Self-assembled, helically stacked anionic aggregates of 2,5,8,11-tetra-tert-butylcycloocta[1,2,3,4-def;5,6,7,8-d'e'f']bisbiphenylene, stabilized by electrostatic interactions

A modified Stokes-Einstein equation for Aβ aggregation

BACKGROUND

In all amyloid diseases, protein aggregates have been implicated fully or partly, in the etiology of the disease. Due to their significance in human pathologies, there have been unprecedented efforts towards physiochemical understanding of aggregation and amyloid formation over the last two decades. An important relation from which hydrodynamic radii of the aggregate is routinely measured is the classic Stokes-Einstein equation. Here, we report a modification in the classical Stokes-Einstein equation using a mixture theory approach, in order to accommodate the changes in viscosity of the solvent due to the changes in solute size and shape, to implement a more realistic model for Aβ aggregation involved in Alzheimer's disease. Specifically, we have focused on validating this model in protofibrill lateral association reactions along the aggregation pathway, which has been experimentally well characterized.

RESULTS

The modified Stokes-Einstein equation incorporates an effective viscosity for the mixture consisting of the macromolecules and solvent where the lateral association reaction occurs. This effective viscosity is modeled as a function of the volume fractions of the different species of molecules. The novelty of our model is that in addition to the volume fractions, it incorporates previously published reports on the dimensions of the protofibrils and their aggregates to formulate a more appropriate shape rather than mere spheres. The net result is that the diffusion coefficient which is inversely proportional to the viscosity of the system is now dependent on the concentration of the different molecules as well as their proper shapes. Comparison with experiments for variations in diffusion coefficients over time reveals very similar trends.

CONCLUSIONS

We argue that the standard Stokes-Einstein's equation is insufficient to understand the temporal variations in diffusion when trying to understand the aggregation behavior of Aβ42 proteins. Our modifications also involve inclusion of improved shape factors of molecules and more appropriate viscosities. The modification we are reporting is not only useful in Aβ aggregation but also will be important for accurate measurements in all protein aggregation systems.

Highly charged supramolecular oligomers based on the dimerization of corannulene tetraanion

The supramolecular dimerization of tetraanionic corannulene is utilized as a self-assembly motif for the construction of highly charged, buckybowl-based supramolecular oligomers. Oligomers of up to 7 units (3900 g mol(-1)) of reduced dicorannulenic monomers are identified and characterized by various NMR techniques, and the reversibility of their assembly is established through the concentration dependence of their molecular weight and the effect of monofunctional chain-stoppers.

Reduction of bowl-shaped hydrocarbons: dianions and tetraanions of annelated corannulenes

[structure: see text] The reduction of several annelated corannulene derivatives was undertaken using lithium and potassium metals. It was found that annelation affects the annulenic character of corannulene by changing its charge distribution; the dianions of derivatives that are annelated with six-membered rings have less annulenic character and are less paratropic than corannulene dianion. This effect is even more pronounced in corannulenes that are peri-annelated with five-membered rings. The alkali metal used in the reduction process has a great influence on the outcome, especially on the degree of reduction. Most derivatives get reduced to tetraanions only with potassium, and not with lithium, the exception being systems that can stabilize the tetraanion with lithium by special means, such as aggregation or dimerization. One such system is cyclopenta[bc]corannulene (acecorannulylene), which gives a coordinative dimer that consists of two cyclopentacorannulene tetraanions, bound together in a convex-convex fashion by lithium cations. The points of contact in this dimer are two rehybridized carbons from each cyclopentacorannulene unit, which are bridged together by two lithium cations.

Ball-and-Socket Stacking of Supercharged Geodesic Polyarenes: Bonding by Interstitial Lithium Ions

Unprecedented supramolecular stacks of highly reduced geodesic pi-systems were prepared by the reduction of the derivatized fullerenes Me(5)C(60)H and Ph(5)C(60)H and corannulene with lithium metal (R(5)C(60)(5)(-)/Cor(4)(-)/9Li(+)). The host--guest assemblies form because of the enhanced electrostatic interactions between the lithium cations and the anionic moieties, in addition to the structural compatibility between the curved hydrocarbons. The high stability of these new supramolecular assemblies (heterodimers) enables the introduction of another organization motif to the system. This is achieved by using tethered corannulenes as host molecules, which leads to the formation of tethered bis-heterodimers ((Me(5)C(60)(5)(-)/Cor(4)(-))(2)(CH(2))(8)/18Li(+)).

