Cyclodextrin-substrate complexation

Also of interest is the model developed by Breslow and Overman (183) where a metal ion is introduced into a cyclodextrin-subtrate complex. With p-nitrophenyl acetate as substrate the presence of Ni(II) in the cyclodextrin-substrate complex results in a further increase in rate of hydrolysis by a 1000-fold. 

From these observations, we have noticed the similarity of the simple lattice inclusions to the more sophisticated assemblies of molecules (e.g. cyclodextrins 76 and proteins 78 where the formation of H-bonded loops was first detected and described. Conclusively the motive for the formation of simple inclusion crystals and of more complex associates between high and low molecular weight compounds, such as enzyme-substrate complexes, can be traced back to the same source. 

As we saw in the previous sections, inclusion compounds have many structural properties which relate them to other systems based on the hierarchy of non-bound interactions, like enzymes or enzyme-substrate complexes. As a matter of fact, most of the so-called artificial enzymes are based on well-known host molecules (e.g. P-cyclodextrin) and are designed to act partly on such bases 108>109). Most of these models, however, take advantage of the inclusion (intra-host encapsulation) phenomena. Construction of proper covalently bound model molecules is a formidable task for the synthetic chemistuo>. Therefore, any kind of advance towards such a goal is welcomed. 

An important advantage of the inclusion complexes of the cyclodextrins over those of other host compounds, particularly in regard to their use as models of enzyme-substrate complexes, is their ability to be formed in aqueous solution. In the case of clathrates, gas hydrates, and the inclusion complexes of such hosts as urea and deoxycholic acid, the cavity in which the guest molecule is situated is formed by the crystal lattice of the host. Thus, these inclusion complexes disintegrate when the crystal is dissolved. The cavity of the cyclodextrins, however, is a property of the size and shape of the molecule and hence it persists in solution. In fact, there is evidence that suggests that the ability of the cyclodextrins to form inclusion complexes is dependent on the presence of water. Once an inclusion complex has formed in solution, it can be crystallized however, in the solid state, additional cavities appear in the lattice, as in the case of the hosts previously mentioned, which enable the inclusion of further guest molecules. ... 

Natural enzymes use the hydrophobic effect as a binding force in forming the enzyme-substrate complex. Artificial enzymes can be used to bind substrates and enhance reactivities in water (Breslow, 1995). Cyclodextrins, which are cyclic compounds composed of glucose units, can be used as the artificial enzymes (Bender and... 

Many plant secondary metabolites produced by cell cultures and substrates used in culture media are often hydrophobic and have low solubility in aqueous medium. Improving the production of metabolites in the plant cell system has also been attempted by enhancing the solubilities of metabolites by using the cyclodextrin-guest complex formation. There are two major approaches, one involves the improvement of substrate solubility and the other improves metabolite. 

Equations are derived which take into account the formation of cyclodextrin to substrate complexes other than simple one to one host guest associations. An equation is also derived which describes the binding of a mono-protic species in which either its ionized or unionized form could bind to one or two cyclodextrin molecules. Because multiple binding constants are difficult to evaluate graphically, a non-linear least squares computer program is utilized. The approach works equally well for the determination of binding constants in micellar media. 

Whereas P- and y-cyclodextrins adopt a round shape in aqueous solution which does not significantly change upon complex formation, a-cyclodextrin is somewhat collapsed and opens to a round shape when a guest molecule enters. This is comparable to the induced fit proposed for enzyme-substrate complexation and is discussed in some detail in Box 18.2. ... 

Complex formation with substrate (S) can proceed directly, by route A, to yield a relaxed a-cyclodextrin with all six 0(2) -0(3 ) hydrogen bonds engaged (as in the a-cyclodextrin methanol complex, Fig. 18.8), or the macrocycle can first open up to a relaxed form, route B, with the enclosed water molecules disordered over several sites so as to fill, statistically, the 5 A diameter a-cydodextrin cavity (as observed in the a-cyclodextrin 7.57H20 crystal struc- ture, Fig. 18.6 b). The water is now in an activated form and can be replaced directly by the j substrate. In a third possible mechanism, route C, the substrate aggregates first at the periphery of tense a-cyclodextrin, and in a second step replaces the two enclosed water molecules. 

Savage, H. Water structure in vitamin B12 coenzyme crystals. II. Structural characteristics of the solvent networks. Biophys. J. 50, 967-980 (1986). Stezowski, J. J. Molecular motion in host-substrate complexes the / -cyclodextrin N-acetylphenylalanine methyl ester system. Trans. Amer. Cryst. Assn. 20, 73-82 (1984). 

