Helical channel

An open-framework zinc phosphate synthesized under mild hydrothermal conditions possesses two interpenetrating helical channels.414 Piperazine phosphate yields a variety of open framework structures in reaction with zinc, including linear chain, layer, and three-dimensional systems.415... 

X-ray diffraction studies on gramicidin commenced as early as 1949 218-219> and this early work pointed to a helical structure 220). Recent work by Koeppe et al. 221) on gramicidin A crystallised from methanol (/%) and ethanol (.P212121) has shown that the helical channel has a diameter of about 5 A and a length of about 32 A in both cases. The inclusion complexes of gramicidin A with CsSCN and KSCN (P212121) have channels that are wider (6-8 A) and shorter (26 A) than the uncomplexed dimer 221 222). Furthermore there are two cation binding sites per channel situated either 2.5 A from either end of the channel or 2.5 A on each side of its centre 222) Unfortunately these data do not permit a choice to be made from the helical models (i)—(iv) and it is not certain if the helical canals studied are the same as those involved in membrane ion transport. 

Major effort has therefore been applied to study of the helical channels by non-erystallographic methods in order to discriminate unambigously between the likely models. This has resulted in the gramicidin ion channel not only being the first clearly identified example but also the most characterised in structural terms. The bulk of... 

In order to understand the mechanisms for fluid flow in the metering channel, it is important to understand the reference frames used in the mathematical analysis of the section. As discussed in many chapters leading up to this point, the extruder is a cylindrical structure, as shown in Fig. 7.1(a), that has a helical channel formed... 

Stadler, A. M., Kyritsakas, N., Lehn, J.-M., Reversible folding/unfolding of linear molecular strands into helical channel-like complexes upon proton-modulated binding and release of metal ions. Chem. Conrnmn. 2004, 2024-2025. 

Example 6.2 The Synthesis of the Inverse Single Screw Pump There is, however, another, more elegant way to relax the channel-length constraint, as shown in Fig. E6.2. After one circumference, a flow channel formed by the moving and stationary planes of a given width can be twisted helically by an amount equal to the width of the channel to create a much longer helical channel on the same roll or shaft. The channel itself can be simply machined onto the inner surface of the stationary cylinder or barrel. 

As in the inverse screw pump, we relax the channel length constraint to one circumference by twisting it and making it helical [Fig. E6.4(d)]. We create the helical channel by machining it onto a solid shaft, resulting in a screw. We now have a single... 

Fig. E6.4 The synthesis of the SSE for building block 1. (a) The building block (b) a rotary hollow cylinder forms the moving plane (c) the shallow channel of certain width spread over one circumference of the cylinder (d) a twisted helical channel relaxes the length constraint (e) the channel machined onto a solid shaft, the rotation of the cylinder interchanged with that of the shaft, and feeding and discharge ports fixed on the cylinder or barrel, resulting in an SSE.

We commence its development by reversing the conceptual synthesis process. The space between a tightly fitting screw and the barrel is a helical channel. We unwind the channel from the screw and lay it on a flat surface. The result is a shallow rectangular straight channel, as in Fig 6.9. 

Fig. 6.49 The unwound helical channel segment forming a single chamber of the counterrotating twin screw.

In screw extruders, as we have seen, the processing takes place in the helical channel formed between the screw and the barrel. In a co-rotating disk processor (CDP), on the other hand, processing takes place in flat doughnut-shaped processing chambers formed by two neighboring disk surfaces, the inner surface of the barrel, the shaft to which the disks are attached, and the channel block attached to the barrel, with very small clearance to the disk surfaces. It thus blocks the channel and separates inlet from outlet, as shown in Fig. 9.44. Processing chambers can be connected in series, parallel, or any other... 

Downstream from the mixing element section there are two separate inverse SSEs (see Section 6.2) that have the helical channel machined into the barrel, thus needing only rotating shafts to convey the material. Two pressure transducers, one diameter apart, record the pressure built up at closed discharge, a parameter that can be used to measure the viscosity of the molten blend at various shear rates. This rheometry section is the forerunner of the Helical Barrel Rheometer (HBR) of the Polymer Processing Institute (118). 

Another even more intensively studied example is alamethicin (Aim), an a-helical channel-forming peptide that forms voltage-activated ion channels in lipid bilayers. It consists of 20 amino acids and exists as two major variants ... 

Figure 7.16. Top view of the helical channels of the zinc phosphate (Neeraj et al. [30]).

As already described, the counterion can template the formation of different oxalate frameworks. For example, the synthesis of a heterobimetallic 3D framework K" [Cr " (ox)3] was achieved with [Cu(trans[14]dien)] " (trans[14] dien = 5,7,7,12,14,14-hexamethyl-l,4,8,11-tetraazacyclotetradeca-4-11-diene) (66). While each and Cr atom is octahedrally coordinated, each Cu " " ion coordinates an oxalate oxygen atom in a monodentate fashion producing 21 X 9 A helical channels (Fig. 4). This generates a ths net (7) structure, while the previous 3D oxalate networks consist of chiral srs nets. Magnetic studies showed an antiferromagnetic interchain interaction between the copper and chromium centers. 

Which of the following toxins fit the helical channel model ... 

The myosin heads are helically distributed and the actin molecules form helical double strands. There are also additional helical elements attached to the actin threads, but they can be ignored in this context. The cross-sectional arrangement of actin and myosin shown in Fig. 8.8 is consistent with the Q surface. The myosin molecviles are centred on the 62 axes and actin on 3l axes, which occur in the proportion 2 1. The Q surface partitioning of space into helical channel systems corresponds to the position of the myosin threads. There is thus no connection between adjacent channel systems, i.e. between neighbouring myosin threads. To vmderstand the connections between these channels, we can consider the rectangular nets, which span this surface, shown in Fig. 8.9. The channels exhibit four-coordination alternatively we can regard the vmits as four-armed. 

Among open-framework metal phosphates, zincophosphates exhibit diverse compositions and fascinating structural topologies.[212] Notable examples are zincophosphates with extra-large pores bound by 16-, 20-, and 24-rings, and helical channels. 

Figure 2.94 [NH3(CH2)2NH2(CH2)2NH3][Zn4(P04)3(HP04)hH20. (a) Helical columns are interconnected via the HP04 groups to form the 8-ring cavities and the helical channels (b) a view of the 8-ring channels along the [010] direction. Reproduced with permission from [215]. Copyright (1999) Royal Society of Chemistry...

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