Synthetic strategies to chiral organosulfur donors related to bis(ethylenedithio)tetrathiafulvalene

Syntheses of enantiopure organosulfur donors by three different strategies requiring only four-six steps are reported. The key step involves either double substitution of an enantiopure cyclic sulfate ester by a dithiolate, attachment of a chiral diol as a ketal, or completely diastereoselective cycloaddition of 1,3-dithiole-2,4,5-trithione to an enantiopure alkene.

Using Diffusion NMR To Characterize Guanosine Self-Association: Insights into Structure and Mechanism

This paper presents results from a series of pulsed field gradient (PFG) NMR studies on lipophilic guanosine nucleosides that undergo cation-templated assembly in organic solvents. The use of PFG-NMR to measure diffusion coefficients for the different aggregates allowed us to observe the influences of cation, solvent and anion on the self-assembly process. Three case studies are presented. In the first study, diffusion NMR confirmed formation of a hexadecameric G-quadruplex [G 1](16)4 K(+)4 pic(-) in CD(3)CN. Furthermore, hexadecamer formation from 5'-TBDMS-2',3'-isopropylidene G 1 and K(+) picrate was shown to be a cooperative process in CD(3)CN. In the second study, diffusion NMR studies on 5'-(3,5-bis(methoxy)benzoyl)-2',3'-isopropylidene G 4 showed that hierarchical self-association of G(8)-octamers is controlled by the K(+) cation. Evidence for formation of both discrete G(8)-octamers and G(16)-hexadecamers in CD(2)Cl(2) was obtained. The position of this octamer-hexadecamer equilibrium was shown to depend on the K(+) concentration. In the third case, diffusion NMR was used to determine the size of a guanosine self-assembly where NMR signal integration was ambiguous. Thus, both diffusion NMR and ESI-MS show that 5'-O-acetyl-2',3'-O-isopropylidene G 7 and Na(+) picrate form a doubly charged octamer [G 7](8)2 Na(+)2 pic(-) 9 in CD(2)Cl(2). The anion's role in stabilizing this particular complex is discussed. In all three cases the information gained from the diffusion NMR technique enabled us to better understand the self-assembly processes, especially regarding the roles of cation, anion and solvent.

Self-Recognition, Structure, Stability, and Guest Affinity of Pyrogallol[4]arene and Resorcin[4]arene Capsules in Solution

In the present study, we used diffusion NMR to probe the structures and characteristics of the products obtained from the self-assembly of resorcin[4]arenes 1a and 1b and pyrogallol[4]arenes 2a and 2b in CDCl(3) solutions. It was found that all four molecules self-assemble into hexameric capsules. The hexameric capsules of pyrogallol[4]arenes 2a and 2b were found to be more stable than the capsules of resorcin[4]arenes 1a and 1b in polar media. We also studied the role of water molecules in the self-assembly of the different capsules and found that water molecules are part of the hexameric capsules of resorcin[4]arenes 1a and 1b but not in the capsules of pyrogallol[4]arenes 2a and 2b. It was found that the self-assembly process between the resorcin[4]arenes and pyrogallol[4]arenes proceeds with self-recognition. When mixing two macrocycles of different types in a chloroform solution, no heterohexamers are formed, only the capsule constructed from the same macrocycle is detected. However, when two resorcin[4]arenes (i.e., 1a and 1b) or two pyrogallol[4]arenes (i.e., 2a and 2b) are mixed, heterohexamers are formed over time. In addition, we found that resorcin[4]arenes and pyrogallol[4]arenes differ significantly in their guest affinity. The capsules of 1a and 1b can accommodate both the tertiary alkylamines and their respective ammonium salts, while the capsules of 2a and 2b encapsulate only the neutral tertiary alkylamines.

Reduction of Strained Polycycles: How Much Strain Can a Pyrene Anion Take?

The reduction of a series of [n](2,7)pyrenophanes (n = 7-10) with lithium or potassium metal shows that the strain in the system, controlled by the length of the tether, determines the nature of the reduction products. The reduction of [7](2,7)pyrenophane (2) and [2]metacyclo[2](2,7)pyrenophane (3) leads to reductive dimerization followed by novel intramolecular sigma-bond formation as a means of escaping strained anti-aromaticity. [8](2,7)Pyrenophane (4) affords only reductive dimerization, and no two-electron reduction is observed. The reduction of [9](2,7) pyrenophane (5) and [10](2,7)pyrenophane (6) leads to reductive dimerization, followed by the formation of a dianionic anti-aromatic species, which eventually cleaves the solvent, THF-d(8). The similarity between the reduction of the latter systems and the reduction of pyrene (1) is discussed.