Self-assembly mechanism solid state structures

The complex formation of PECs and PE-surfs is closely linked to self-assembly processes. A major difference between PECs and PE-surfs can be found in their solid-state structures. PE-surfs show typically highly ordered mesophases in the solid state [15] which is in contrast to the ladder and scrambled-egg structures of PECs [2]. Reasons for the high ordering of PE-surfs are i) cooperative binding phenomena of the surfactant molecules onto the polyelectrolyte chains [16-18] and ii) the amphiphilicity of the surfactant molecules. A further result of the cooperative zipper mechanism between a polyelectrolyte and oppositely charged surfactant molecules is a 1 1 stoichiometry. The amphiphilicity of surfactants favors a microphase separation in PE-surfs that results in periodic nanostructures with repeat units of 1 to 10 nm. By contrast, structures of PECs normally display no such periodic nanostructures. 

Artificial membranes are used to study the influence of drug structure and of membrane composition on drug-membrane interactions. Artificial membranes that simulate mammalian membranes can easily be prepared because of the readiness of phospholipids to form lipid bilayers spontaneously. They have a strong tendency to self-associate in water. The macroscopic structure of dispersions of phospholipids depends on the type of lipids and on the water content. The structure and properties of self-assembled phospholipids in excess water have been described [74], and the mechanism of vesicle (synonym for liposome) formation has been reviewed [75]. While the individual components of membranes, proteins and lipids, are made up of atoms and covalent bonds, their association with each other to produce membrane structures is governed largely by hydrophobic effects. The hydrophobic effect is derived from the structure of water and the interaction of other components with the water structure. Because of their enormous hydrogen-bonding capacity, water molecules adopt a structure in both the liquid and solid state. 

As indicated by the opening paragraphs, the emphasis of this review is on advanced solid-state NMR experiments suitable for providing insight into the mechanisms of self-assembly which are of much relevance in modern polymer science. As such, we restrict our attention to the spin /= 1/2 nuclei found in organic solids, in particular, H and 13C, and focus on recently developed, i.e., within the last 10 years, solid-state NMR methods that probe the structural and dynamic information inherent to the dipolar coupling. Importantly, we will show that H NMR of rigid solids is now feasible. 

Stoddart and co-workers have developed molecular switch tunnel junctions [172] based on a [2]rotaxane, sandwiched between silicon and metallic electrodes. The rotaxane bears a cyclophane that shuttles along the molecular string toward the electrode and back again driven by an electrochemical translation. They used electrochemical measurements at various temperatures [173] to quantify the switching process of molecules not only in solution, but also in self-assembled monolayers and in a polymer electrolyte gel. Independent of the environment (solution, self-assembled monolayer or solid-state polymer gel), but also of the molecular structure - rotaxane or catenane - a single and generic switching mechanism is observed for all bistable molecules [173]. 

Supramolecular interactions are susceptible to being deformed and destroyed in the liquid phase and gas phase, while structural consequences are frozen and accentuated in the solid state. Both the flexibility and repetition of supramolecular forces are favored by biological and crystallization processes to accomplish self-correction. A main challenge for supramolecular chemists is to understand how to employ weak noncovalent interactions to construct supramolecular compounds. The construction and properties of a supramolecular ensemble will involve molecular recognition and self-assembly of constituent components. In line with the enzyme-substrate lock-and-key mechanism, the interacting partners can comprise host and guest species. 

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