Written by Rahul Dev Mukhopadhyay and Ayyappanpillai Ajayaghosh
The paper in Nature Communications is here: https://rdcu.be/OzNa
In complex biological processes, precise host-guest interactions occur inside subcellular compartments with absolute control. Many of these enzyme mediated transformations occur in a step-wise fashion guided by various chemical (pH, ions, sugar, phosphates) and physical (heat) stimuli. Unfortunately, having such flawless control in artificial stimuli-responsive systems is a difficult task using temperature as a stimulus. In the above communication, it has been demonstrated that a step-wise control of host-guest interaction is possible with temperature when cyclodextrin is complexed with a rationally designed coordination polymer.
Cyclodextrin, a naturally occurring cyclic compound consisting of glucopyranoside units (six to eight members) tethered at their 1- and 4-positions in a head to tail fashion, has a hydrophobic interior and a hydrophilic exterior. Cyclodextrins form stable host-guest complexes with molecules such as adamantane, azobenzene, ferrocene etc. in an aqueous environment. In a recent development, cyclodextrins have also found use in the treatment of neurodegenerative disorders and obesity. Having been used conventionally as a supramolecular host, so far the design of a synthetic host for the controlled delivery and uptake of cyclodextrins (CDs) as a guest has been broadly neglected. Moreover due to their optical transparency CDs remain stealth and cannot be detected by conventional spectroscopic techniques. Nevertheless, due to their chiral nature they can be detected using induced circular dichroism (ICD) spectroscopy, once they bind with a light absorbing molecule.
Keeping in mind the above issues, we designed a water soluble coordination polymer (host) from azobenzene ligands and magnesium oxide based polymeric inorganic cluster. The hydrophobic azobenzene units can form stable inclusion complexes with cyclodextrins (guest) in water. We next thought of exploiting the Le Châtelier's principle, which predicts the shifting of a thermodynamic equilibrium, in order to undo the change caused by an external stimulus. We found that the ICD signal corresponding to the bound CD molecules remains constant at a particular temperature for any desired amount of time. Additionally, by astutely monitoring the temperature, formation (capture) or dissociation (release) of CD molecules from the host coordination polymer is possible in a step-wise and reversible fashion. The ICD signal in each case can be controlled the way, one wants it to. Such control is even extended in the quasi-solid state when the host-guest complex is incorporated inside a hydrogel matrix.
The present study may be considered as a cornerstone in the design of smart thermo-responsive materials, and drug delivery platforms in the near future and useful in detecting the delivery profile of spectroscopically silent molecules like CDs and drugs covalently functionalized with CDs. However, the challenge is to design a biocompatible coordination polymer that can act as a host of an appropriate guest for their thermally controlled step-wise release. It will also be interesting to exploit this chemistry in the design of energy dissipative self-assembling systems (systems chemistry) and in controlled supramolecular polymerization.