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chem research paper 1

Directly be ejected into t phase at a rate su cient

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Unformatted text preview: ied addition of water at the Zn. Pentacoordinated Zn co are possible intermediates of this process.6 3 After this has occurred, the second stage of catalysis is the trans proton from the active site to bu ers in bulk solvent. A p the Zn-bound water cannot directly be ejected into t phase at a rate su cient to support the catalyti c tur 106 s 1 6 4 ,6 5 but is transferred in a proton shuttle to a L represents organic nitrogen ligands. For further explanation, see B.6 2 ,6 6 This proton transfer in the second stage of cataly the text. rate-limiting event in the maximal velocity of catalysis a step associated with substantial H/ D kinetic isotope e solution as illust rated andthe dashed line in Figure 4. In other generally applicable6 for all types by mechanism described above are magnitude near 4. 6 There is a pertinent conformationa All of the processes words, if ε is increased, the e ective acidit y of the oxygen acid is when the base B is the imidazole side chain of His64. I increased. The mismatch then disappearsfor water, where ε = 80. structures this side chain of carbonic anhydrases, and the differences between variable zinc contain active site is observed in inward and conformations, corresponding to whether the imid Finall y, we note that for strong acids zwitterionic compl exes are always obtained independent of the permittivit y, as illust rated by His64 is oriented toward or away from the Zn.6 7 , 6 8 In the mthe overlap of the examined. In theline on the left side of imics have been dashed and solid Satcher study, scaffoldingconformation, the protonated base of His64 comes in structural differences were Figure 4. contact with the bulk solvent into which the proton is e In the rst order, Kt of Scheme 3 is independent of p The data point of 5 [open triangle, pKa (AH) = 1, pKa ≈ 6.3] in Figure 4 was obtained here at 180 K using CD2 Cl2 as this reaction no proton exchange with the solvent takes p indirect dependence is possible, for example induced b solvent, exhibiting at this temperature a dielectric permittivity of nation deprotonation processes of nearby amino a ε ≈ 16.3 0 The data point is well-located both on the dashed and the soli d correlation curves. This means that (C6 F5 ) 3 BOH2 chains which alter the local electrostatics. For example behaves in a similar way as a very strong carboxylic acid. In other increase if an anion is created nearby the base B, thus mi examined through their impacts on CO2 hydration rates. Cyclen-Zn2+ and Cyclam-Zn2+ have similar zinc binding constants, but have different Zn2+ coordination environments due to varying pKa values of 8.02 and 9.77 respectively.x The difference between the two conformers is the staggering of the hydrogen atoms around the metal center shown in Figure 4. Through experiments, specifically pH trials, it is seen that the cyclen isomer has a higher rate of CO2 hydration at a lower pH. It is observed that the cyclam isomer does hydrolyze CO2 at a slower rate, just not as effective...
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