Crystalline fibril model

Models of fringed micelles (a) and fringed fibrils (b). C, crystalline fibril A, amorphous domain. 

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.

Lateral growth occurs in real systems but is not accounted for in the model of Flory. What allows its incorporation into these new calculations is the assignation of the chains to their most probable positions the chains continuously seek positions of equilibrium as crystallization proceeds. This means that all amorphous links have the same propensity for crystallization, which therefore tends to eliminate a distinction between lateral and longitudinal crystal growth (keep in mind that different levels of crystallinity favor one growth pattern over the other -low crystallinity favors fibrils, high crystallinity favors lamellae). 

Frequency-selective REDOR (fsREDOR) is a very powerful technique developed for the study of 13C and 15N uniformly labeled peptides or proteins [92]. The basic idea of this technique is to combine REDOR and soft n pulses to recouple a selected 13C-15N dipole-dipole interaction in a multiple-spin system. Usually one could use Gaussian shaped pulses to achieve the required selective n inversions. Other band selective shaped pulses have been developed for a more uniform excitation profile [93]. In its original implementation, fsREDOR was used to extract the intemuclear distances of several model crystalline compounds [92], In the past few years, this technique has proven to be very useful for the study of amyloid fibrils as well. For the Ure2p10 39 fibril samples containing 13C and 15N uniformly... 

Sikorski, P., and Atkins, E. (2005). New model for crystalline polyglutamine assemblies and their connection with amyloid fibrils. Biomacromolecules 6, 425-432. 

Fig. 4. A model for the possible relationship between crystalline and disordered regions within a collagen fibril. The cross-sectional model of a 50-nm diameter fibril shows regions of crystallinity interfaced by grain boundaries. The individual crystalline unit cells are shown and the gap region is represented by a darker color. The axial projection of a single microfibrillar unit is also shown. Based on die structures developed by Hulmes et al. (1995) and adapted with permission from Hulmes et al (2002).

A recent model of the BC structure in the never-dried state was given by Fink et al. [13]. Anhydrous nano-fibrils in the range 7 x 13nm appear hydrated as a whole and are aggregated to flat microfibrils with a width of 70-150 nm. This means that the water is outside of the crystalline cellulose nano-units and between these elements. A shell of noncrystalline cellulose chains passes around neighboring microfibrils to produce a microfibril band... 

Figure 4 Self-assembling structures and liquid crystalline phase behavior observed in solutions of Pu-2 in water with increasing peptide concentration c (log scale). Electron micrographs (a) of ribbons (c = 0.2mM), (b) and (c) of fibrils (c = 6.2mM), and (d) fibers. The curves in (e) were calculated with the generalized model described in the text (see also Figure 5d). The polarizing optical micrograph (f) shows the thick thread-like texture observed for a solution with c = 3.7mM in a 0.2 mM pathlength microslide ...

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