Cyclodextrins ribonuclease mimics

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]. 

R. Breslow, P. Bovy, C. Lipsey Hersh, Reversing the selectivity of cyclodextrin bisimidazole ribonuclease mimics by changing the catalyst geometry, J. Am, Chem. Soc., 1980, 102, 2115-2117. 

We applied this test to our ribonuclease mimic, the 6A,6B isomer of cyclodextrin bisimidazole, cleaving the bound cyclic phosphate 21. We found that there was indeed a square dependence of the kinetic isotope effect on the mole fraction of deuterium in the water solvent, and interestingly the values of the isotope effect for the two protons in flight were almost identical with those that had been seen with the enzyme itself and its normal substrate.As described above, the protonation in the model system involves an imidazolium ion putting a proton on the substrate phosphate anion as the imidazole delivers a water molecule to the phosphorus. 

Ribonuclease A is a member of a group of enzymes that cleave RNA using general acid-base catalysis without a metal ion in the enzyme. In ribonuclease A, such catalysis is performed by two imidazoles of histidine units, one as the free base (Im) and the other, protonated, as the acid (ImH+). To mimic this in an artificial enzyme, we prepared (3-cyclodextrin bis-imidazoles 41 [124]. The first one was a mixture of the... 

Cyclodextrin derivatives can act as catalysts, not just as reagents. We are focussing on an attempt to develop a mimic for the enzyme ribonuclease A that incorporates the functional groups of the enzyme, binds an appropriate substrate, and then catalyzes the hydrolysis of such a substrate by a mechanism used by the enzyme itself. Although we want to imitate the mechanism, the selectivity, and the rate of the enzyme, our systems do quite well only with the first two points. They are still quite slow compared with the real enzyme. 

So far, cyclodextrin (CD 4) derivatives have been the most successful artificial enzymes based on small synthetic host molecules. Since CD derivatives form inclusion complexes with various hydrophobic molecules, they have been utilized as binding pockets." " Examples are illustrated by 5 and 6. The CD derivative indicated in 5 is a mimic of ribonuclease in which the two histidyl imidazoles of the active site are believed to act as a general acid and a general base catalyst. The CD dimer indicated in acts as an artificial metalloesterase manifesting selectivity toward esters with two hydrophobic groups. 

A different kind of selectivity has been observed with cyclodextrin bisimidazoles. These species were synthesized as mimics of the enzyme ribonuclease, and indeed they do perform the cleavage of certain bound phosphate esters with a mechanism closely related to that of the natural enzyme [26]. Of course in the natural enzyme there is considerable selectivity, including regioselectivity. For instance, the natural enzyme can cleave a cyclic phosphate ester between C-2 and C-3 of a ribose unit in such a way as to leave the phosphate group entirely on C-3. Ordinary chemical hydrolysis of such a molecule would lead to a mixture of C-2 and C-3 phosphates. Our cyclodextrin bisimidazole is able to imitate such selectivity. 

Even before this work was started, we had prepared a mimic 20 of ribonuclease consisting of j8-cyclodextrin with two imidazole rings attached to the C-6 primary methylenes of two different glucose residues of the cyclodextrin.The substrate was not an RNA, but a cyclic phosphate 21 whose cleavage mimicked the cleavage of the 2 -3 cyclic phosphate of RNA. This substrate, 4-t-butylcatechol cyclic phosphate, bound well into the cyclodextrin in water, and the catalyst hydrolyzed the cyclic phosphate with a bell-shaped pH vs rate profile, showing that both the imidazole and the imidazolium ion were playing a catalytic role. 

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