Templating a polymer-scaffolded dynamic combinatorial libraryw

Dynamic combinatorial chemistry (DCC) has emerged in recent years as a powerful tool for the discovery of molecular receptors. DCC uses reversible covalent reactions to link together building blocks, forming libraries of compounds whose product distributions are under thermodynamic control. The reversible nature of the linkages enables the library members to reconfigure their structures by exchanging and reshuffling their building blocks. Equilibrium perturbations, such as the addition of a template, can result in the structural adaptation of the library, amplifying the concentration of those library members which are stabilized by the template. The library can be kinetically ‘fixed’—often by simply quenching the catalyst involved in catalysing the dynamic exchange—allowing library members to be isolated and characterized. The appeal of the dynamic combinatorial approach lies in its inherent ability to combine library synthesis and affinity screening into a single step, making it a potentially rapid, simple and cost-effective method for the discovery of receptors. DCC has demonstrated great promise for the discovery of small molecule receptors, but its potential for the discovery of macromolecular counterparts has yet to be investigated. Several years ago, one of us described a new class of DCL—the so-called ‘‘polymer-scaffolded dynamic combinatorial library’’ (PS-DCL)—which was designed to apply concepts from the field of DCC towards the discovery of macromolecular receptors. PS-DCLs are based upon a synthetic polymer scaffold where functionalized residues are grafted onto the scaffold through dynamic covalent linkages. The reversibility of these linkages allows residues to exchange with one another and reshuffle their positions upon the polymer scaffold, presenting a mechanism for the members of the library to adapt their primary structures. We hypothesize that a polymer-scaffolded DCL should be able to respond to the addition of a template in the same manner as that demonstrated by other DCLs, inducing the system to re-equilibrate and amplifying the concentration of library members which best bind the template. This wholly thermodynamically controlled templation process provides a mechanism to correct and refine binding sites, a virtue absent in molecularly imprinted polymers (MIPs) where the molecular imprinting process is kinetically controlled and there is no scope for errorcorrection during the templating procedure. This deficiency leads to significant heterogeneity and binding pockets which are not optimised, with detrimental effects on molecular recognition. The PS-DCL concept can be contrasted to the conventional combinatorial approach utilised by Schrader and co-workers for the discovery of polymers for use in protein recognition. In this work, a small set of functionalized monomers was used to prepare libraries of copolymers which were screened successfully to find selective binders for argininerich proteins in aqueous solution, a process requiring extensive synthesis and screening. As with MIPs, the recognition units are incorporated irreversibly into the polymer scaffold and there is no available mechanism for the optimisation of binding sites. Recently Ghadiri and co-workers have arguably demonstrated the validity of aspects of the PS-DCL concept within their impressive work on sequence-adaptive oligonucleotides. We report here the generation of an aqueous PS-DCL based on acylhydrazone exchange upon a simple synthetic polyacrylamide scaffold, and demonstrate that this library can re-equilibrate in the presence of macromolecular templates, work which we believe represents important first steps towards the realisation of the PS-DCL concept as a general method for the discovery of truly macromolecular receptors. Acylhydrazone exchange was chosen as the dynamic reaction to append organic residues to suitably functionalized polymer scaffolds as it is a well-studied and successful reaction in dynamic covalent chemistry. Acylhydrazone linkages are formed from the acid-catalysed condensation of acylhydrazides with aldehydes, and the resulting acylhydrazone bonds can readily undergo component exchange, with optimum rates of formation and exchange observed at pH 4.5. Many DCLs have been reported based around acylhydrazone exchange in organic media, however, there have been relatively few reports of dynamic systems based upon acylhydrazone exchange in aqueous solution. Our water-soluble PS-DCL was constructed upon the polymer scaffold P1, which is a polyacrylamide derivative featuring Chemical Nanoscience Laboratory, School of Chemistry, Newcastle University, Newcastle upon Tyne, NE1 7RU, United Kingdom. E-mail: [email protected]; Fax: +44 (0) 191 222 6929; Tel: +44 (0) 191 222 7065 w Electronic supplementary information (ESI) available: Experimental procedures, spectroscopic data, GPC traces. See DOI: 10.1039/ c1cc11998b ChemComm Dynamic Article Links