McCurdy, Alison (1995) Cation-pi and polarizability effects in biomimetic catalysis and the design of a photoactive donor-cyclophane-acceptor triad. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-10172007-094703
The role of the cation-[pi] effect and polarizability in ground-state and transition-state stabilization by a cyclophane host P was explored. The family of guests that bind well to this host was expanded to include sulfonium salts and sulfoxides. The catalysis of the demethylation of aryldimethylsulfonium salts with thiocyanate by host P and the related host C was observed. This reaction is a model for the demethylation of S-adenosylmethionine (SAM), a cofactor of methyltransferase enzymes. The effect of aryl substituents on reaction rates was presented in linear Hammett plots for the uncatalyzed reaction in aqueous buffer, the host-catalyzed reaction, and the uncatalyzed reaction in acetonitrile. The data revealed that the cation-[pi] effect alone cannot be responsible for the biomimetic catalysis. Simple polarity effects were ruled out. To explain the catalysis data, the polarizability of the cyclophanes' cavities was invoked as the additional stabilizing factor. Sulfonium-aromatic interactions appear to be present in some methyltransferases, and the cation-[pi] effect and polarizability of the SAM binding site are possibly catalytic mechanisms found in Nature.
Additional work involved the synthesis and study of other cyclophane macrocycles to further explore the cation-[pi] effect. A host PHOS which uses phosphate groups as water-solubilizing groups was designed. This host should have superior solubility properties to host P. It also may be used to quantitatively assess the effect on cation binding affinities of negative charges on the macrocycle. The last synthetic step was unsuccessful. Small organic soluble cyclophanes O and S were synthesized to bind alkali metal cations. Although potassium-aromatic interactions have been invoked to explain selectivity in voltage-gated potassium channels, this kind of interaction was not detected in these cyclophanes.
Finally, progress toward the synthesis of a photoactive donor-cyclophane-acceptor triad has been made. This triad is a model for a macrocycle-crosslinked conducting polymer-based sensor. A guest analyte would act as a conductivity switch by facilitating interchain hopping of charge carriers. A photoinduced electron donor ruthenium complex moiety and an acceptor quinone moiety will be linked to the host, fixed away from the binding cavity. The acceptor portion was successfully synthesized. However, while monodentate, bidentate, and tetradentate ligands for ruthenium have been synthesized, the formation of a stable ruthenium metal complex has been unsuccessful. Further work will be required to synthesize this triad.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Degree Grantor:||California Institute of Technology|
|Division:||Chemistry and Chemical Engineering|
|Thesis Availability:||Restricted to Caltech community only|
|Defense Date:||4 August 1994|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Imported from ETD-db|
|Deposited On:||26 Oct 2007|
|Last Modified:||26 Dec 2012 03:05|
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