Fibril extension

The processes of both seed formation and fibril extension are dependent on temperature and on peptide concentration, with 37°C being required for establishing equilibrium within 24 h with 30 pM Pi 4o- A full description of the assay system may be found elsewhere [97,117], A 4 h reaction time is typically within the linear portion of the time course. This nucleus-dependent assay detects mainly inhibitors that are substoichiometric with the monomeric peptide, which is present at high concentration. It is relatively insensitive to inhibitors that target the monomeric peptide. Whether the inhibitors interact with the growing end of a seed or with a low abundance conformational form of the p peptide that is competent to add to the seed is difficult to determine at this time. Similar dose-response curves are obtained for Congo Red as an inhibitor with either thioflavin T (ThT) fluorescence or filtration of radioiodinated peptide readouts (Fig. 4) Caveats in the interpretation of both the ThT and radiometric filtration assays for the evaluation of putative inhibitors are discussed elsewhere [97]. 

Figure 4 Inhibition of nucleated fibril extension by Congo Red fibril... 

Naiki H, Nakakuki K. First-order kinetic model of Alzheimer s beta-amyloid fibril extension in vitro. Lab Invest 1996 74 374-383. 

In vitro, fibril formation by several proteins displays an initial lag phase, followed by a rapid increase in aggregation (reviewed in Rochet and Lansbury, 2000). Introduction of fibrillar seeds eliminates the lag phase. These cooperative aggregation kinetics suggest that fibril formation begins with the formation of a nucleus and proceeds by fibril extension. The structure of the nucleus must therefore act as a template for the protein s conformation in the fibril. As the structural requirements for templating are unclear, it is difficult to assess the consistency of the model classes with this feature of fibril formation. We have described one possible templating mechanism for the cross-/ spine of GNNQQNY (Nelson et al., 2005). 

Effect of UV Light on Amyloidogenic Proteins Nucleation and Fibril Extension... 

Fig. 5. Craze fibril extension ratio vs. distance from crack tip in a craze grown in a polyftert-butylstyrene) film (From Ref. courtesy of John Wiley Sons.)...

As implied by the discussion above craze fibril extension ratio or its inverse the fibril volume fraction of the craze is an important parameter of the microstructure. Fibril volume fractions can be measured by several different methods. The refractive index n of the craze can be measured by measuring the critical angle for total reflection of light by the craze surface. Using the Lorentz-Lorenz equation Vf then can be computed from The method is difficult because small variations... 

To this point the craze fibril volume fraction Vf and fibril extension ratio X have discussed as if they were true constants of the craze. While this view is approximately correct, one would expect the draw ratio of the polymer fibrils to depend somewhat on the stress at which they are drawn, since the polymer in the fibrils should have a finite strain hardening rate. Experimental evidence for just such stress effects on X, is discussed below. 

Fig, 13. Fibril extension ratio profile l.(x) and surface stress profile S(x) along an air craze in PS. From Ref courtesy Taylor and Francis, Ltd... 

Craze fibril extension ratio plotted against weight fraction PPO for two types of PS/PPO blends. 0, high M PS blend. A, low M PS blend. The theoretical curves for are also shown as solid lines... 

Weibull scale parameter monomeric friction coefficient craze fibril extension ratio (=l/vj) average extension ratio in stretch zone... 

The first molecular variable to be considered is chain entanglement density. It is known that this factor, as well as a related parameter, viz. the average chain contour length between entanglements, can have an appreciable effect on deformation modes observed in thin films and on various aspects of crazing such as the craze stress and the craze fibril extension ratio. Examples of these effects will be presented and dis-... 

As the chain contour length between physical entanglements is reduced, so too is the maximum craze fibril extension ratio, Provided the entanglement network remains intact upon straining, X is given by... 

Fig. 7. Craze stress and craze fibril extension ratio vs. chain entanglement contour length in thin films of PTBS, PS, PSAN and PPO. Data from Ref.

This result is consistent with Kramer s results showing that the fibril extension ratio (which is just the inverse of the fibril volume fraction) is equal to the bulk polymer network full extension ratio. As a matter of fact, it is unlikely that the toluene vapor changes the physical and chemical structure of the bulk it just makes the fibril drawing easier . On the other hand, it is generally admitted that the fibril diameter times the craze surface stress is constant. Therefore, the craze surface stress being lower in toluene vapor, the fibrils are probably thicker. 

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