Nature's use of a simple genetic code to enable life's complex functions is an inspiration for supramolecular chemistry. DNA nucleobases carry the key information utilizing a variety of cooperative and non-covalent interactions such as hydrophobic, van der Waals, pi-pi stacking, ion-dipole and hydrogen bonding. This tutorial review describes some recent advances in the form and function provided by self-assembly of guanine (G) based systems. We attempt to make connections between the structures of the assemblies and their properties. The review begins with a brief historical context of G self-assembly in water and then describes studies on lipophilic guanosine analogs in organic solvents. The article also focuses on examples of how G analogs have been used as building blocks for functional applications in supramolecular chemistry, material science and nanotechnology.

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G-quadruplex DNA (G4-DNA) structures are four-stranded helical DNA (or RNA) structures, comprising stacks of G-tetrads, which are the outcome of planar association of four guanines in a cyclic Hoogsteen hydrogen-bonding arrangement. In the last decade the number of publications where CD spectroscopy has been used to study G4-DNAs, is extremely high. However, with very few exceptions, these investigations use an empirical interpretation of CD spectra. In this interpretation two basic types of CD spectra have been associated to a single specific difference in the features of the strand folding, i.e. the relative orientation of the strands, "parallel" (all strands have the same 5' to 3' orientation) or "antiparallel". Different examples taken from the literature where the empirical interpretation is not followed or is meaningless are presented and discussed. Furthermore, the case of quadruplexes formed by monomeric guanosine derivatives, where there is no strand connecting the adjacent quartets and the definition parallel/antiparallel strands cannot apply, will be discussed. The different spectral features observed for different G-quadruplexes is rationalised in terms of chromophores responsible for the electronic transitions. A simplified exciton coupling approach or more refined QM calculations allow to interpret the different CD features in terms of different stacking orientation (head-to-tail, head-to-head, tail-to-tail) between adjacent G-quartets irrespectively of the relative orientation of the stands (parallel/antiparallel).