Fibrillar model

The shortcomings of such investigations can be interpreted as (a) the starting fibrillar model is of insufficient accuracy to mimic the changes imposed by the mechanisms of distortion or (b) the emphasis, or even the entire basis, for the mechanisms of distortion is incorrect. 

The first fibrillar model was described by Krimm and Schor (1956), and in later publications (Enomoto and Krimm, 1961 Krimm, 1961, 1962 Schor and Krimm, 1961a,b Westover et al, 1962) it was claimed that this model accounted for a large body of experimental evidence. Nevertheless, the /3-helix model, as it was called, is so much at variance with the observed X-ray pattern that it is unlikely to be correct in its present form (Fraser and MacRae, 1963). Despite this, the recognition by Krimm and his colleagues that the X-ray diagram was consistent with a helical structure was an important step forward. 

A Fibrillar Model Derived from Small-Angle X-Ray Diffraction. 99... 

The general, qualitative properties just described as expected of the fibrillar model are used to illuminate various aspects of problems which arise subsequently. 

Figure 17.2 Representation of the fringed fibrillar model for the fibre structure. ...

Hierarchical fibrillar model [80] This model was proposed for drawn TLCP fibers. They are composed of bundles of macro fibrils (5 p,m), fibrils (0.5 (Jim), and micro fibrils (0.05 p.m) in a hierarchical order (see Figure 8.17). This type of highly oriented fibrillar morphology is also found in the inner skin region of the injection-molded parts, as illustrated in Figure 8.18. 

Fig. 8.19. Fibrillar model of fibrous structure with practically all ends of microfibrils concentrated on the outer boundary of fibrils (after 1581).

Model peptides that could build up quarternary fibrillar structures are not yet known. Though complete explanation of the interdependence between the primary structure and the stability of the quarternary structure has not yet been possible, i.e. the role of the different amino acids in collagen could be understood completely only in correlation with the fibril formation (formation of polar and hydrophobic clusters ). 

A model of lamellae formation In stretched networks is proposed. Approximately one-half of the chains do not fold. Formation of such lamellae Is accompanied by declining stress. Highly folded systems (high crystallinity), however, can cause a stress Increase. In the calculations crosslinks are assigned to their most probable positions through the use of a characteristic vector. A contingent of amorphous chains Is also Included. The calculations suggest that the concept of fibrillar-lamellar transformations may be unnecessary to explain observed stress-temperature profiles In some cases. 

It has recently been shown (7) that a transformation from fibrillar to lamellar morphology is not required to replicate the force-temperature profile of stretched networks in the crystallization region. This latest work shows that a close duplication of the behavior of gutta percha (8) can be predicted with a model (7) of fibrillar crystallization that Incorporates several new features omitted in earlier theories, specifically ... 

Refolding models propose that the fibril-forming protein exists in two distinct states the native state and the fibrillar state (Fig. 2). In converting from one to the other, the protein must unfold, then refold. As the fibrillar state is common to proteins with dissimilar sequences, Fandrich et al. (2001) have suggested that refolding into fibrils is dominated by backbone interactions, which are available to all protein sequences. 

The parallel /f-helix model does not provide an 8.8-A sheet-to-sheet spacing, suggested by the 8.8-A reflection of the prion rod diffraction pattern (Nguyen et al., 1995). However, the authors (Govaerts et al, 2004) note that a number of jS-sandwich folds are consistent with the general structural requirements for a model of fibrillar PrP, although they do not attempt to model it as such. 

This direct-stacking model (Olofsson et al., 2004 Serag et al., 2002) therefore proposes that TTR maintains much of its native structure, including the native dimer interface, in the fibrillar state. A new interaction interface is gained with the shifting of /(-strands at the ends of two sheets, driving fibril formation. 

In summary, two different Gain-of-Interaction models have been proposed for the fibrillar structure of /12m. The cross-(3 spine model (Ivanova et al., 2004) proposes a core composed of C-terminal /1-hairpins, and the direct-stacking model (Benyamini et al., 2003) proposes a core of native-like /12m molecules with their N- and C-terminal strands displaced. 

In a later publication, Kishimoto et al. (2004) proposed the water-filled nanotube as a model for the fibrillar N-terminal domain of the yeast prion Sup35p. The authors find that hydrated Sup35p fibrils show no 10-A equatorial reflection in the fiber diffraction pattern, but that dried fibrils... 

Fay et al. (2005) have proposed a completely different model for Ure2p fibril structure. Their model is based on data which suggest that Ure2p fibrils do not have a cross-/ structure (Bousset et al., 2003) and that the C-terminal globular domain is tightly involved in the fibrillar scaffold (Bousset et al.,... 

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