Fibrillation model

Direct observation of the fibrillated chain formation process in ER fluids indicate that the chain length increases with the electric field strength and the particle volume fraction, and decreases with field frequency. These trends may be explained by considering the polarization forces between chained particles within a single chain and between fibrillated chains. The magnitude of the polarization force thus determines the mechanical strength of the fibrillated structure. 

In a word, the fibrillation model is based on the observation of fibrillated chain structure in ER fluids, This model was further expanded for quantitatively calculating the mechanical strength of ER suspensions, which is called the polarization model and will be addressed in detail in a future section. Sometimes the fibrillated chain model may be called a primary polarization model, because the particle polarization is emphasized. 

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Fig. 1. Structure of amyloid fibrils formed by the human amylin peptide. Negatively stained (A) and metal shadowed (B) fibrils formed by human amylin (adapted from Goldsbury et al., 2000a). (C) A human amylin fibril model formed by three protofibrils having a superpleated /i-structure (adapted from Kajava et al., 2005). Only Ca traces of the polypeptide chains are shown. (D) Atomic model of the cross-/ motif formed by the human amylin peptide (adapted from Kajava et al, 2005). Scale bar, 100 nm (A and B).

C) Cross section of the Ure2p amyloid-like fibril model, showing the parallel superpleated -structure at the N-terminus, and various positions possibly occupied by the globular C-terminus (gray ovals). Panels B and C are based on Fig. 4 of Kajava et al. (2004). 

A related fibril model for A/ o was proposed based on scanning proline mutagenesis (Williams et al, 2004) and molecular modeling (Guo et al., 2004). This model proposes that residues 15-21, 24-28, and 31-36 form 3 /-strands, with 2 intervening turns formed by residues 22-23 and 29-30 (Fig. 17G). Residues 17 and 34 are placed in close proximity, as double cysteine mutants at these positions form disulfide bonds on oxidation after fibrillization (Shivaprasad and Wetzel, 2004). Since fibrils with this triangular cross section would not be expected to show an H0-A... 

The Refolding, Gain-of-Interaction, and Natively Disordered classes of fibril models are at least partly consistent with the common properties of amyloid and amyloid-like fibrils. We summarize consistencies, inconsistencies, and uncertainties linking model class and amyloid property in Table I. In the following paragraphs, we describe these properties and discuss the extent to which they may be explained by the various classes of models. 

The extreme stability of amyloid and amyloid-like fibrils is difficult to understand in terms of the three classes of fibril models. For the Refolding models, it has been suggested that the amyloid conformation is a default conformation for a polypeptide chain (Dobson, 1999). However, these models do not give a clear indication of what types of interactions differ in the amyloid conformation versus the native conformation, and so it is unclear why the amyloid conformation should be more stable. Also, it seems that the elevated protein concentrations associated with fibril formation might disproportionately favor nonspecific aggregation of the destabilized intermediate over amyloid fibril formation. 

Benyamini, H., Gunasekaran, K., Wolfson, H., and Nussinov, R. (2003). Beta2-micro-globulin amyloidosis Insights from conservation analysis and fibril modelling by protein docking techniques./. Mol. Biol. 330, 159-174. 

Figure 3 Molecular packing in collagen fibrils Model for a transverse section. The red dots correspond to molecules ending in the gap. Reproduced from Hulmes, D. J. S. Wess, T. J. Prockop, D. J. Fratzl, P. Biophys. J. 1995, 68,1661-1670. Copyright (1995), with permission from Elsevier.

Figure 9.1. A. Fringe-fibril model of cellulose after Hearle [4] see also Zugenmaier [1], The right figure B. shows a schematic of a macro-fibril as existing in plant cells begin a composite of micro-fibrils. These consist of elementary fibrils which are made of 30-40 polymeric linear cellulose chains (picture based on the botany visual resource library [5]). The picture in figure A. is observed in crystalline cellulose, grown either artificially as for instance in textile fibers [1] or can be thought to mimic the structure of elementary fibrils.

S. A. Arzhakov Folded fibril model, with folded chains perpendicular to fibrillar axis (o)... 

The yield stress and the apparent viscosity of an ER suspension are largely dependent on the particle volume fraction. A linear relationship between the yield stress and the particle volume fraction was derived on the basis of the fibrillation model [103] and compared with the experimental data of the hydrated poly(mclhacrylalc) particle in a chlorinated hydrocarbon suspension obtained by Marshall [104]. The theoretical prediction was only valid in high particle volume fractions, and failed in low particle volume fractions, as stated in this paper and shown in Figure 41. 

Both the EDL and water bridge mechanisms lost the physical ground when anhydrous ER fluids were invented in 1985. The fibrillation model received much attention again and many attempts were made to quantitatively calculate the electrostatic polarization force between particles within a chain structure. Several review articles have addressed the various slightly different polarization models and tried to compare the calculated results with the experimental data [2, 15-18]. Since the polarization model has strong limitations for explaining the ER phenomena, only a brief description will be presented in this section. 

Modified Fringed Micelle Model and Fringed Fibril Model... 

In synthetic fibers, the micelles are different from those in many other polymer products. The micelles in synthetic fibers often have the fibril shape, with diameters ranging from several nm to 100 nm. The lengths of fibrils depend on the polymer type and the processing conditions. This leads to a fringed fibril model. Figure 3.14 shows a basic fringed fibril model. The bonding between fibrils is relatively weak, and hence fibrils actually can be observed in fracture studies of some synthetic fibers. 

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