Cycloamyloses inclusion complexes

Cycloamylose forms inclusion complexes stereoselectively with the enantiomers of isopropyl methylphosphinate (124) from which it was possible to isolate one enantiomer with an optical purity of 66%. The absolute configuration of menthyl methylphosphinate has been revised to the opposite of that previously assigned. 

A final source of evidence for the formation of inclusion complexes in solution has been derived from kinetic measurements. Rate accelerations imposed by the cycloamyloses are competitively inhibited by the addition of small amounts of inert reagents such as cyclohexanol (VanEtten et al., 1967a). Competitive inhibition, a phenomenon frequently observed in enzymatic catalyses, requires a discrete site for which the substrate and the inhibitor can compete. The only discrete site associated with the cycloamyloses is their cavity. 

The conclusions of the preceding discussion can be briefly summarized as follows. The formation of inclusion complexes in both the crystalline state as well as in solution has been convincingly demonstrated by spectral and kinetic techniques. Whereas the crystalline complexes are seldom stoichiometric, the solution complexes are usually formed in a 1 1 ratio. Although the geometries within the inclusion complexes cannot be accurately defined, it is reasonable to assume that an organic substrate is included in such a way to allow maximum contact of the hydrophobic portion of the substrate with the apolar cycloamylose cavity. The hydrophilic portion of the substrate, on the other hand, probably remains near the surface of the complex to allow maximum contact with the solvent and the cycloamylose hydroxyl groups. The implications of inclusion complex formation for specificity and catalysis will be elucidated in subsequent sections of this article. 

It is revealing to note that inclusion complexes are apparently formed only in aqueous solution. Attempts to induce precipitation of cycloamylose adducts from organic solvents have failed (Schlenk and Sand, 1961 Lach and Chin, 1964a). This observation suggests that water is intimately involved in the association process or, more accurately, that in water solvation of the cycloamylose-substrate adduct is energetically more favorable... 

With the realization that the cycloamyloses form stable monomolecular inclusion complexes in solution came the idea that the inclusion process might affect the reactivity of an organic substrate. This idea was initially pursued by Cramer and Dietsche (1959b) who discovered that the rates of hydrolysis of several mandelic acid esters are enhanced by the cycloamyloses. More recently, the inclusion process has been shown to exert both accelerating and decelerating effects on the rates of a variety of organic reactions. The remainder of this article will be devoted to a discussion of these reactions in an attempt to review, compare, and unify the many intriguing facets of cycloamylose catalysis. 

Values of /c2, the maximal rate constant for disappearance of penicillin at pH 10.24 and 31.5°, and Ka, the cycloheptaamylose-penicillin dissociation constant are presented in Table VII. Two features of these data are noteworthy. In the first place, there is no correlation between the magnitude of the cycloheptaamylose induced rate accelerations and the strength of binding specificity is again manifested in a rate process rather than in the stability of the inclusion complex. Second, the selectivity of cycloheptaamylose toward the various penicillins is somewhat less than the selectivity of the cycloamyloses toward phenyl esters—rate accelerations differ by no more than fivefold throughout the series. As noted by Tutt and Schwartz (1971), selectivity can be correlated with the distance of the reactive center from the nonpolar side chain. Whereas the carbonyl carbon of phenyl acetates is only two atoms removed from the phenyl ring, the reactive center... 

In contrast to the reactions of the cycloamyloses with esters of carboxylic acids and organophosphorus compounds, the rate of an organic reaction may, in some cases, be modified simply by inclusion of the reactant within the cycloamylose cavity. Noncovalent catalysis may be attributed to either (1) a microsolvent effect derived from the relatively apolar properties of the microscopic cycloamylose cavity or (2) a conformational effect derived from the geometrical requirements of the inclusion process. Kinetically, noncovalent catalysis may be characterized in the same way as covalent catalysis that is, /c2 once again represents the rate of all productive processes that occur within the inclusion complex, and Kd represents the equilibrium constant for dissociation of the complex. 

Finally, we come to enzyme models. D. W. Griffiths and M. L. Bender describe the remarkable catalytic property of certain cycloamyloses which act through formation of inclusion complexes, and in this respect recall the clefts containing the active sites in enzymes such as lysozyme and papain. 