The cause of the scatter is the non-systematic influence of the substituent on the microscopic environment of the transition structure. The linear free energy relationship between product state XpyH (Equation 22) and the transition structure (Xpy. .. PO32 . . . isq) will be modulated by second-order non-systematic variation because the microscopic environment of the reaction centre in the standard (XpyH ) differs slightly from that (Xpy-PO ) in the reaction under investigation giving rise to specific substituent effects. These effects are mostly small. An unusually dramatic intervention of the microscopic medium effect may be found in Myron Bender s extremely scattered Hammett dependence of the reaction of cyclodextrins with substituted phenyl acetates.22 The cyclodextrin reagent complexes the substrate and interacts... 

The ratios of the rate constants of the cleavages of the substrates incorporated in the a-cyclodextrin inclusion complexes to those in the absence of a-cyclodextrin are determined kinetically in the 1 1 (v/v) mixture of pH 8.5 buffer and dimethyl sulfoxide. 

As shown previously, cyclodextrins in their native forms show many features characteristic of enzymes (1) specificity (2) the formation of catalyst-substrate complexes prior to chemical transformation and (3) large accelerations. 

R. Breslow, R Campbell, Selective aromatic substitution within a cyclodextrin mixed complex, J. Am. Chem. Soc., 1969, 91, 3085 R. Breslow, P. Campbell, Selective aromatic substitution by hydrophobic binding of a substrate to a simple cyclodextrin catalyst, Bioorg. Chem., 1971,... 

A palladium complex with cyclodextrin modified with propionitrile and benzoylnitrile groups 73-74 was active in Wacker oxidation of higher 1-alkenes (Experiment 11-4, Section 11.7), and its activity was much higher than the activity of a catalj ic system prepared as a mixture of cyclodextrin and the palladium complex owing to the cooperative substrate binding and to the increase in the stability constant of the catalyst-substrate complex. As in hydroformylation, the catalyst was more active in the reaction with an aromatic substrate, styrene, than with linear alkenes [59,210-211], The catalyst activity depended on the 1-alkene chain length and was maximum for 1-heptene. 

Hydrogen bonding frequently makes an important contribution to fixing the enzyme-metal-substrate complex before enzymic action can take place. Hydrogen bonding is of major importance in stabilising cyclodextrins and many natural polysaccharide structures and their phosphorylated derivatives (Chapter 10.1). 

Formylations of phenol, resorcinol and indole, dichloromethylations of 4-methylphenol and 5,6,7,8-tetrahydro-2-naphthol, carboxylation of phenol, and allylation of 2,4,6-trimethylphenol proceed site-selec-tively in high yields by using 3-cyclodextrin as catalyst. The formation of ternary inclusion complex composed of cyclodextrin, substrate, and dichlorocarbene, trichloromethyl cation or allyl cation in the reaction mixture is an important factor of the site-selective reactions. The cyclodextrin is also effective by limiting the molecular size of the reaction intermediate. 

Maltopentaose competitively inhibits the degradation of y-cyclodextrin with Asp. oryzae a-amylase. Due to its similar structure this oligomer occupies the substrate-bihd-ing sites of the enzyme and thus the dissociation constant of the enzyme-substrate complex is changed the value is increasing and remains constant. 

B) The empty a-cyclodextrin molecule gains activation energy, i.e. it transforms into a molecule of almost hexagonal, low energy conformation while the included water molecules pick up energy and become disordered (like the krypton atom in the a-cyclodextrin-Kr complex). The disordered water molecules are replaced by the substrate. 

This scheme for an inclusion mechanism can explain why a-cyclodextrin forms complexes with such a diversity of substrate molecules The driving force is independent of the nature of the substrate. The only requirement is that the substrate molecule should be small enough to fit into the a-cyclodextrin cavity. Of course the other a-cyclodextrin-substrate interactions mentioned above will also play a role, depending on the nature of the substrate. The main and general energy contribution, however, will come from the a-cyclodextrin itself. In agreement with this statement are also the kinetic data presented in Table VIII [21]. 

Fig. 18. Schematic representation of the a-cyclodextrin - substrate inclusion process. The empty a-cyclodextrin molecule in the upper left hand corner corresponds to the a-cyclodextrin (H20)2 complex found in the crystal structure [12, 13]. Only four of the six 0(2) 0(3) interglucosidic hydrogen bonds are formed and the molecule is in a tense state with high conformational and low hydrogen bonding energy. Upon adduct formation via routes A, B or C it goes into a relaxed , com-plexed state with all 0(2) 0(3) hydrogen bonds formed and with low conformational energy.

As a final example we consider noncovalent molecular complex formation with the macrocyclic ligand a-cyclodextrin, a natural product consisting of six a-D-glucose units linked 1-4 to form a torus whose cavity is capable of including molecules the size of an aromatic ring. Table 4-3 gives some rate constants for this reaction, where L represents the cyclodextrin and S is the substrate ... 

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