Quantum Chemical Study of Guanin Supramolecular Structures

Abstract

Guanine and its derivates tend to self-assemble into various different supramolecular architectures in both biological and non-biological environments. Presence of the monovalent or divalent cations in the system dictates self-assembly into guanine quadruplexes or G-ribbons which represents promising supramolecular platforms for molecular electronics application. In vivo, guanine quadruplexes are identified as therapeutic regions in tumorous cells whose stabilization with different ligands can prevent abnormal recombination of the cell. Modern quantum chemical methods such as (timedependent) density functional theory and density functional based tight binding can provide valuable atomistic insights into the characteristics of diverse guanine quadruplex and ribbon nanostructures. In this thesis, several key characteristics of the guanine-based quadruplexes are addressed as well as the rylene dye decorated guanine ribbons. Firstly, infrared spectral fingerprint of the potential anti-tumor drug metalloporphyrin complexes has been studied and results suggested that the presence of the different cations within the metalloporphyrin system can modulate spectral characteristics of the guanine quadruplex carbonyl stretching mode, which can serve as a marker for future studies of the similar promising anti-tumor drug candidates. Affinity of the synthesized quadruplex structures towards divalent cations imposed between every adjacent quadruplex layer has been examined since this binding mode of the divalent cations is able to modulate excited state properties of these systems. In aqueous solution, the supramolecular design associated with water-mediated cytosine quartet imposed between guanine quartets could be able to host divalent cations in the unusual binding mode which makes these quadruplexes interesting candidates for optoelectronic applications. Therefore, special attention is dedicated to understand interplay between the cation types and the charge separation states of these systems. It has been determined that biologically abundant alkali metal cations are not able to significantly modulate excited state properties of the studied quadruplexes. The same holds for the alkaline earth cations when imposed between every other layer within the quadruplex. On the other hand, maxima of the fluorescence spectra could be modulated using alkaline cations. When using divalent cations imposed between every quadruplex layer, charge transfer states are significantly modulated (stabilized), which is also important from the aspects of optoelectronics. At the end of the thesis, self-assembly and the optical properties of the rylene dye decorated guanine supramolecular architectures on the graphene surface is studied. Both rylene decorated guanine ribbon and reference hydrogen bonded organic frameworks tend to form molecularly thin films that cover large surface area with similar electronic absorption properties. These findings suggest a presence of the prospective long-living charge-separated excited states, which can be significant for the optoelectronic applications.