The Advantage of Covalent Capture in the Combinatorial Screening of a Dynamic Library for the Detection of Weak Interactions

In this paper we address the advantage of screening a dynamic library by covalent capture in comparison with an approach in which the target is not covalently bound to the molecular receptor. The aim is the selection of recognition units for the binding of an anion (or polyanion) by relatively weak binding interactions, a situation typically found in supramolecular chemistry. To compare the two approaches, two model systems have been studied both based on the functionalization of a molecular platform by reversible imine formation. In the case of the noncovalently bound substrate, the platform PI is a trisubstituted benzene unit, 2,4,6-trimethylbenzene-1,3,5-tricarbaldehyde, to select three recognition arms for the binding of the trisodium salt of benzene-1,3,5-tricarboxylate. For the covalent-capture-based approach the platforms P2 and P4 are benzene derivatives with a tethered phosphonate target (tetrabutylammonium 2-formylphenyl ethylphosphonate) for the selection of a single recognition unit. The library of recognition elements comprises phenyl- and ammonium-functionalized amines. We show that the selection of recognition units for the binding of the substrate with weak to medium binding constants may encounter, by using a noncovalently bound substrate, serious problems. This is because the best conditions for the amplification of the library, that is, a large excess of variable recognition elements and target, lead also to competitive binding of the elements not bound to the platform with the target. This may result in negligible amplification of the best-fit members of the library. In contrast, upon tethering the target to the platform and using the covalent-capture strategy for the selection of the recognition elements, significant amplification is observed, even for systems with much lower binding constants. Although competition with excess recognition units may also become an issue in the case of the tethered target, there is a way to overcome the problem by working at low concentrations.