Thermodynamic aspects of dicarboxylate recognition by simple artificial receptors.
Recognition of dicarboxylates by bis-functional hydrogen-bonding receptors displays divergent thermodynamics in different solvent systems. NMR titration and isothermal titration calorimetry indicated that neutral bis-urea and bis-thiourea receptors form exothermic complexes with dicarboxylates in DMSO, with a near zero entropic contribution to binding. The increased binding strength of bis-guanidinium receptors precluded quantitative measurement of binding constants in DMSO, but titration calorimetry offered a qualitative picture of the association. Formation of these 1:1 complexes was also exothermic, but additional endothermic events occurred at both lower and higher host-guest ratios. These events indicated multiple binding equilibria but did not always occur at a discrete 2:1 or 1:2 host-guest molar ratio, suggesting higher aggregates. With increasing amounts of methanol as solvent, bis-guanidinium receptors form more endothermic complexes with dicarboxylates, with a favorable entropy of association. This switch from association driven by enthalpy to one driven by entropy may reflect a change from complexation involving the formation of hydrogen bonds to that promoted by solvent liberation from binding sites.
Linton
B R
BR
Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520-8107, USA.
Goodman
M S
MS
Fan
E
E
van Arman
S A
SA
Hamilton
A D
AD
D001665:Binding Sites; D002151:Calorimetry; D003998:Dicarboxylic Acids / Q000737:chemistry*; D006860:Hydrogen Bonding; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D011956:Receptors, Cell Surface / Q000737:chemistry; D012997:Solvents / Q000737:chemistry; D013816:Thermodynamics; D013890:Thiourea / Q000737:chemistry; D014508:Urea / Q000737:chemistry
D003998:Dicarboxylic Acids; D008024:Ligands; D011956:Receptors, Cell Surface; D012997:Solvents; D014508:Urea; D013890:Thiourea