Helical ribbon

P = pitch of a helical ribbon impeller, m W = blade width, m... 

FIGURE 5.8 Two structural motifs that arrange the primary structure of proteins into a higher level of organization predominate in proteins the a-helix and the /3-pleated strand. Atomic representations of these secondary structures are shown here, along with the symbols used by structural chemists to represent them the flat, helical ribbon for the a-helix and the flat, wide arrow for /3-structures. Both of these structures owe their stability to the formation of hydrogen bonds between N—H and 0=C functions along the polypeptide backbone (see Chapter 6). 

Figure 7.20 shows some of the impellers which are frequently used. Propellers, turbines, paddles, anchors, helical ribbons and screws are usually mounted on a central vertical shaft in a cylindrical tank, and they are selected for a particular duty largely on the basis of liquid viscosity. By and large, it is necessary to move from a propeller to a turbine and then, in order, to a paddle, to an anchor and then to a helical ribbon and finally to a screw as the viscosity of the fluids to be mixed increases. In so doing the speed of agitation or rotation decreases. 

Figure 7,20. Commonly used impellers (a) Three-bladed propeller ( >) Six-bladed disc turbine (Rushton turbine) (c) Simple paddle (d) Anchor impeller (e) Helical ribbon (/) Helical screw with draft tubs...

Anchors, helical ribbons and screws, are generally used for high viscosity liquids. The anchor and ribbon are arranged with a close clearance at the vessel wall, whereas the helical screw has a smaller diameter and is often used inside a draft tube to promote fluid motion throughout the vessel. Helical ribbons or interrupted ribbons are often used in horizontally mounted cylindrical vessels. 

Carreav. P, J.. Patterson, I., and Yap, C. Y. Can J. Chem. Eng. 54 (1976) 135. Mixing of viscoelastic fluids with helical ribbon agitators 1. Mixing time and flow pattern. 

Bourne, J. R. and Butler. H. Trans. Inst. Chem. Eng. 47 (1969) Til. On analysis of the flow produced by helical ribbon impellers. 

This patent then calls for two or more reflux cooled conical CSTR s with helical ribbon agitators operating in series in an "intermediate conversion zone" ranging from 65 to 85% conversion. The conversion in succeeding reactors in this zone should show a relative difference of 15-25%. As with the earlier patent,... 

Paddle, anchor and helical ribbon agitators (Figures 10.56a, b, c), and other special shapes, are used for more viscous fluids. 

Figure 10.56. Low-speed agitators (a) Paddle (b) Anchor (c) Helical ribbon...

Figure 5.16 Cryo-transmission electron micrograph of (a, b) helical ribbons and (c, d) multi-lamellar tubules in aqueous dispersions of A-dodecanoyl-L-serine (28) at pH 6.4 (a-c) and 4.9 (d). Reprinted with permission from Ref. 79. Copyright 2001 by the American Chemical Society.

Figure 5.21 Transmission electron micrographs showing right-handed helical ribbons of L-Glu-Bis-3 (37) in aqueous environment. Reprinted with permission from Ref. 97. Copyright 2002 by Elsevier Science.

Helical ribbons were found to be metastable intermediates in the process of cholesterol crystallization from bile in the gallbladder.160 Since gallstones result from the formation of cholesterol monohydrate crystals in supersaturated... 

In further studies, Zastavker et al. established that the formation of helical ribbons with two distinct pitch angles is a general phenomenon observed in a wide variety of multicomponent systems containing a sterol.162 High-pitch (54°) and low-pitch (11°) helices were observed in almost all of the... 

Figure 5.43 Phase contrast optical micrographs of typical helical structures in chemically defined lipid concentrate system, (a) Low-pitch helical ribbon with pitch angle i r = 11 2°. (b) High-pitch helical ribbons with pitch angle t t = 54 2°. (c) Intermediate-pitch helical ribbons with pitch angle i r = 40.8 3.8°. Reprinted with permission from Ref. 162. Copyright 1999 by the National Academy of Sciences, U.S.A.

Furthermore, Oda et al. pointed out that there are two topologically distinct types of chiral bilayers, as shown in Figure 5.46.165 Helical ribbons (helix A) have cylindrical curvature with an inner face and an outer face and are the precursors of tubules. These are, for example, the same structures that are observed in the diacetylenic lipid systems discussed in Section 4.1. By contrast, twisted ribbons (helix B) have Gaussian saddlelike curvature, with two equally curved faces and a C2 symmetry axis. They are similar to the aldonamide and peptide ribbons discussed in Sections 2 and 3, respectively. The twisted ribbons in the tartrate-gemini surfactant system were found to be stable in water for alkyl chains with 14-16 carbons. Only micelles form... 

Figure 5.46 Schematic representation of helical and twisted ribbons as discussed in Ref. 165. Top Platelet or flat ribbon. Helical ribbons (helix A), precursors of tubules, feature inner and outer faces. Twisted ribbons (helix B), formed by some gemini surfactant tartrate complexes, have equally curved faces and C2 symmetry axis. Bottom Consequences of cylindrical and saddlelike curvatures in multilayered structures. In stack of cylindrical sheets, contact area from one layer to next varies. This is not the case for saddlelike curvature, which is thus favored when the layers are coordinated. Reprinted with permission from Ref. 165. Copyright 1999 by Macmillan Magazines.

The experiments discussed in this chapter have shown that a variety of chiral molecules self-assemble into cylindrical tubules and helical ribbons. These are indeed surprising structures because of their high curvature. One would normally expect the lowest energy state of a bilayer membrane to be flat or to have the minimum curvature needed to close off the edges of the membrane. By contrast, these structures have a high curvature, with a characteristic radius that depends on the material but is always fairly small compared with vesicles or other membrane structures. Thus, the key issue in understanding the formation of tubules and helical ribbons is how to explain the morphology with a characteristic radius. 

To address this issue, several researchers have developed models of chiral self-assembly. In this section, we review these models. We begin with models based on nonchiral mechanisms and argue that they all have limitations in identifying a mechanism to select a particular tubule radius. We then discuss models based on the elastic properties of chiral membranes and argue that they provide a plausible approach to understanding the formation of tubules and helical ribbons. Most of this discussion was previously presented in our recent theoretical review article.139... 

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