G-quadruplexes (GQs) are non-canonical DNA structures composed of stacks of stabilized guanine tetrads (G-tetrads). GQs are highly diverse structures that can be categorized by their strand directionality, number of G-tetrads, and loop types among other parameters. Due to their high guanine content, GQs are expected to fold in regions such as telomers or oncogene promoters. They are thus firmly established as biologically viable targets for the development of anticancer therapeutics. In order to harvest GQs’ therapeutic potential, extensive structural studies are required to elucidate the structures’ diverse topologies alone and in complex with selective ligands. Towards this goal, this thesis studies nine variants of the telomeric repeat (TTGGGG)ₙ from Tetrahymena thermophila (designated as TET) alone and in complex with the highly selective GQ ligand, N-Methyl Mesoporphyrin IX (NMM). Our biophysical characterization shows that almost all TET GQs are highly heterogenous and can form multiple conformations. The addition of NMM, however, converts all the sequences to a parallel conformation and increases their thermal stability. To gain further insight into the molecular structures of the variants, we sought out to solve the crystal structures of TET26-2, TET22-NMM, TET24A-NMM, and TET25-NMM. TET26-2, solved to 1.97 Å, shows a parallel GQ with a four G-tetrad core and three TT propeller loops. Preliminary solutions of TET22-NMM, TET24A-NMM, and TET25-NMM complexes show parallel GQs with NMM π − π stacking on top of the 3’- terminal G-tetrad. Significantly, the first example of an NMM-NMM dimer was observed in these preliminary solutions which contributes to the understanding of GQ folding and the structures’ interactions with small molecule ligands. Overall, T. thermophila telomeric variants display unusual structural diversity by forming very distinct GQ structures. Our results provide insight into the many GQ topologies available to telomeric repeats and how they interact with the selective ligand NMM, which is important for the design of optimized GQ-selective anticancer therapeutics
