Hydrogen bonding in water using synthetic receptors

Four water-soluble adenine receptors were synthesized to study the influence of hydrophobic interactions and hydrogen bonding on molecular recognition in aqueous solution. Association constants were measured in aqueous solution at five temperatures from 3-27 degrees C (pH 6, 51 mM ionic strength). For the mono(imide) receptors, delta H was -5.8 kcal/mol (carbazole) and -9.2 kcal/mol (naphthalene). The entropy of association for these was -13 cal.mol-1.K-1 (carbazole) and -26 cal.mol-1.K-1 (naphthalene). The carbazole bis(imide) receptor showed a binding enthalpy of -7.4 kcal/mol and entropy of -18 cal.mol-1.K-1. From this the free energy at 298 K of a single hydrogen bond is estimated to be only 0.2 kcal/mol. The enthalpy of a single hydrogen bond in this solvent-exposed system is estimated to be, at most, 0.8 kcal/mol, indicating that enthalpy just compensates for the unfavorable entropy in this system. These values reflect stronger hydrophobic interactions with the more polarizable naphthalene, as well as enthalpy-entropy compensation effects.