A supramolecular-hydrogel-encapsulated hemin as an artificial enzyme to mimic peroxidase.

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: [email protected] [email protected]