation of its catalytic activity.[2] A potential solution to this problem is to synthetically modify the porphyrin structure to achieve a variety of iron porphyrin derivatives for improved catalytic activity or stability.[3] An alternative approach is to use high surface area materials such as zeolites, nanoparticles, silica or natural clay to support hemin to achieve improved stability or activity in epoxidation or other reactions in organic solutions.[4] For reactions in aqueous solutions, hydrogel-embedded hemin[5] or more elaborate hemin complex obtained by conjugating with specific antibodies[6] have shown activity significantly better than free molecules, which is, however, still orders of magnitude inferior to natural enzymes, not to mention the difficulties in the synthesis of such kinds of complex hemin conjugates. Therefore, the discovery and development of novel materials as supports to achieve biomimetic catalysts with enzyme-like activity is highly desired.

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Graphene‐Supported Hemin as a Highly Active Biomimetic Oxidation Catalyst

An assessment of drug-haematin binding as a mechanism for inhibition of haematin polymerisation by quinoline antimalarials

Mössbauer studies on protoporphyrin IX iron(III) solutions

Mössbauer studies on ferrocene complexes IX. Phosphaferrocenes and their protonated derivatives

Mössbauer studies on protoporphyrin IX iron(II) frozen solutions containing ligands that cause the iron to be in a five coordinate high spin iron(II) environment

Conductometric studies on protoporphyrin IXiron(III) alkali metal solutions. Evidence for the alkali metals binding to the protoporphyrin IXiron(III) moiety

B. Lukas

Papers

Mössbauer studies on protoporphyrin IX iron(III) solutions

Mössbauer studies on ferrocene complexes IX. Phosphaferrocenes and their protonated derivatives

Mössbauer studies on protoporphyrin IX iron(II) frozen solutions containing ligands that cause the iron to be in a five coordinate high spin iron(II) environment

Conductometric studies on protoporphyrin IXiron(III) alkali metal solutions. Evidence for the alkali metals binding to the protoporphyrin IXiron(III) moiety

Studies on metalprotoporphyriniron(III) complexes