LAUSR

Allostery, often referred to as the ‘second secret of life’, is a process in which ligands binding to a biomolecule influence activity at a remote site. The causal sequence of binding and conformational change is, however, much debated. Do ligands bind to a sub-optimal conformer, reshaping the receptor in their own image? Or do they select a preferred conformer from a pre-existing ensemble of interchanging structures? Analysis of artificial host–guest complexes can potentially offer an analytically tractable way to answer this question. But such studies are rare, and those performed to-date typically suggest that conformational selection mechanisms predominate over induced-fit pathways. Now, a team led by Jovica Badjić at the Ohio State University report a thorough investigation of the relative contributions of these pathways to the host–guest chemistry of an artificial supramolecular host (Pavlović, R. Z. et al. Angew. Chem. Int. Ed. https://doi.org/10.1002/ anie.202107091 (2021)). The system in question is a twisted capsule (pictured) that adopts one of two conformers with opposite helical twist sense. Upon binding of a tetrahalomethane guest molecule, the conformational distribution of the capsule changes, providing a minimal model of allostery. By using a flux-based analytical approach derived from studies of biomolecular receptors, the contributions of conformational selection and induced-fit mechanisms as a function of guest concentration can be quantified. Untangling the fast dynamics of the system requires extensive analysis by NMR spectroscopy at –95 °C, enabling characterization of each elementary step in the network of equilibria. The rate constants for guest binding and conformational changes are obtained through 2D exchange spectroscopy (EXSY) and 1D titration studies. The experimental work is complemented by DFT calculations, which help rationalize the kinetic parameters and experimentally obtained barriers to exchange. In contrast to prior studies, this flux-based analysis suggests that both pathways operate, with lower guest concentrations favouring the conformational selection mechanism previously thought to be generally dominant in abiotic systems. At higher guest concentrations, induced-fit pathways come to dominate — both pathways are in fact viable, a result previously observed for biological systems. It may be that the methodology established here can help clarify the contributions of these pathways in other host–guest ensembles and thereby guide the design of more complex systems in the future. ❐