Hydrolysis, cyclodextrin catalysis

Figure 5.5. The cyclodextrin catalysis of phenylester hydrolysis (Saenger, 1980).

In nucleophilic catalysis, an anion of a secondary hydroxyl group of the cyclodextrin (CD-OH) attacks at the electrophilic center of the ester substrate included in the cavity of the cyclodextrin, resulting in the formation of acyl-cyclodextrin (2) together with the release of the leaving group (see Scheme 1 for ester hydrolysis). The catalysis is completed by the regeneration of the cyclodextrin through the hydrolysis of 2. 

An artificial metalloenzyme (26) was designed by Breslow et al. 24). It was the first example of a complete artificial enzyme, having a substrate binding cyclodextrin cavity and a Ni2+ ion-chelated nucleophilic group for catalysis. Metalloenzyme (26) behaves a real catalyst, exhibiting turnover, and enhances the rate of hydrolysis of p-nitrophenyl acetate more than 103 fold. The catalytic group of 26 is a -Ni2+ complex which itself is active toward the substrate 1, but not toward such a substrate having no metal ion affinity at a low catalyst concentration. It is appearent that the metal ion in 26 activates the oximate anion by chelation, but not the substrate directly as believed in carboxypeptidase. 

Numerous examples of modiflcations to the fundamental cyclodextrin structure have appeared in the literature.The aim of much of this work has been to improve the catalytic properties of the cyclodextrins, and thus to develop so-called artificial enzymes. Cyclodextrins themselves have long been known to be capable of catalyzing such reactions as ester hydrolysis by interaction of the guest with the secondary hydroxyl groups around the rim of the cyclodextrin cavity. The replacement, by synthetic methods, of the hydroxyl groups with other functional groups has been shown, however, to improve remarkably the number of reactions capable of catalysis by the cyclodextrins. For example, Breslow and CO workersreported the attachment of the pyridoxamine-pyridoxal coenzyme group to beta cyclodextrin, and thus found a two hundred-fold acceleration of the conversion of indolepyruvic acid into tryptophan. 

Preparation and catalysis of disubstituted cyclodextrin as an excellent enzyme model is demonstrated by the RNAase model reported by Breslow et al. (68, 83). The enzyme models 10 and II, derived from 1, show a bellshaped pH versus rate profile for the hydrolysis of the cyclic phosphate of 4-terf-butylcatechol, indicating the cooperative catalysis by two imidazole groups (Fig. 21). The reactions catalyzed by 10 and II give exclusively 12 and 13, respectively. This interesting specificity indicates that the geometry of the P—O bond cleavage is quite different from each other. Another interesting enzyme-like kinetic behavior that these hosts exhibited is successful demonstration of the so-called bell-shaped pH profile. 

Alkaline hydrolysis rates of a series of thiophenyl 4-X-benzoates (47 X = H, Me, N02) was significantly enhanced in the presence of cyclodextrins (CDs), and this was attributed to strong binding of the benzoyl moiety within the CD cavity and covalent catalysis by secondary hydroxy groups of the CDs (48).63 The effect of MeCN and MeOH on the alkaline hydrolysis of acetylsalicylic acid in aqueous micellar solutions was reported.64 Butylaminolysis of p-nitrophenyl acetate in chlorobenzene in the presence of different kinds of phase-transfer catalysts (crown ethers and gly-mes) supported the existence of a novel reaction pathway exhibiting a first-order dependence on the concentration of the phase-transfer catalyst and a second-order... 

Cyclodextrins and their derivatives are already known to catalyse an enormous variety of biochemical and non-biochemical transformations. The basis of the catalysis by native (unmodified) cyciodextrins is the positioning of the reactive secondary hydroxyl groups directly at the entrance to the molecular cavity. One of the most effective reactions catalysed by cyclodextrins is the hydrolysis of aryl and phosphate esters (esterase activity). For example, the rate of hydrolysis of p-nitrophenol esters is increased by factors of up to 750 000 by /TCD. The mechanism of action of the cyclodextrin is shown in Scheme 12.2.1... 

Catalyst 17 is effective only with substrates that can bind to the metal ion, so we attached it - coordinated as its Ni2+ derivative - to the secondary face of a-cyclodextrin in catalyst 21 [102]. This was then able to use the metallo-oxime catalysis of our previous study, but with substrates that are not metal ligands, simply those that bind hy-drophobically into the cyclodextrin cavity. As hoped, we saw a significant rate increase in the hydrolysis of p-nitrophenyl acetate, well beyond that for hydrolysis without the catalyst or for simple acetyl transfer to the cyclodextrin itself. Since there was full catalytic turnover, we called compound 21 an artificial enzyme - apparently the first use of this term in the literature. The mechanism is related to that proposed earlier for the enzyme alkaline phosphatase [103]. 

