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).

Single crystals of a lipophilic G-quadruplex formed by 5′-tert-butyl-dimethylsilyl-2′,3′,-di-Oisopropylidene G 2 were obtained from a CH 3 CN solution containing potassium picrate and cesium picrate. The X-ray structure showed that 16 units of G 2 and 4 equiv of alkali picrate form the lipophilic G-quadruplex. The quadruplex has a filled cation channel, with three K + ions and one Cs + ion located along its central axis. The quadruplex is formed by a pair of head-to-tail (G 2) 8 -K + octamers. Both octamers use eight carbonyl oxygens to coordinate K + . The two (G 2) 8 -K + octamers are of opposite polarity, being coaxially stacked in a head-to-head orientation. A Cs + cation, with an unusual coordination geometry, caps the cation channel. The Cs + is coordinated to four acetonitrile solvent molecules in an η 2 -fashion. Within an octamer the two tetramers are stacked so that they are 3.3 Å apart and twisted by 30°. A second stacking interaction is defined by the head-to-head arrangement between the two (G 2) 8 -K + octamers. This stacking, with a 90°twist, positions the exocyclic amines of the central two quartets so that both exocyclic NH2 B protons can hydrogen bond to the picrate anions that rim the quadruplex equator. The four picrates form an anionic belt that wraps around the cation channel. The sugars are well ordered in the structure. Circular dichroism spectra indicate that the G-quadruplex retains its helical structure in chlorinated solvents. Some diverse compounds have been proposed to form ion channels. These include magainin, cecropin, and gramicidin 1 and synthetic peptides that form bundles and nanotubes. 2,3 Various organic compounds also conduct ions across membranes. 4 Lipophilic ion pairs, modified phospholipids, bouquet molecules, unusual macrocycles, sterols, bolaamphiles, rigid rods, and crown-peptides all may form ion channels. 5-12 For many of these compounds, self-assembly in the membrane presumably gives channels with hydrophobic exteriors and hydrophilic interiors.Nucleobases self-associate via hydrogen bonding and base stacking. Thus, artificial ion channels are conceivable from lipophilic nucleobases. 13 Guanosine (G) is notorious for its propensity to aggregate. In a cation-templated process, G derivatives self-associate in water to give the G-quartet ( Figure 1). 14,15 The planar G-quartet is stabilized by hydrogen bonds between the NH1 amide and NH2 amino donors on one purine and the O6 and N7 acceptor atoms on a neighboring base. The G-quartet, with four oxygens surrounding a cavity, binds alkali cations with a selectivity of K + > Na + , Rb + . Cs + , Li + . 16 For example, K + forms a sandwich with two G-quartets. 17 These

Self-assembly plays an important role in the formation of many chiral biological structures and in the preparation of chiral functional materials. Therefore the control of chirality in synthetic or biological self-assembled systems is important either for the comprehension of recognition phenomena or to obtain materials with predictable and controllable properties. Circular dichroism was developed to study molecular chirality, however, because of its outstanding sensitivity to chiral perturbations of the system under investigation; it has been extended more recently to supramolecular chemistry. In particular, self-assembly processes leading to the formation of chiral supramolecular architectures (and eventually to gels or liquid crystal phases) can be monitored by CD. Furthermore, CD spectroscopy often allows one to obtain structural information on the assembled structures. This review deals with representative contributions to the study of supramolecular chirality by means of circular dichroism.

When a chiral dopant is dissolved in an achiral liquid crystal medium, the whole sample organizes into a helical structure with a characteristic length-scale of the order of microns. The relation between chirality at these quite different length-scales can be rationalized by a relatively simple model, which retains the relevant factors coming into play: the molecular shape of the chiral dopant, which controls the chirality of short range intermolecular interactions, and the elastic properties of the nematic environment, which control the restoring torques opposing distortion of the director. In this tutorial review the relation between molecular and phase chirality will be reviewed and several applications of the chiral doping of nematic LCs will be discussed. These range from the exploitation of the amplified molecular chirality for stereochemical purposes (e.g., the determination of the absolute configuration or the enantiomeric excess), to newer applications in physico-chemical fields. The latter take advantage of the periodicity of the chiral field, with length-scales ranging from hundreds to thousands of nanometres, which characterise the cholesteric phase.