Hierarchical formation of helical supramolecular polymers via stacking of hydrogen-bonded pairs in water

Abstract

Bifunctional ureido-s-triazines provided with penta(ethylene oxide) side chains are able to self assemble in water, leading to helical columns via cooperative stacking of the hydrogen-bonded pairs (DADA array). Monofunctional ureido-s-triazines do not form such helical architectures. The presence of a linker, covalently connecting the two ureido-s-triazine units, is essential as it generates a high local concentration of aromatic units, favorable for stacking interactions. This hydrophobic stacking of the aromatic units occurs at concentrations as low as 5⅐10 ؊6 M and can be visualized by using fluorescence spectroscopy. The stacking generates a hydrophobic microenvironment that allows intermolecular hydrogen bonding to occur at higher concentrations because the hydrogen bonds are shielded from competitive hydrogen bonding with water. This hierarchical process results in the formation of a helical self-assembled polymer in water at concentrations above 10 ؊4 M. Chiral side chains attached to the ureido-striazine units bias the helicity of these columns as concluded from CD spectroscopy and ''Sergeants and Soldiers'' experiments.I n DNA, the well known double helical motif originates from self assembly and is directed by lateral hydrogen bonds between bases and stabilized by solvophobic interactions between the covalently linked bases perpendicular to the hydrogen bonds (1). Control over the intrinsic helicity of the structure is governed by the peripheral chirality in the sugar-phosphate backbone. In synthetic systems, noncovalent interactions have been used to obtain well defined self-assembled architectures in organic solvents (2-7). Peripheral chiral centers in assemblies (8-11) and chiral side chains attached to a polymer backbone (12-20) have been shown to bias chirality at the supramolecular level. Highly ordered multimolecular supramolecular structures stable in water, held together by hydrophobic interactions or with additional hydrogen bonding, are also known (21-25). However, it remains difficult to exploit directional noncovalent interactions in a cooperative way for the formation of discrete multimolecular assemblies stable in water. In this solvent, water molecules strongly compete with directional polar interactions such as hydrogen bonds, resulting in low association constants.The formation of helical self-assembled polymers in solution by both stacking and hydrogen bonding has recently been demonstrated by us (26, 27). Self-complementary apolar molecules 1 and 2 dimerize via strong cooperative 4-fold hydrogen bonding (ADAD) in chloroform (28), giving rise to the formation of a dimer by 1 and a random coil polymer by 2, the latter featuring viscous solutions at higher concentrations. Additional to the hydrogen bonding, solvophobic interactions between the aromatic surfaces arise for 1 and 2 in alkanes. Association via hydrogen bonding of the ureido-s-triazine functional groups leads to the formation of a large and planar aromatic core surrounded by six flexible chains, a structure that is conduciv...

Chiralitätsverstärkung in dynamischen supramolekularen Aggregaten

“…[94] An Ureidotriazinderivate wurden anstelle der lipophilen Alkylketten auch polare chirale Penta(ethylenoxid)-Seitenketten geknüpft, um das Aggregationsverhalten in Wasser studieren zu können. [90] Monofunktionelle Derivate bildeten in Wasser keine Stapel, difunktionelle hingegen schon. Dieser Stapelungsvorgang beruht auf der hohen lokalen Konzentration hydrophober Einheiten wegen der Verknüpfung durch eine Hexamethylenbrücke.…”

Chiral structures from achiral liquid crystals in cylindrical capillaries

“…When the chiral symmetry of an achiral system is broken, a handedness is established, and materials with different handedness commonly exhibit distinct and useful properties (10)(11)(12)(13)(14) relevant for applications ranging from chemical sensors (15,16) to photonics (17)(18)(19). To date, considerable effort has been expended to control handedness in materials (for example, by chiral separation of racemic mixtures or chiral amplification of small enantiomeric imbalances) (1,8,(20)(21)(22). Recently and in a different vein, identification and elucidation of pathways by which achiral building blocks spontaneously organize to create chiral structures have become an area of active study.…”

New Materials Derived from Heterocyclic Systems

“…Linear, even polymeric [79], structures have also been described. For example, Timmerman has reported the formation of the linear discrete assembly 81 through hydrogen-bond interactions between a trimer of the triazine derivative melamine and a bis(barbituric acid) derivative [80].…”