Artificial enzymes with metal ions can also hydrolyze phosphate esters (alkaline phosphatase is such a natural zinc enzyme). We examined the hydrolysis of p-nitro-phenyfdiphenylphosphate (29) by zinc complex 30, and also saw that in a micelle the related complex 31 was an even more effective catalyst [118]. Again the most likely mechanism is the bifunctional Zn-OH acting as both a Lewis acid and a hydroxide nucleophile, as in many zinc enzymes. By attaching the zinc complex 30 to one or two cyclodextrins, we saw even better catalysis with these full enzyme mimics [119]. A catalyst based on 25 - in which a bound La3+ cooperates with H202, not water - accelerates the cleavage of bis-p-nitrophenyl phosphate by over 108-fold relative to uncatalyzed hydrolysis [120]. This is an enormous acceleration. 

Cyclodextrin bis-imidazole catalyzes enolization by a bifunctional mechanism in which the ImH+ is hydrogen-bonded to the carbonyl oxygen while the Im removes the neighboring methyl proton (cf. 50). As expected from this, there was a bell-shaped pH vs. rate profile for the process. In the transition state two protons will move simultaneously, as in the hydrolysis reaction described above. Thus we indeed have a powerful tool to determine the geometric requirements for simultaneous bifunctional catalysis, a tool that could be of quite general use. 

The catalytic activity of artificial chymotrypsin in the hydrolysis of m-tert-butylphenyl acetate (k = 2.8xl02 s 1, KM = 13xl05M) was found to be close to the activity of chymotrypsin in the hydrolysis of p-nitrophenyl acetate (k,.at = l.lxlO2 s 1, KM = 4x105M). Another example of mimicking enzyme catalysis by P-cyclodextrin is general acid-base-catalyzed hydrolysis and nitrosation of amines by alkyl nitrites (Iglesias, 1998). 

Yano, H., Hirayama, F., Arima, H., Uekama, K. Hydrolysis behavior of prednisolone 21-hemisuccinate/ 3-cyclodextrin amide conjugate involvement of intramolecular catalysis of amide group in drug release. Chem. Pharm. Bull. 2000, 48, 1125-1128. 

Classically, the bell-shaped dependence of rate of the enzymic reaction on pH has been attributed to general acid and base catalysis by the two histidine residues in the active site, His-12 and His-119 (66). Support for this explanation based on the kinetic properties of a model system was first provided by an observation by Breslow and co-workers that 8-cyclodextrin functionalized with two imidazole groups will catalyze the 1,2-cyclic phosphate of 4-rert-butylcatechol (67). The dependence of hydrolysis rate on pH mimics that of RNase A, and this behavior demonstrates that the presence of two imidazole functional groups on a nonionizable framework is the simplest kinetic mimic of the enzyme. 

General base catalysis involves enhancement of the nucleophilicity of the water molecule by the abstraction of a proton. In the cyclodextrin case, general base catalysis was found for the first time in the hydrolysis of trifluoroethyl ester of p-nitrobenzoate [13], since no expected covalent intermediate is formed in the course of the reaction and since there is a 1,7-fold DjO effect. 

There have been no examples of reactions proceeding via general acid catalysis alone by cyclodextrin. In the hydrolysis of trifluoroacetanilide, however, general acid catalysis enhances the cleavage of the tetrahedral intermediate (5) formed by nucleophilic attack by a secondary alkoxide ion. General acid catalysis serves to convert the leaving group from an extremely unstable anion of aniline to a stable neutral aniline molecule (Scheme 2) [14]. 

Phosphate esters can be cleaved by template catalysts, especially those with cyclodextrin binding groups and linked catalytic groups. Catalysis of the hydrolysis of a bound cyclic phosphate by ribonuclease mimics has been extensively studied [92-98], as has catalysis by enzyme mimics carrying bound metal ions [99-102]. 

The participation of the secondary alcoholic functions to the increase in association constant of the ketone recalls the basic catalysis of various esters in aqueous medium. Indeed, this hydrolysis is clearly assisted by j6-CD and Tee et al. present a mechanism in which an ionized hydroxy group of the cyclodextrin acts as a nucleophile towards the guest ester [56]. In several cases the kinetics studies are consistent with a hydrolysis process in which a complex formed from the ester and two molecules of cyclodextrin is involved [57]. 

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