Combinatorial Chemistry & High Throughput Screening, 2010, 13, 45-53 45 1386-2073/10 $55.00+.00 © 2010 Bentham Science Publishers Ltd. Tagging Molecules with Linear Polymers: Biocatalytic Transformation of Substrates Anchored on Soluble Macromolecules Claudio Cornaggia and Dario Pasini * Department of Organic Chemistry, University of Pavia -Viale Taramelli, 10-27100 Pavia, Italy Abstract: With the increasingly available technology in automated synthesis and screening protocols, the combination of polymer-supported chemistry and biocatalysis, with their respective advantages over classical organic synthesis, has become more scientifically attractive, yet remains challenging. Since the development of solid-phase synthesis and its rapid expansion in combination with the advent of combinatorial techniques, a variety of alternative methodologies have been proposed and demonstrated to be viable for applications in high-throughput and multistep syntheses of the desired products. These alternative methodologies overcome the disadvantages of crosslinked polymer beads, which, as a consequence of their insolubility and their being necessarily heterogeneous in the reaction mixture, do have operational drawbacks. They often rely on a common strategy: tagging the target substrate of interest with other fragments (fluorous synthons, macromolecules, "precipitons") in such a way that the tag-substrate covalent ensemble is then easily separated from the reaction mixture by physical methods (liquid-liquid extraction, precipitation, etc.). The efficiency of enzymes in transforming substrates is often enhanced when the stability limitations of the biocatalyst in unnatural conditions (i.e. organic solvents, high temperatures) are avoided by the use of immobilization-stabilization techniques. We comment here, with recent examples, on the use of linear macromolecules as recyclable tags capable of acting as covalent supports in combination with a biocatalyzed reaction. Keywords: Biocatalysis, linear polymers, solution-phase synthesis, macromolecules, organic tags, high-throughput synthesis, organic chemistry, combinatorial synthesis. 1. INTRODUCTION Within the perspective of the development of green, sustainable chemical processes, the use of catalysts, derived or extracted from renewable sources (biocatalysts), is certainly attractive. Specific enzymes have been demons- trated to be viable for the catalysis of a wide variety of organic transformations, even on an industrial scale [1-3]. Their complex structures, perfected during millions of years of evolution, often achieve unsurpassed activities combined with very high organic chemo-, regio- and stereoselectivities. The use of enzymes also offers mild reaction conditions (physiological pH and temperature if in aqueous environ- ment) and a biodegradable catalyst at the end of its operational use. Since different enzymes often operate under similar conditions, two or more of them can be combined in one pot. All these advantages have prompted studies towards the utilization of enzymes in nonaqueous solvents. In this context, immobilization of enzymes is often the key for the optimization of their performance in industrial processes; in fact, immobilization techniques are often essential for both a) the recyclability of the biocatalysts by simple filtration of the reaction mixture thus allowing high-throughput synthesis particularly desirable on an industrial scale; and b) enhancing the stability of the biocatalyst in organic solvents, where most organic transformations are carried out. Different methods for the immobilization of enzymes have been recently reviewed [4]. The methods can be divided into *Address correspondence to this author at the Department of Organic Chemistry, University of Pavia -Viale Taramelli, 10-27100 Pavia, Italy; Tel: +39 0382 987835; Fax: +39 0382 987323; E-mail: dario.pasini@unipv.it three main categories as follows: (i) binding to a prefabricated support (carrier); (ii) entrapment in organic or inorganic polymer matrices; and (iii) cross-linking of enzyme molecules. Although these methods have obvious advantages and disadvantages when compared with each other, they share a common, important feature: the biocatalyst is heterogeneous (insoluble in the reaction mixture). The immobilization of catalysts and of organic substrates in general has its historical origin in the introduction of solid-phase synthesis, pioneered by Merrifield and coworkers in the 1960s [5], since it became necessary, in the organic synthesis of complex biopolymers to introduce easy work-up procedures in a repetitive growth scheme, and to render simpler the use of excess organic reagents. Since then, solid-phase synthesis has experienced a tremendous growth in terms of availability of supports, linkers and methods of analysis, particularly in combination with its use in combinatorial chemistry; innovative approaches such as "mix-and-split" for the creation of complex libraries of several thousand or more organic compounds have been conceived with the use of crosslinked beads, since the positive lead identified on the bead can then be mechanically separated, and the chemical structure of the "positive" determined by deconvolution or encoding methodologies [6- 9]. However, crosslinked, insoluble polymer supports do present disadvantages: generally speaking, the characteristic which is their main advantage, their insolubility, is also the origin of many operational drawbacks. Soluble polymers, on the contrary, can provide the primary advantage of heterogeneous systems, i.e. facile product/reagent separation