Halogen bonding is a recently rediscovered secondary interaction that shows potential to become a complementary molecular tool to hydrogen bonding in rational drug design and in material sciences. Whereas hydrogen bond symmetry has been the subject of systematic studies for decades, the understanding of the analogous three-center halogen bonds is yet in its infancy. The isotopic perturbation of equilibrium (IPE) technique with (13)C NMR detection was applied to regioselectively deuterated pyridine complexes to investigate the symmetry of [N-I-N](+) and [N-Br-N](+) halogen bonding in solution. Preference for a symmetric arrangement was observed for both a freely adjustable and for a conformationally restricted [N-X-N](+) model system, as also confirmed by computation on the DFT level. A closely attached counterion is shown to be compatible with the preferred symmetric arrangement. The experimental observations and computational predictions reveal a high energetic gain upon formation of symmetric, three-center four-electron halogen bonding. Whereas hydrogen bonds are generally asymmetric in solution and symmetric in the crystalline state, the analogous bromine and iodine centered halogen bonds prefer symmetric arrangement in solution.

Halogen bonding is the electron density donation based weak interaction of halogens with Lewis bases. Its applicability for molecular recognition processes long remained unappreciated and has so far mostly been studied in silico and in solid state. As most physiological processes and chemical reactions take place in solution, investigations in solutions are of highest relevance for its use in the pharmaceutical and material scientific toolboxes. Following a short discussion of the phenomenon of halogen bonding, this tutorial review presents an overview of the methods hitherto applied for gaining an improved understanding of its behaviour in solutions and summarizes the gained knowledge in order to indicate the scope of the techniques and to facilitate exciting future developments.

Halonium ions are particularly strong halogen bond donors, and are accordingly valuable tools for a variety of fields, such as supramolecular and synthetic organic chemistry.

It has been hypothesised that drugs in the chemical space "beyond the rule of 5" (bRo5) must behavea smolecular chameleons to combine otherwisec onflicting properties, including aqueous solubility,c ell permeability and target binding. Evidence for this has, however,b een limited to the cyclic peptidec yclosporine A. Herein, we show that the non-peptidic and macrocyclic drugs roxithromycin, telithromycin and spiramycin behave as molecular chameleons, with rifampicin showing al ess pronounced behaviour.I n particular roxithromycin, telithromycin and spiramycin display am arked, yet limited flexibility and populate signifi-cantly less polar and more compact conformational ensembles in an apolart han in ap olar environment. In addition to balancingo fm embrane permeability and aqueous solubility, this flexibility also allows binding to targetst hat vary in structure between species. The drugs' passivec ell permeability correlates to their 3D polar surface area and corroborate two theoretical models for permeability,d eveloped for cyclic peptides. We conclude that molecular chameleonicity should be incorporated in the design of orally administered drugs in the bRo5 space.[a] Dr.