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Hydrophobic Effect and Beyond

Cyclodextrins (CDs) are the best known and most frequently investigated supramolecular hosts for hydrophobic species in aqueous solutions, characterized by their high water solubility and solubilizing ability. The latter is found to be of great interest in pharmaceutical industry and formulation development, implying the importance of thermodynamic investigations of the guest(CD) complex formation, particularly in storage conditions. During the recent years, cucurbit[n]urils (CBn) were recognized as outstanding receptors for non-polar species characterized by significantly higher affinity and selectivity, with complex stabilities even surpassing the one of biotin-avidin pair (log K > 15). The cyclodextrin and cucurbituril inclusion complex formation is attributed to favorable desolvation of the guest and the host cavity. In order to investigate the thermodynamical background of hydrophobically driven complexation in detail, the binding of a variety of hydrophobic and amphiphilic guests with α-, β-, γ-CD and CB7 was explored in several structured solvents (water, formamide, ethylene glycol) in a wide temperature range by means of isothermal microcalorimetry. The complex structure was investigated by NMR spectroscopy which revealed interesting information concerning/regarding the inclusion kinetics. Thus obtained results provided valuable insights into solvent and structural factors governing the complexation process, as well as its temperature dependence. For most of the explored systems, the temperature dependence of ΔrG° was weak (or negligible), which was the consequence of the enthalpy-entropy compensation. The finding is in line with the classical rationale of the hydrophobic effect at lower temperatures, and the non-classical explanation at higher ones.

Hydrophobic effect ; solvophobic effect ; inclusion complex ; complexation thermodynamics ; cyclodextrins ; cucurbit[7]uril

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