Cyclodextrins intramolecular hydrogen bonds

In di- and trisaccharide crystal structures and in cyclodextrins, intramolecular hydrogen bonds have been observed between adjacent monosaccharides, as shown by the representative examples given in Thble 9.3 and shown in Fig. 9.1. If these... 

The ESIPT of 2-(2 -hydroxyphenyl)-4-methyloxazole (HPMO) (27) has been explored by Douhal and co-workers [166] for its probe characteristics in a variety of organized media which include cyclodextrin, calixarene, micelle, and HSA. The incorporation of HPMO into hydrophobic cavities in an aqueous medium involves the rupture of its intermolecular hydrogen bond to water and formation of an intramolecular hydrogen bond in the sequestered molecule. Upon excitation (280-330 nm) of this entity, a fast intramolecular proton-transfer reaction of the excited state produces a phototautomer (28), the fluorescence of which (Xm = 450 170 nm) shows a largely Stokes-shifted band. Because of the existence of a twisting motion around the C2—C bond of this phototautomer, the absorption and emission properties of the probe depend on the size of the host cav-... 

Table 9.5 b. The intramolecular hydrogen bonds in y5-cyclodextrin undecahydrate at 120 K are all of the three-center type, one is a flip-flop. Data taken from a neutron diffraction study [455]. (For atom numbering see Part IIB, Chap. 18)... 

Intramolecular hydrogen bonding stabilizes the macrocycle. As in a-maltose and in -maltose monohydrate, the separation of the 0(2)H and 0(3)H groups on adjacent glucose residues in the cyclodextrin molecules is such that intramolecular interglucose hydrogen bonds can be formed. 

Intramolecular Hydrogen Bonds in the a-Cyclodextrin Molecule Are Variable - the Induced-Fit Hypothesis... 

A simple calixarene built up with four phenol units linked via methylene bridges is shown in Fig. 10 (substitution of the methylene junctions with S atoms gives rise to the class of thiocalixarenes). Cooperative networks of intramolecular hydrogen bonds (circular array of hydrogen bonds, observed also in cyclodextrins) play a capital role in the cavity shape as well as in the conformational features of the macrocyclic skeleton [291-293]. Encapsulation of other molecules in the cavity is also controlled by hydrogen bonds [294]. 

The effect of the permethylation on the macrocyclic conformation appears most clearly in the change of 0(2) 0(3 ) distances and tilt-angles, The average 0(2) 0(3 ) distances of permethylated cyclodextrins are about 0.6 A larger than those of parent cyclodextrins. In a- and 3-cyclodextrin, secondary hydroxyl groups form intramolecular 0(2) 0(3 ) hydrogen bonds, as indicated by the 0(2) 0(3 ) distances of 2.79-2.89 A. These intramolecular hydrogen bonds may maintain the round macrocyclic... 

Cyclodextrins are a class of chiral, cyclic oligosaccharides thaf have moleculesized cavities. They are preorganised and have a defined bowl shape fhaf is held together by an intramolecular hydrogen bonding network. Cyclodextrins are the... 

These intramolecular hydrogen bonds modulate the solubility of cyclodextrins in water (145 gl" for a-cyclodextrin 1, 18.5 gl for fS-cyclodextrin 2, and 232 gH for y-cyclodextrin 3) [3]. The structure of fi-cyclodextrin 2 is particularly favorable to the intramolecular hydrogen bonds, which explains its particularly poor solubihty in water. a-Cyclodextrin 1 is partially distorted, and only four hydrogen bonds can... 

Kim YH, Cho DW, Yoon M, Kim D (1996) Observation of hydrogen-bonding effects on twisted intramolecular charge transfer of p-(N, N-diethylamino) benzoic acid in aqueous cyclodextrin solutions. J Phys Chem 100(39) 15670-15676... 

Fig. 18.5a-c. Hydrogen-bonding structures in a-cyclodextrin hydrates, a a-Cyclodextrin 6H20, form I b a-cyclodextrin 6 0, form II c a-cyclodextrin 7.57 H20 , intramolecular. Data in a are from neutron and in b, c from X-ray analyses. In c covalent O-H bonds are normalized to 0.97 A. 

Of the 7.57 water molecules per asymmetric unit, four are fully ordered and one, W(5), shows two positions at 0.64 and 0.36 occupation. These water molecules are located in intermolecular voids, and the remaining 2.57 waters are statistically distributed over four sites in the a-cyclodextrin cavity (see Fig. 18.6 b). Associated with this disorder in the a-cyclodextrin cavity is a round shape of the macrocycle, where in contrast to the other two a-cyclodextrin hydrate forms, all six intramolecular interglucose 0(2) 0(3 ) hydrogen bonds are formed. Because hydrogen atoms attached to the disordered water molecules in the 7.57 hydrate could... 

Fig. 18.7a-e. Cyclic and chain-like hydrogen-bonding patterns O-H- -O observed in a a-cyclo-dextrin-6H20, form I b a-cyclodextrin 6 H20, form II , intramolecular c,d,e a-cyclodextrin-7.57 H20. Infinite homodromic chains are marked chain, and cycles are indicated by round arrows with one (homodromic) and two (antidromic) heads. Only atoms of water molecules and of hydroxyl groups are drawn, the former denoted by W and the latter by the nomenclature described in Fig. 18.1. Each of the pictures a to e shows only a section of the respective crystal structure [78, 109, 575]...

The tense and relaxed forms of a-cyclodextrin mimic the induced-fit model proposed for enzymes. In the inclusion complexes of a-cyclodextrin, the macrocycle adopts a round, relaxed shape with all the 0(2),0(3) hydroxyls engaged in intramolecular 0(2)- (3 ) hydrogen bonds (Fig. 18.8) [577]. The average O - - O distance of around 3.0 A is consistent with an H 0 separation of about 2.0 A, indicating relatively weak interactions. This form of a-cyclodextrin persists even when an included guest molecule, such as krypton (3.8 A diameter) is too small to fully occupy the 5.0 A cavity and is disordered over several sites [578], analogous to the gas hydrate structures discussed in Part IV, Chapter 21. 

When a-cyclodextrin is free in aqueous solution, it adopts the tense form found in the two hexahydrate crystal structures (Fig. 18.6 a) one glucose unit is rotated inwards to reduce the size of the a-cyclodextrin cavity so that the enclosed water molecules are held more tightly. The two intramolecular 0(2) 0(3 ) hydrogen bonds to this glucose unit are broken, the other four remain intact. 

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