Supramolecular and covalent chiral capsules based on 1,1’-binaphthol and exploring the enzyme kinetics of the Weimberg pathway via NMR spectroscopy

Ehlers, Martha Charlotte GND

The work described in this dissertation was split into four main parts, each one described in the corresponding chapters 3-6. For the first project, it was envisioned to synthesize hydrogen-bonded capsules via self-assembly. Two novel chiral capsules based on phosphoric acid – amidine hydrogen bonds were successfully synthesized and fully characterized. Synthetic pathway optimization lead to stable, enantiomerically pure capsules that remained intact even in competitive polar media. The capsules (all-R)-18 and (all-R)-19 remained in their self-assembled [3+2] or [2+2] structure, respectively, even in concentrations as low as 100 μM, where intermolecular interactions could be observed in NOESY spectroscopy, and a single diffusion coefficient was determined using DOSY spectroscopy, thereby proving the structure to be a single supramolecular structure rather than a mixture of the building blocks. These capsules were used for the application in host-guest chemistry of substituted C70-[IPH], where the capsules facilitated the solubilization of the highly hydrophobic fullerene in chloroform : methanol (8 : 2 v/v). Fluorescence measurements showed the combination of static and dynamic quenching observed when C70-[IPH] was titrated to a solution of the capsules (all-R)-18 and (all-R)-19. High association constants of up to 20200 M-1 were observed between the C70-[IPH] and the [3+2] capsule (all-R)-18. The fluorescence observed in the capsule on its own was quenched by 98 % after the addition of three equivalents of C70-[IPH]. It was proven by reference experiments using bisphosphoric acid (R,R)-11 and the C70-[IPH] guest that this quenching was due to the encapsulation of the guest into the cavity of the host molecule, and not due to association of the guest to the outside of the structure. Further investigations by NOESY spectroscopy showed intermolecular interactions between the aliphatic protons of the derivated guest fullerene and the aromatic protons of the capsule structure, thereby proving that the guest was in close spacial proximity to the host. In a second project, a novel approach to linearly substituted BINOL-derivatives was demonstrated. Building upon the synthetic approaches taken in the first project, a new pathway for the achievable substitution of BINOL in a not previously reported 5-position was derived. From that starting point, a versatile building block for various coupling reactions, such as Sonogashira and Glaser couplings, was be successfully synthesized. Employing different linkers, 2-BINOL systems could successfully be established. Trials for the formation of boronic ester-based capsules were unsuccessful. A third project saw the synthesis of a novel type of BINOL-based bisaldehyde building block for the formation of reversible covalent imine-based capsules via a metal template. The synthesis for the building block was optimized and gave high yields of pure compound, which was found to be stable under ambient conditions and was analyzed by X-Ray crystallography, showing that the suspected structure of the molecule was indeed observed. Ultra-high dilution conditions for the formation of the templated imine capsule were optimized to give the corresponding product, while demonstrating an extraordinary stability for an assembly based on labile bonds. The capsule could be stored under argon for several months and showed no sign of bond breakdown, which would have been easily observed by the parent aldehyde peak in NMR spectroscopy. DOSY spectroscopy also showed a single diffusion coefficient for the capsule structure, thus proving the formation of the reversible bonds. Efforts have been made to functionalize the imine bond based capsule via phosphorylation, and effective synthetic strategies have been derived to protect the aldehyde functionality from unwanted ester formation during the functionalization. While the phosphorylated bisaldehyde has not been fully characterized to this day, it has been detected using mass spectrometry. The optimization of the synthetic procedure for the functionalization should be carried out, and trials should be run on a metal-templated imine capsule formation analogously to the unfunctionalized version that was effectively synthesized. Further crystallization attempts should be taken in order to prove the spacial arrangement within the capsule and thus gather more information about the size of the internal cavity. The phosphorylated capsule should be used in catalysis, such as in transfer hydrogenations. In a final project, the enzyme kinetics of the Weimberg pathway of D-xylose conversion were investigated via NMR spectroscopy. After the intermediates were characterized by Niemeyer, a method was developed for the quantification of intermediates in the Weimberg pathway. In several cases, different isomers of the intermediates have been be identified and quantified. The quantification of substrates could be carried out via analysis of the NMR data measured at any point of both stepwise and one-pot reactions. NMR measurements were also used to identify potential bottlenecks in the cascade reaction, and thus served as the basis for cascade optimization efforts, showing the versatility of the NMR spectroscopy approach for following enzymatic cascades. This in-vitro data presented a breakthrough in the field of metabolic engineering and further work into achieving an NMR-based approach for in-vivo studies should be pursued.


Citation style:
Ehlers, M.C., 2020. Supramolecular and covalent chiral capsules based on 1,1’-binaphthol and exploring the enzyme kinetics of the Weimberg pathway via NMR spectroscopy.
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