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. 2009 Mar 4;131(8):3069-77.
doi: 10.1021/ja8100566.

Substituent effects on xenon binding affinity and solution behavior of water-soluble cryptophanes

P Aru Hill  1 , Qian Wei, Thomas Troxler, Ivan J Dmochowski
Affiliations

Substituent effects on xenon binding affinity and solution behavior of water-soluble cryptophanes

P Aru Hill et al. J Am Chem Soc. .

Abstract

A water-soluble triacetic acid cryptophane-A derivative (TAAC) was synthesized and determined by isothermal titration calorimetry and fluorescence quenching assay to have a xenon association constant of 33,000 M(-1) at 293 K, which is the largest value measured for any host molecule to date. Fluorescence lifetime measurements of TAAC in the presence of varying amounts of xenon indicated static quenching by the encapsulated xenon and the presence of a second non-xenon-binding conformer in solution. Acid-base titrations and aqueous NMR spectroscopy of TAAC and a previously synthesized tris(triazole propionic acid) cryptophane-A derivative (TTPC) showed how solvation of the carboxylate anions can affect the aqueous behavior of the large, nonpolar cryptophane. Specifically, whereas only the crown-crown conformer of TTPC was observed, a crown-saddle conformer of TAAC was also assigned in aqueous solution.

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Figures

Figure 1
Figure 1
Water-soluble cryptophanes TAAC and TTPC
Figure 2
Figure 2
Fluorescence quenching of TAAC (15 µM) by Xe in 1 mM, pH 7.2 phosphate buffer, 293 K. Inset: Curve fit for a single-site binding model.
Figure 3
Figure 3
Enthalpogram of 5.05 mM aqueous xenon solution titrated into 0.77 mM TAAC at 293 K.
Figure 4
Figure 4
Stern-Volmer plots of fluorescence quenching by xenon of TAAC (blue diamonds) and TTPC (pink squares).
Figure 5
Figure 5
Initial slopes of steady-state Stern-Volmer plots of TAAC (blue diamonds) and TTPC (pink squares).
Figure 6
Figure 6
Fluorescence decays of TAAC with increasing amounts of xenon and the deconvolved instrument response function (IRF).
Figure 7
Figure 7
Initial slope of time-resolved Stern-Volmer plot of TAAC.
Figure 8
Figure 8
600 MHz 1H NMR spectrum of TTPC in 10% D2O.
Figure 9
Figure 9
600 MHz 1H NMR spectrum of TAAC in 10% D2O.
Scheme 1
Scheme 1
13-step synthesis of water-soluble triacetic acid cryptophane TAAC* *Conditions: (a) Cs2CO3, DMF, 55 °C, 12 h, 60%; (b) CHCl3, HCOOH, reflux, 9 h, 40%; (c) Pd(OAc)2, P(Ph)3, (Et)2NH, THF, H2O, 80 °C, 4 h, 84%; (d) ethylbromoacetate, Cs2CO3, DMF, 60 °C, 12 h, 79%.; (e) KOH (2 M), THF, 70 °C, 12 h, 87%.
Scheme 2
Scheme 2
Proposed model of fluorescence quenching of TAAC by encapsulated xenon. The excitation (ex) and emission (em) wavelengths are 280 nm and 313 nm, respectively.
See this image and copyright information in PMC

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