Strand Model

A particular frailty of one-dimensional systems is the elfect of a defect or impurity within the chain itself. The interrupted strand model was developed for the situation where the blockages dominate the electron motion (253, 345, 346). The model was originally applied to K2Pt(CN)4Bro.3(H20)s and was also 

This differs qualitatively from a conductivity increasing with decreasing temperature for uninterrupted strands. Similarly, it was shown (120, 342) that the specific heat and magnetic susceptibility for the interrupted strands can differ greatly both in magnitude and temperature dependence from that expected for continuous strands. The predicted optical conductivity and dielectric constant for the interrupted strand model (120,173,344) is more complicated than that for the continuous strands. 

Thus good crystal morphology is of crucial importance for experimentally establishing the intrinsic nature of highly conducting one-dimensional materials. Impurities, disorder, and defects can drastically change the measured material properties. 

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The dimensions of the xylan unit cell are slightly different a = b = 1.340 nm, (fibre axis) = 0.598 nm.) Atkins and Parker T6) were able to interpret such a diffraction pattern in terms of a triple-stranded structure. Three chains, of the same polarity, intertwine about a common axis to form a triple-strand molecular rope. The individual polysaccharide chains trace out a helix with six saccharide units per turn and are related to their neighbours by azimuthal rotations of 2ir/3 and 4ir/3 respectively, with zero relative translation. A similar model for curdlan is illustrated in Figure 6. Examinations of this model shows that each chain repeats at a distance 3 x 0.582 = 1.746 nm. Thus if for any reason the precise symmetrical arrangement between chains (or with their associated water of crystallization) is disrupted, we would expect reflections to occur on layer lines which are orders of 1.746 nm. Indeed such additional reflections have been observed via patterns obtained from specimens at different relative humidity (4) offering confirmation for the triple-stranded model. 

When the sample is dehydrated the X-ray diffraction pattern obtained is of poorer quality (Fig. 4b) and is similar to that reported for lentinan (8). We suggest that the removal of water causes a twisting of the chains back toward the six-fold triple-stranded model. On annealing, the sample completes this transition (Fig. 4c) by exhibiting a pattern similar to Figure 3. The reflections observed in Figure 4c index on a hexagonal unit cell with dimensions a = b = 1.530 nm, c (fibre axis) = 1.76 nm and the measured densTty Ts 1.52 g/cm . 

The X-ray evidence favours a model for the curdlan gel based on triple-stranded molecules. This structure is present in both the gel prepared from aqueous suspension and from alkali solution. The alkali gel contains the proposed seven-fold triple-stranded model (Figs. 7 and 14a) which converts to the more symmetric six-fold triple helix by annealing (Figs. 6 and 14b). This... 

Figure 14. Comparison of triple-stranded models involve (a) seven-fold helices ...

The cylindrically averaged Fourier transform of the sevenfold and six-fold triple-stranded structures are shown in Figure 15. The Fourier transform of the six-fold triple-stranded model illustrates the symmetry of the system by the total absence of intensity on layer lines with index i 3 n, where n is an integer. The Fourier transform of the seven-fold triple-stranded structure shows that in destroying this precise symmetry relationship intensity occurs on all layer lines which are orders of the 2.27 nm spacing. This reinforces the concept of an indigenous triple-stranded structure which is perturbed slightly by the interaction of solvent. 

Not only have you got all those negatively charged phosphates in the middle, she said, but your molecule is too dense. Her own studies had shown that in the body, DNA soaked up a great deal of water, too much to be accounted for by the three-stranded model. She was convinced that DNA existed in two forms, one dry and more dense, the other wet and fully extended. The phosphates need to be on the outside, she said, where they could be encased in water. She eventually showed Watson and Crick her X-ray... 

Crick (1952) pointed out that this difficulty could be overcome by supposing that the a-helices in a-keratin were distorted in a helical manner to form coiled coils as illustrated in Fig. 14. This distortion, which was claimed to require only about 0.1 kcal per residue, enabled the side chains to pack more neatly. In subsequent papers Crick obtained an expression for the Fourier transform of a coiled coil (Crick, 1953a) and was able to show that this type of distortion could account in a general way for some previously unexplained features of the X-ray pattern of a-keratin (Crick, 1953b) including the simultaneous appearance of 1.5 and 5.15 A meridional reflections. Detailed descriptions of two-strand and three-strand ropes of these coiled coils were given in which the pitch of the major helix was 186 A, and it was suggested that the three-strand model was appropriate... 

Weidner, T., Apte, J.S., Gamble, L.J., Castner, D.G. Probing the orientation and conformation of ot-helix and p-strand model peptides on self-assembled monolayers using sum frequency generation and nexafs spectroscopy. Langmuir 26, 3433-3440 (2010)... 

Example 1 A simple double-strand model of a protein polymer is shown in Figure 14(a). 

Figure 6 Simplified molecular representations (a) strands model of the backbone of enzyme trypsin color coded by secondary structure elements (yellow beta strands magenta alpha helices blue and white coils and turns) (b) ribbon model of the backbone of enzyme trypsin and (c) Richardson style representation of crambin (arrows beta strands cylinders alpha helices tubes coils and turns)...

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