Revisiting the internal conformational dynamics and solvation properties of cyclodextrins

Molecular View into the Cyclodextrin Cavity: Structure and Hydration

We find, through atomistic molecular dynamics simulation of native cyclodextrins (CDs) in water, that although the outer surface of a CD appears like a truncated cone, the inner cavity resembles a conical hourglass because of the inward protrusion of the glycosidic oxygens. Furthermore, the conformations of the constituent α-glucose molecules are found to differ significantly from a free monomeric α-glucose molecule. This is the first computational study that maps the conformational change to the preferential hydrogen bond donating capacity of one of the secondary hydroxyl groups of CD, in consensus with an NMR experiment. We have developed a simple and novel geometry-based technique to identify water molecules occupying the nonspherical CD cavity, and the computed water occupancies are in close agreement with the experimental and density functional theory studies. Our analysis reveals that a water molecule in CD cavity loses out about two hydrogen bonds and remains energetically frustrated but possesses higher orientational degree of freedom compared to bulk water. In the context of CD-drug complexation, these imply a nonclassical, that is, enthalpically driven hydrophobic association of a drug in CD cavity.

The hydrophobic effect revisited--studies with supramolecular complexes imply high-energy water as a noncovalent driving force

Traditional descriptions of the hydrophobic effect on the basis of entropic arguments or the calculation of solvent-occupied surfaces must be questioned in view of new results obtained with supramolecular complexes. In these studies, it was possible to separate hydrophobic from dispersive interactions, which are strongest in aqueous systems. Even very hydrophobic alkanes associate significantly only in cavities containing water molecules with an insufficient number of possible hydrogen bonds. The replacement of high-energy water in cavities by guest molecules is the essential enthalpic driving force for complexation, as borne out by data for complexes of cyclodextrins, cyclophanes, and cucurbiturils, for which complexation enthalpies of up to -100 kJ mol(-1) were reached for encapsulated alkyl residues. Water-box simulations were used to characterize the different contributions from high-energy water and enabled the calculation of the association free enthalpies for selected cucurbituril complexes to within a 10% deviation from experimental values. Cavities in artificial receptors are more apt to show the enthalpic effect of high-energy water than those in proteins or nucleic acids, because they bear fewer or no functional groups in the inner cavity to stabilize interior water molecules.

Hydration properties of α-, β-, and γ-cyclodextrins from molecular dynamics simulations

Atomistic molecular dynamics (MD) simulations of α-, β-, and γ-cyclodextrins (ACD, BCD, and GCD) in aqueous solutions have been performed. Detailed analyses were carried out to compare the microscopic properties of water confined within the cavities of these macromolecules and in the hydration layers around them. It is noticed that reduced tetrahedral ordering of water in and around the CD molecules are associated with their restricted motions. Interestingly, unlike the translational motions, the rotational motions of cavity water molecules are found to be highly dependent on cavity dimensions. Additionally, it is found that severely hindered rotational motion of cavity water molecules is the origin of drastically restricted structural relaxation of hydrogen bonds involving those water molecules. It is demonstrated that the geometrical constraints within the cavities of the CD molecules enhance the rate of reformation of broken hydrogen bonds, thereby resulting in rapid establishment of the breaking and reformation equilibria for hydrogen bonds involving cavity water molecules.