Fibrous aggregation

Figure 5.7 AFM images of fibrous aggregates of D-Glu-8 (1) lying directly on silicon surfaces (a) network of fibers (b) high-resolution phase image of quadruple helices. Reprinted with permission from Ref. 42. Copyright 2000 by the American Chemical Society.

Based on the same principle, there are monomeric / -helical proteins that carry at their extremities a cluster of helical or nonrepetitive structures that could act as a capping element covering their exposed ends (Emsley et al., 1996 Lietzke et al, 1994 Petersen et al, 1997 Steinbacher et al, 1994). For example, the last 40 residues of pectate lyase C form a large loop that partially covers the surface of the /Hielix (Yoder et al, 1993). The fibrous (or otherwise elongated) domain of these natural /f-stranded proteins is not stable in isolation, as for example in the case of the P22 tailspike where bacterially expressed isolated /Hielix domain, at high concentrations, forms fibrous aggregates that bind Congo red (Schuler et al, 1999). 

Schuler, B., Rachel, R., and Seckler, R. (1999). Formation of fibrous aggregates from a non-native intermediate The isolated P22 tailspike beta-helix domain./. Biol. Chem. 274, 18589-18596. 

A paper by Meijer et al. described the synthesis and electronic properties of a series of PPE-PPV hybrid oligomers. The approach is shown in Scheme 11 [70]. Starting from a brominated stilbene, reaction with a series of diethynyl-benzenes furnished pentameric PE-PV hybrids in yields around 80%. The materials are used as emitters in LEDs and other electrooptical devices. It was noted that fibrous aggregates formed upon melting and recooling of thin films of these pentameric PE-PV hybrids. 

The most common, and certainly the most familiar, inorganic fibers are the mineral fibers known as asbestos. Several minerals have been mined as asbestos (Fig. 1.1 A, C see chapter 2 for a detailed discussion of the asbestos minerals). These minerals are widespread in nature, but mining is economical only when they occur as continuous fibrous aggregates (veins) crosscutting rock masses. The veins are made up of hairlike fibers in parallel array that, on close inspection, appear bent (Fig. I.IB). Not surprisingly, a mineralogical term, asbestiform, has been used to describe this subset of inorganic materials. 

Over the past several decades six different silicate minerals have been mined as asbestos and processed for industrial and commercial applications. The most commonly encountered asbestos mineral today is chrysotile. The five other minerals are tremolite, actinolite, anthophyllite, grunerite, and rie-beckite. All five are members of the amphibole group of minerals, and each can occur as chunky, acicular, or equant crystals, as well as in fibrous form. When found as fine fibrous aggregates, in quantities appropriate for mining, they are usually distinguished as a special variety—for example, tremolite-asbestos. 

As might be anticipated for minerals with chain structures, pyroxenes commonly occur in columnar, prismatic, rodlike, and acicular forms. Enstatite has been found in the form of rosettes of fine-fibrous crystals. Special names such as victorite, chladnite, and shepardite were assigned to different occurrences in this distinctive morphology, possibly because the fibrous aggregates were located in iron meteorites. However, the composition and crystal... 

These few examples illustrate the range of habits exhibited by minerals whose basic structural units are single chains. In addition, the pyroxenes, which have compositions quite similar to those of the amphiholes, are often found intimately intergrown with the latter double-chain minerals in what appears to be a single large crystal. An intergrowth of amphibole and pyroxene as a fiber, or in a fibrous aggregate, has not yet been described, but it is a likely occurrence. 

Fig. 2 Schematic diagram of a monomer isomers studied b monomer self-assembly equilibria and fibrous aggregate polymerization scheme with horseradish peroxidase. Raman scattering intensities as a function of unpolymerized monomer concentration are presented for c DEDT at pH 6.0, and d DELT at pH 6.0. SEM images are presented for the enzymatically polymerized e DEDT at pH 6.0 Scale bar = 2 jxm), and f DELT at pH 6.0 Scale bar = 2 xm). This figure was reprinted with permission from [68]...

Figure 11.16. (a) SEM image of the fibrous aggregates formed in the self-assembled LC gel with 1% of the shown azobenzene gelator and 5% chiral dopant (b) photomicrograph of the LC gel before irradiation through a photomask (c) after 5-min irradiation and (d) after 10-min irradiation. Source Zhao and Tong, 2003. Reprinted with permission. See color insert. 

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