Supramolecular assemblies, stability

Fig. 7. Formation of a supramolecular aggregate composed of a compound containing nine melamine rings (the three-layered nonamelamine derivative A) and nine molecules of neohexylisocyanurate (B). Of the 16 possible conformers that can result, two are shown The first has the nine molecules of B arranged in three rosettes of three molecules each, stacked atop each other in the second, the rosettes are staggered with respect to each other such that the rosettes in the first and third layers of A are aligned with each other, but not with the rosette in the second layer. The supramolecular assembly is stabilized...

In this context it is interesting to note that archaea, which possess S-layers as exclusive cell wall components outside the cytoplasmic membrane (Fig. 14), exist under extreme environmental conditions (e.g., high temperatures, hydrostatic pressure, and salt concentrations, low pH values). Thus, it is obvious one should study the effect of proteinaceous S-layer lattices on the fluidity, integrity, structure, and stability of lipid membranes. This section focuses on the generation and characterization of composite structures that mimic the supramolecular assembly of archaeal cell envelope structures composed of a cytoplasmic membrane and a closely associated S-layer. In this biomimetic structure, either a tetraether... 

Douki T, Ravanat J-L, Angelov D, Wagner JR, Cadet J (2004) Effects of Duplex Stability on Charge-Transfer Efficiency within DNA. 236 1-25 Drabowicz J, Mikolajczyk M (2000) Selenium at Higher Oxidation States. 208 143-176 Dutasta J-P (2003) New Phosphorylated Hosts for the Design of New Supramolecular Assemblies. 232 55-91... 

Siderophore-ionophore supramolecular assembly formation via host-guest complexation of the pendant protonated amine arm of ferrioxamine B has been confirmed by X-ray crystallography (Fig. 28) (203). The stability and selectivity of this interaction as a function of ionophore structure, metal ion identity, and counter anion identity were determined by liquid-liquid extraction, isothermal calorimetry, and MS (204 -211). Second-sphere host-guest complexation constants fall in the range 103— 106M-1 in CHC13 and methanol depending on ionophore structure. 

The strength of fibrous proteins is enhanced by covalent cross-links between polypeptide chains within the multihelical ropes and between adjacent chains in a supramolecular assembly. In a-keratins, the cross-links stabilizing quaternary structure are disulfide bonds (Box 4-2). In the hardest and toughest a-keratins, such as those of rhinoceros horn, up to 18% of the residues are cysteines involved in disulfide bonds. 

As a reference, the NA hydrolysis catalyzed by Zn2+—cyclen 2b has been determined in the presence of 10 mM Triton X-100 by the same method in 10% (v/v) CH3CN aqueous solution (pH 9.2 with 20 mM CHES buffer) with / = 0.10 (NaN03) at 25°C. The rate constant of 4.6 X 10-2 M-1 sec-1 is smaller than that of 0.10 M-1 sec-1 for the same reaction in the absence of Triton X-100. Provided that only the NA partitioned in aqueous phase was subjected to attack by the hydrophilic 2b, it can be estimated that approximately 50% of NA is partitioned in the micellar phase, since the NA value in the presence of Triton X-100 was decreased by half. The comparison of these NA values (5.0 vs. 4.6 X 10 2M-1 sec-1) clearly shows that the lipophilic cyclen complex lib is a 100-fold better catalyst in a micellar system. In the micellar phase, the lipophil-icity of both the substrate NA and catalyst lib would contribute to stabilization of the supramolecular assembly, leading to more frequent collisions between them. 

This chapter deals with formation of cyclic 2D and 3D structures in solution by self-assembly of two or more components using hydrogen bonds as the major interactions. Obviously, the formation of a defined supramolecular aggregate stabilized by non-covalent forces is a thermodynamically driven process which reflects a balance between enthalpy and entropy. Consequently, the product of a non-covalent macrocyclic synthesis must be evaluated and predicted in terms of thermodynamic minima in an equilibrium mixture. 

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