Cyclodextrin water molecules

JOC3338, 2003OL3839>. The addition of ot-cyclodextrin (lequiv) increased the yield, but did not influence the translcis-selectivity <1996J(P1)1113>. The higher yield of 79 in aqueous medium was explained as a result of the hydrophobic effect on a reactant encapsulated in a cavity surrounded by a hydrogen-bonded network of water molecules <2001JOC3338>. 

We begin with an excerpt from Environmental Science Technology (excerpt 4B). In a combined R D section, the authors tell us what happened when they coated different types of soil with randomly methylated P-cyclodextrins (RAMEB). Cyclodextrins are highly water-soluble, crystalline sugars their shape (referred to as toroidal) resembles a water pail without a bottom. The outer surfaces of the pail are hydrophilic (water-loving), which accounts for their solubility in water and their ability to attract water molecules. RAMEB alone adsorbs water molecules hence, the authors predicted that RAMEB-coated soils would adsorb more water than their noncoated counterparts. 

Empty ft- (11) and y-cyclodextrins (12) also take normal torus shapes, as revealed by X-ray crystallography, and no significant deformations are observed for these two cyclodextrins when they bind guest molecules. It is noteworthy that all empty cyclodextrins include water molecules in their cavities as shown in Table IV and Fig. 4. Since no water molecules were observed in inclusion complexes of cyclodextrins with organic guest molecules, it is evident that the expulsion of these water molecules in the cyclodextrin cavities is one of the important factors for formation of the inclusion complexes. 

Fig. 4. Schematic representation of water molecules in cyclodextrin cavities (a) a-cyclo-dextrin (Form I) (b) /i-cyclodextrin.

Release of two water molecules from the cavity of a-cyclodextrin (form I) (19) is accompanied not only by the loss of van der Waals interaction (/fj jdw) and hydrogen bonding ( —2 A/fH.bond), but also by the gain of motional freedoms of two water molecules as to translation (2S ans) and three-dimensional rotation (2S ,I(3 D)). At the same time, a change in conformational energy of a-cyclodextrin is involved which is estimated by the use of Allinger s method (49). 

Water molecules in cyclodextrin s cavity (H dw + 2//H bond = —16 kcal mol-1) comprise high-energy water, as suggested by Hingerty and Saenger (9), compared with those in the liquid phase (2 A/f p = —20.92 kcal mol-1). 

Lanthanide complexes of mono- and tetra-amide /1-cyclodextrin derivatives of DOTA have been characterized [140]. The proton NMR spectra of the Eu3+ complexes in methanol-d, show that, while the tetra-amide complex occurs in solution exclusively as a C4-symmetry SAP structure, the mono-amide complex, with less than C4-symmetry, occurs predominantly as two SAP isomers (A/XXXX and Al8885), with the presence of a small amount of the twisted SAP isomer. Luminescence and relaxivity measurements confirm that the Eu3+, Tb3+ and Gd3+ complexes of the eight-coordinate mono-amide ligand possess one bound water molecule, while the tetra-amide complexes have q = 0. The relaxivity of the /LCD mono-amide Gd3+ complex is enhanced when non-covalently bound to a second Gd3+ complex bearing two phenyl moieties (MS-325, AngioMARK , EPIX/Mallinckrodt). 

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