Cycloamyloses have been separated by h.p.l.c. on a /u-Bondapak-carbohydrate column using acetonitrile-water mixtures as eluant. The molecular dynamics of the inclusion complexes formed between cyclohexa-amylose and some aromatic amino-acids and dipeptides have been studied by n.m.r. spectroscopy. The forces binding the complexes were found to be weak. The c.d. spectra of cyclohepta-amylose which had been complexed with 2-substituted naphthalenes were measured at various concentrations of cyclohepta-amylase and temperatures between 10-70 C. The complex with 2-naphthoxyacetic acid showed 1 1 stoicheiometry. The molar ellipticity and thermodynamic parameters were determined and enthalpy and entropy ranges calculated. The correlation was explained by a cyclohepta-amylose guest molecule interaction where the guest molecule was highly solvated. The induced c.d. spectra of cyclohepta-amylose complexes with substituted benzenes confirmed that an axial inclusion... 

Inclusion complex formation with cycloamylose has been shown to be a technically simple process by which the ratio of saturated to unsaturated fatty acids may be altered under mild conditions in fatty acid mixtures obtained from natural sources. Complex formation was demonstrated by the results of X-ray diffraction spectra, the low solubility of the formed complexes, and the protection of the complexed fatty acids against atmospheric oxidation. 

Closely related to the helical amyloses are the cycloamyloses which are produced by enzymatic degradation of amylose by glucosyl transferase enzymes. Cycloamyloses, better known as cyclodex-trins (Table 10.5), have been characterised with 6,7,8 and 9 glucose units (a,p,Y,8, varieties). The p form (10.28a) is the most studied and most used to date. Like the helical amyloses, the toroidalshaped cyclodextrins can form inclusion complexes with various molecules. These are usually formed by adding guest molecules to saturated solutions of the latter. 

FIGURE 10.9 Cyclodextrin inclusion complexes. Toroidal units arranged with axes (a) parallel, (b) perpendicular to Zr(HP04)2 H2O layers and (c) nucleotide chain inside cycloamylose toroidal units. 

Cycloamyloses (cyclic a-l,4-linked oligomers of D-glucose) have a toroidal or doughnuf -shaped structure. The primary hydroxy groups are located on one side of the torus while the secondary ones lie on the other side. Relative to water the interior of the cycloamylose torus is apolar. The catalytic properties of cycloamyloses depend on the formation of inclusion complexes with the substrate and subsequent catalysis by either the hydroxy, or other groups, located around the circumference of the cavity (Komiyama and Bender, 1984 Page and Crombie, 1984). 

The cyclodextrins (cycloamyloses) are torus-shaped molecules that can form crystalline inclusion compounds, recently attracting much attention as enzyme-site models. Their history has been seen in three phases. From 1891 to 1935 they were known as natural products, but with no recognition of their exact chemical structure. This recognition emerged in the second period, to about 1970, when most of their characteristics were also elucidated. The period from 1970 to the present has seen considerable research into their industrial use and production.239 Their inclusion compounds or complexes have found employment in such diverse fields as explosives, insecticides, pharmaceutical products, rust-prevention agents, and even baking powder. 

The cycloamyloses are probably best known for the noncovalent complexes they form by inclusion of organic molecules within the cycloamylose annuli. The formation of such inclusion compounds, including gas adducts, and their interesting properties have been reviewed. Reports in this field include the partial resolution of chiral sulfoxides and isopropyl methylphosphinate by stereospecific inclusion into cyclo-... 

A n.m.r. spectroscopic study of 1 1 complexes between cycloamyloses and azo dyes having a naphthalene nucleus has been reported.Almost all carbons of the guest molecules showed low field shifts on inclusion, consistent with n.m.r. spectroscopic results. The shift may be due not to a simple steric polarization but to hydrophobic interaction with the host molecule, whose cavity uniformly affects the guest molecule as if it is dissolved in a hydrophobic solvent. The host molecule was shown to be deformed by the bulky naphthalene nucleus at the glycosidic linkage rather than at the small diameter side of the cavity. 

The X-ray structure of a cyclohexa-amylose-kryptin complex has been reported and a mechanism proposed for the inclusion process.Cycloamyloses have been shown to form complexes with nitroxyl radicals the complexes were examined by e.s.r. spectroscopy. ... 

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