The anatomy of the energetics of molecular recognition by calorimetry: chiral discrimination of camphor by alpha-cyclodextrin.
The molecular recognition of both camphor enantiomers 2 with the chiral alpha-cyclodextrin (alpha-CD) 1 in water and D(2)O was examined by calorimetry. On the basis of statistically supported determinations the thermodynamics of 2:1 host-guest binding and chiral discrimination was evaluated. The energetic signature strongly supports hydrophobic interaction as the dominant driving force for camphor encapsulation by alpha-CD in water. The solvent isotope effect on the binding equilibrium served to dissect the experimental enthalpy DeltaH(ass) into direct interaction (DeltaH(intr)) and solvent reorganization (DeltaH(solv)) terms. From this analysis the mutual interaction of two cyclodextrin and one camphor molecules contributes only 25 % to the observed enthalpy of binding DeltaH(ass), all the rest is attributed to solvent restructuring. Furthermore, the dramatic change in the pattern of thermodynamic state functions on solvent transfer from water to D(2)O is taken as compeling evidence for the involvement of water as a structural tectone in the supramolecular architecture of the 2:1 complex. As a corollary, bilateral host-guest interactions as conveyed by the lock-and-key metaphor of molecular recognition provide an inadequate description of this seemingly simple artificial host-guest system.
Institute of Organic Chemistry and Biochemistry Technical University of Munich Lichtenbergstrasse 4 85747 Garching, Germany. schmidtchen@ch.tum.de
Franz P
FP
Schmidtchen
D002151:Calorimetry / Q000379:methods; D002164:Camphor / Q000737:chemistry*; D003505:Cyclodextrins / Q000737:chemistry*; D017666:Deuterium Oxide; D013816:Thermodynamics; D047391:alpha-Cyclodextrins
D003505:Cyclodextrins; D047391:alpha-Cyclodextrins; D002164:Camphor; D017666:Deuterium Oxide; C032613:alpha-cyclodextrin