Thermodynamics of Formation of Host−Guest Supramolecular Polymers

The interactions between three beta-cyclodextrin hosts (having 1-3 binding sites) and two adamantyl guests (having 1-2 binding sites) have been studied by ITC, ROESY, static and dynamic light scattering (SLS and DLS), and AFM and TEM techniques. The enthalpy and free energy values (determined from ITC experiments) evidence that the single interaction between one binding site of the guest and one binding site of the host is independent of the number of binding sites of the interacting species. The average values are deltaH degrees = -26.6 +/- 2.3 kJ mol(-1) and deltaG degrees = -30.4 +/- 3.2 kJ mol(-1), indicating that the process is mainly enthalpy driven. In all cases, the experimental molar ratio (from ITC experiments) agrees with the expected one from the number of binding sites of both the host and guest. The formation of polymer-like entities was demonstrated by SLS, DLS, AFM, and TEM measurements. The structure of polymers is linear when both the host and the guest are ditopic entities and dendritic (or Cayley tree type) when the host and the guest have three and two binding sites, respectively.

Diastereomeric Molecular Recognition and Binding Behavior of Bile Acids byl/d-Tryptophan-Modifiedβ-Cyclodextrins

[reaction: see text] Binding behavior of L- and D-tryptophan-modified beta-cyclodextrins (L/D-Trp-beta-CD) (1 and 2) with four bile acids, i.e., cholate (CA), deoxycholate (DCA), glycocholate (GCA), and taurocholate (TCA), has been investigated by fluorescence, circular dichroism, and 2D-NMR spectroscopies and fluorescence lifetime measurement, as well as isothermal titration microcalorimetry. From the induced circular dichroism (ICD) and 2D NMR spectra, it is deduced that the D-Trp moiety of 2 attached to beta-CD is more deeply self-included in the cavity than that of the antipodal L-Trp moiety of 1, indicating appreciably enantioselective binding of the chiral sidearm by beta-CD. Interestingly, the original difference in conformation between 1 and 2 led to quite a large difference in affinity toward DCA, giving 3.3 times higher binding ability for 2 than for 1. Thermodynamically, the inclusion complexation of 1 and 2 with bile acids was entirely driven by favorable enthalpy change (DeltaH degrees) with accompanying negative entropy change (DeltaS degrees). The stronger binding of bile acids by L/D-Trp-beta-CD is attributable to higher enthalpic gains. The combined use of the calorimetric and NMR ROESY spectral examinations revealed the correlation between the thermodynamic parameters and the role of sidearm conformation in modified beta-cyclodextrins.