pH-dependent binding of guests in the cavity of a polyhedral coordination cage: reversible uptake and release of drug molecules

Binding of organic guests containing acidic or basic groups inside a water-soluble coordination cage host shows strong pH dependence.


Experimental details
The host cage H was prepared according to ref. 13a (main text). All pH measurements were made using a Hamilton Spintrode pH combination electrode at 298 K and calibrated with calibration standards at pH 4.01, 7.0 and 10.01. 400 MHz 1 H NMR spectra were recorded on a Bruker AV3-400 instrument.
For all host/guest binding experiments samples were prepared as follows. Host solution: the host cage (16 mg) was dissolved in 10 ml D 2 O to make a 0.2 mM stock solution. Guest solutions: the guests were individually dissolved in 5 ml of the host solution (to keep the host concentration constant during the titration); the mass used varied with the desired concentration. All binding experiments were performed three separate times; the values for binding constants quoted are the average of the three measurements with the error quoted being two standard deviations from the mean.

pH titrations -guests in slow exchange
The guest solutions were equilibrated using a water bath thermostatted at 298 K. The pH was measured and then adjusted to the desired value by addition of NaOD or DCl (1 M). The 1 H NMR spectrum for each addition was measured.
The pKa was calculated by plotting pH against the 1 H NMR chemical shift of a selected guest signal; the resultant curve was fitted using the Microsoft Excel add-on 'Solver' by using a least squares fitting of the calculated curve to the data.
For a guest with a single pK a : Where δ is the chemical shift of the signal in question (ppm), δ(G 1 ) is the chemical shift of the charged species; and δ(G 0 ) is the chemical shift of the neutral species. α 0 and α 1 describe the speciation of the neutral and mono-charged species respectively, and follow the relationship outlined below: Electronic Supplementary Material (ESI) for Chemical Science. This journal is © The Royal Society of Chemistry 2014 For a guest with two pK s (adamantane-1,3-dicarboxylic acid): The binding constant at each pH point was calculated by measuring the integral of the free host and host-guest peaks relative to each other using Topspin's deconvolution feature. This was done for several pairs of peaks at each pH and the average value was taken.

pH titrations -fast exchange
The host and guest solutions were equilibrated using a water bath thermostatted at 298 K. The pH of both solutions was adjusted to the same desired value by the addition of NaOD or DCl (1 M). 12 samples were then prepared with varying host:guest ratios (varying from pure host to pure guest), with a total volume of 600 µl per sample. Measurements were performed using 8 inch NORELL 507-HP NMR tubes, sealed with pressure caps to ensure that no solvent loss occurred during the measurements. From the resulting 1 H NMR spectra, the concentration of guest was plotted against change in chemical shift (ppm) for a selected signal, and the resultant curve was fitted to a 1:1 binding isotherm to obtain a value for the binding constant. This was repeated with several signals from the same set of spectra and the value of K averaged.

Binding constants of the neutral and charged forms
In both cases (whether the guest was in fast or slow exchange) separate host/guest titrations were also performed at two fixed pH values -corresponding to the neutral and charged forms of the guest -to get the binding constants for the two extremes. In cases of weak binding for the charged guests (e.g. aspirin, detomidine, nicotine) where binding could not be detected in the NMR spectra, an upper limit for K has been estimated by assuming that formation of host/guest complex occurs to an extent of < 5% in these cases.

Additional data for NMR measurements
These figures follow the formats of Fig. 2 -5 in the main text. In all cases the % cage occupied is the red curve; (ppm) for a selected 1 H signal is the blue curve.