Co-immobilization of an Enzyme and a Metal into the Compartments of Mesoporous Silica for Cooperative Tandem Catalysis: An Artificial Metalloenzyme**

As a result of natural evolution for millions of years, enzymes constitute the state of the art in catalysis, with their ability to efficiently catalyze a wide range of important chemical reactions under mild conditions. Among the enzymes, metalloenzymes stand out as the most impressive examples, and they are involved in several important processes in nature, such as water oxidation, photosynthesis, and nitrogen fixation. In a metalloenzyme, a metal-containing cofactor is integrated into the protein structure to enable new chemical reactions that would not be possible to achieve with the chemistry of the protein alone. In modern society, the utilization of enzymes for large-scale production of chemicals and other industrial applications is well-established. Along with the high efficiency of enzymes, they are also ideal sustainable catalysts, as they are obtained from renewable sources, and are biodegradable and non-toxic. Unfortunately, one major drawback of naturally derived enzymes is that they often exhibit a narrow substrate scope, which limits their use in organic synthesis. Ever since the visionary report by Wilson and Whitesides in 1978, a longsought dream has been to construct metalloenzymes with non-natural cofactors that open up for new efficient catalytic reactions. Several approaches to generate artificial metalloenzymes have so far been explored, but in general most of them involve the anchoring of an organometallic species to a protein, by either covalent, dative, or supramolecular interactions. In many of these cases the catalytic activity of the metalloenzyme originates from the introduced metal species, and the surrounding protein only serves as a ligand determining the stereoselectivty. Recently, highly active heterogeneous catalysts, and in particular those based on nanometallic species, have attracted considerable attention for selective chemical transformations. It has been found that by decreasing the cluster size below 2 nm a unique selectivity and activity may be obtained. Herein, we propose an approach for the construction of an artificial metalloenzyme with two different modes of reactivity: one exhibited by a nanometallic component and one by an enzyme. By the stepwise immobilization of an enzyme and a nanometal species into the same cavity of a mesoporous heterogeneous support, an environment is created where these species can reside and work in close proximity to one another in a cooperative fashion. This environment allows a simple design of cascade reactions in which the enzyme and the metal catalyst can work in cooperative tandem catalysis. Moreover, this approach would provide access to the advantages of heterogeneous catalysis with simple separation and recycling of the catalyst. The approach of developing catalytic entities that make use of the potential of both biocatalysis and transition metal catalysis has so far only been sparingly explored. Foulkes et al. engineered Escherichia coli bacteria to simultaneously express monoamine oxidase intracellularly and bind Pd nanoparticles on the outer membrane, which were then used for amine deracemization. Another promising strategy of generating multifunctional metalloenzyme mimics was recently reported by Kim and co-workers, where Pt nanoparticles were introduced into an aminopeptidase. This artificial metalloenzyme was successfully employed in a twostep cascade reaction, involving enzyme-catalyzed amide bond cleavage and Pt-catalyzed hydrogenation. Very recently, the same approach was used by Filice et al. to introduce Pd nanoparticles into the interior of a lipase to obtain a biohybrid catalyst. In our group, recent research has been devoted to exploring siliceous mesocellular foams (MCFs) as a support for various catalytic species. These mesoporous silica materials are simple to prepare and they possess many desirable properties, such as large pore volumes, large surface areas, and they enable for high loadings of catalytic species that are shielded from mechanical grinding and leaching. The material also has a high surface concentration of silanol groups that allow grafting of various functional groups. [*] Dr. K. Engstrçm, Dr. E. V. Johnston, O. Verho, K. P. J. Gustafson, Dr. M. Shakeri, Prof. J.-E. B ckvall Department of Organic Chemistry, Arrhenius Laboratory Stockholm University, 10691 Stockholm (Sweden) E-mail: [email protected]