Jozef Hodyl PhD Thesis - Theses - Flinders University

The logK values obtained for this study demonstrate that addition of water to thesystem does reverse the trend from p-hydroxybenzoate > m-hydroxybenzoate > o-hydroxybenzoate found in DMSO-d 6 to o-hydroxybenzoate > m-hydroxybenzoate >p-hydroxybenzoate. This is markedly apparent for 2,6-dihydroxybenzoate where thelogK value increases from 2.05 in DMSO-d 6 by ca. three orders of magnitude to 4.64in 20% aqueous acetonitrile. The logK values obtained correlate well with thoseobtained for receptor 29 by fluorescence monitored titration. 129Therefore, theordering of the percent guest uptakes measured in water with immobilised receptor94 is consistent with the findings for these types of receptors. The reversal of thetrend shows that regulation of the system is dependent on specific solvent effects, inthis case, most likely, the ability of water molecules to mediate hydrogen bondingbetween the guest hydroxy groups and the host aromatic rings. Examples of 1 H NMRmonitored titration curves obtained in this series of experiments are shown for m-hydroxybenzoate in Figure 6.10 and 2,6-dihydroxybenzoate in Figure 6.11.7.4207.4007.380δ obs ppm7.3607.3407.3207.3007.2800 1 2 3 4 5 6 7 8 9 10[H]/[G]Figure 6.10 Example of 1 H NMR monitored titration curve for m-hydroxybenzoate with[Cd(TracHP12)] 2+ , 49, in 20% D 2 O/CD 3 CN buffered at pH 7 with HEPES buffer,showing the downward arc due to the upfield perturbation in the chemical shift of the p-proton.119