Sugar modified guanosine analogs as versatile tools for shaping G quadruplex architectures Magnetische Kernresonanz Nucleinsäuren Struktur guanosine analogs hydrogen bond quadruplex sugar pucker G quadruplexes (G4s) have been in the focus of research in the last decades for their regulatory roles in vivo and for their use in nano and biotechnology. However an understanding of the various factors that drive a particular quadruplex fold remains limited challenging rational therapeutic targeting and design of these tetrahelical structures. In this regard insights from modified G quadruplexes may help to deepen our knowledge of G quadruplex structure. In this dissertation sugar modified guanosine analogs are exploited for their altered conformational preferences regarding both glycosidic bond angle and sugar pucker by their incorporation into different syn positions of the G core of a model G quadruplex. Induced structural perturbations as characterized by NMR spectroscopy range from a local change in tetrad polarity to a complete refolding into an unusual structure with a V shaped loop a unique G4 structural element in the focus of this work. Detailed conformational analysis of the introduced G analogs and high resolution structures of the modified quadruplexes reveal a complex interplay of glycosidic torsion angle sugar pucker preferences and local interactions which may all play a leading role in driving G4 folding.