Controlled Diffusion of Photoswitchable Receptors by Binding Anti-electrostatic Hydrogen-Bonded Phosphate Oligomers

Abstract

Dihydrogen phosphate anions are found to spontaneously associate into anti-electrostatic oligomers via hydrogen bonding interactions at millimolar concentrations in DMSO. Diffusion NMR measurements supported formation of these oligomers, which can be bound by photoswitchable anion receptors to form large bridged assemblies of approximately three times the volume of the unbound receptor. Photoisomerization of the oligomer-bound receptor causes a decrease in diffusion coefficient of up to 16%, corresponding to a 70% increase in effective volume. This new approach to external control of diffusion opens prospects in controlling molecular transport using light.Article pubs.acs.org/JACS

A Comprehensive Picture of the Structures, Energies, and Bonding in the Alanine Dimers

“…The classical description of hydrogen bonds invokes an electrostatic Y H −X interaction which would become a destabilizing Y X −H factor in the non conventional cases. Weinhold and Klein [72] reported for the first time anti‐electrostatic hydrogen bonds and showed that orbital interactions more than compensate for the local Coulomb repulsion, other studies soon followed [73–76] . For the particular case of the Alanine dimers, in the NBO orbital interaction picture, it is the π * C=O orbital that acts as the acceptor of charge for most of the non conventional contacts.…”

“…Weinhold and Klein [72] reported for the first time antielectrostatic hydrogen bonds and showed that orbital interactions more than compensate for the local Coulomb repulsion, other studies soon followed. [73][74][75][76] For the particular case of the Alanine dimers, in the � d ! � a NBO orbital interaction picture, it is the π* C=O orbital that acts as the acceptor of charge for most of the non conventional contacts.…”

Search for an exothermic halogen bond between anions

“…Nonetheless, this intuitive idea has been countered by a growing number of observations, from both experimental and theoretical quarters, that like-charged ions can engage in H-bonds with one another under certain conditions. Examples of such bonds involve H 2 PO 4 − , 12–15 (RPO 3 H − ), 16 HSO 4 − , 17,18 HCO 3 − , 19,20 carboxylates, 21,22 and aspartate, 23 as well as an assortment of others. 24–29…”

A Photoresponsive Receptor with a 105 Magnitude of Reversible Anion‐Binding Switching

“…Flood and co‐workers have taken this concept further by using UV light to sequester a wide range of anions, controlling the size of the helical quaternary structure and function by anion binding through azo −N=N− bonds installed on the foldamer [9a,b] . Other groups have been able to photoisomerize stiff‐stilbene oligomer scaffolds decorated with urea anion‐binding motifs to form weakly binding E forms and strongly binding Z forms with dihydrogenphosphate using near‐UV light, with moderate photoconversion ratios [12] . Aside from bioinspired receptors, an acylhydrazone‐based photoswitch with two separate binding sites for cations and anions has been reported; along with a multistep synthesis, cations Li + , Na + , K + , and Bu 4 N + and anions Ph 4 B − , Cl − , Br − , and I − were studied, revealing limited selectivity between cation and anion combinations [13] .…”

“…Aside from bioinspired receptors, an acylhydrazone‐based photoswitch with two separate binding sites for cations and anions has been reported; along with a multistep synthesis, cations Li + , Na + , K + , and Bu 4 N + and anions Ph 4 B − , Cl − , Br − , and I − were studied, revealing limited selectivity between cation and anion combinations [13] . Notable photoswitchable systems by Feringa and co‐workers incorporate the photoactive stiff‐stillbene group to effect a conformational switch with light, while urea groups participate in binding [12,14] . This general framework and its analogs have been used to study a wide range of anions including Cl − , Br − , NO 3 − , CH 3 CO 2 − , H 2 PO 4 − , and HSO 4 − with maximum binding constants of 7.5×10 3 M −1 for H 2 PO 4 − in DMSO.…”