Structural Elucidation of Guanosine Self-assemblies Using Spectroscopic and Computational Methods

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Kwan, Irene Ching Man
Nucleic acid , Supramolecular , NMR Spectroscopy , Chemistry
In this thesis, we document a comprehensive study of the cation-directed self-assembly of three guanosine derivatives: i) guanosine 5'-monophosphate (5'-GMP), ii) guanosine 5'-thiomonophosphate (5'-GSMP), and iii) 2',3',5'-O-triacetylguanosine (TAG). We discovered that, under the neutral pH condition, Na2(5'-GMP) molecules self-assemble into a right-handed helix structure consisting of alternating all-C2'endo and all-C3'endo planar G-quartets stacking on top of each other with a 30° twist. This self-assembled supramolecular structure uses multiple non-covalent forces (e.g., hydrogen-bond, phosphate-hydroxyl, pi-pi (base-base) stacking, ion-carbonyl, and ion-phosphate) to align individual monomers in a way that resembles RNA and DNA sequences in which covalent bonds are used to link monomers. Na+ ions are located in the channel and surface sites of the G-quadruplex. In contrast, under acidic pH conditions, Na2(5'-GMP) molecules self-assemble into a continuous right-handed helix where guanine bases are hydrogen-bonded in a lock-washer fashion with only C3'-endo monomers. Na+ ions are absent in the channel site due to smaller channel radius and lesser repulsions between phosphate groups (-1 vs. -2 charge under neutral pH) contribute to the stronger stacking mechanism. In Na2(5'-GSMP), a longer phosphate bond compared with Na2(5'-GMP) allows stronger P-O-…Na+…-O-P interactions to occur, thus enhancing self-assembly. Solid-state NMR, FT-IR, powder x-ray diffraction, model building, and calculation showed that Na2(5'-GSMP) forms the same self-assembled structure as Na2(5'-GMP) but with significantly greater tendency. This study proves that single-bond modification can enhance stacking in G self-assemblies, and shows direct evidence that Na+ ions reside at the surface (phosphate) sites. Lastly, using lipophilic TAG, we were able to show for the first time that trivalent lanthanide metal ions can facilitate G-quartet formation. A new mode of metal ion binding in G-quartet structures (i.e., a triple-decker G dodecamer containing a single metal ion in the central G-quartet) is reported. We also report the first 1H and 43Ca NMR characterization of Ca-templated G-quartet formation in a [TAG8-Ca]2+ octamer.
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