Jozef Hodyl PhD Thesis - Theses - Flinders University

percent inclusion with the pK a values for each set of benzoate guest molecules it isclear that the hydroxy groups are having some effect on the binding mode in aqueousconditions. This inclusion study with methoxybenzoate guests, which was conductedunder identical conditions to the previous study with hydroxybenzoate guests,demonstrates the proposition that non-classical hydrogen bonding interactionscontribute to host-guest binding strength only in the presence of water.Another question that could be asked with regard to the studies withhydroxybenzoate and methoxybenzoate guests is why p-methoxybenzoate uptake isca. double that of p-hydroxybenzoate. Obviously, there is something other thanhydrogen bonding contributing to the increase in percent inclusion for p-methoxybenzoate as their pK a values are similar. The discrepancy observed could bethe result of a hydrophobic effect driving the p-methoxybenzoate molecule into thecavity of receptor 94, and away from the aqueous surroundings, as its methoxy groupis less hydrophilic than the hydroxy group of p-hydroxybenzoate. Conversely, thehydroxy group of p-hydroxybenzoate could be drawing it out of the cavity ofreceptor 94 by a hydrophilic effect. A lesser rendering of these hydrophobic andhydrophilic effects could describe the decrease in difference between percentinclusion values of m-methoxybenzoate and m-hydroxybenzoate since thesubstituents on these molecules are in the neighbourhood of the boundary dividingthe interior and exterior regions of the host cavity.Hydroxybenzoate guest species with various other substitution patterns were alsoused in these inclusion studies to see if any variation in the location of the hydroxysubstituent would affect binding with receptor 94. The guest molecules chosen were2,5-dihydroxybenzoate, 3,5-dihydroxybenzoate, 2,4,6-trihydroxybenzoate and 3,4,5-trihydroxybenzoate (gallate). Table 6.4 displays the percent inclusion values for103