Parallel-down model

Using the two-chain unit-cell,3 with a = 0.817 nm, b = 0.785 nm, c = 1.034 nm, andy = 96.38°, the modified intensity-data of Mann and coworkers,37 and several residue-geometries, the structure of native ramie cellulose was refined. The resulting R factors were 15.8%, 18.5%, and 17.5% for, the antiparallel, parallel-up, and parallel-down models, respectively. A temperature factor of 0.23 nm2 was necessary in order to obtain a good fit with the observed data. It was suggested that the antiparallel packing of the chains cannot be discounted for cotton and ramie celluloses. 

The favored models are anti-parallel or parallel-up. In both models, one chain has two intramolecular inter-residue hydrogen bonds as in cellulose I, while the other chain has only the 0(3)H- 0(5 ) observed in cellobiose. All hydroxyls are utilized. The parallel-down models appear to be inconsistent with this criterion however, this does not necessarily exclude them from consideration [515]. 

Table XII. Parallel Down Models from Clemson University...

Hydrodynamics Well-stirred model, i.e. uniform concentration inside intestine Parallel tube model, i.e. concentration decrease exponentially down the length of the intestine... 

Some Idea of the ultimate ability of x-ray data to distinguish among models can be obtained by substituting calculated data from one model for the observed data in the program. This simulates having "perfect" data that arise from one chain polarity. The other models (parallel down and antiparallel) are then adjusted to produce the calculated patterns that best match the calculated Intensities from the parallel up model. Table IX shows that the number of reflections observed by RMF should be sufficient to provide a differentiation among the models If the data and chain models are accurate. 

Table IX. Fit of Parallel Down and Antlparallel Models to Intensities Calculated from Parallel Up Model with 06 tg.

All Models Parallel-Down, Sarko-Muggll Geometry, SRRC Computer Program, RMF Data... 

The crystal structure of native ramie cellulose was shown to be similar to that recently reported for Valonia cellulose. The earlier conclusion that the ramie diffraction data could not be satisfied by a conventional cellulose model is refuted. R (an indication of diffraction error) values of 0.158, 0.185, and 0.175 were obtained for anti-parallel, parallel up, and parallel down alignments respectively. The author considers that the anti-parallel model provides the best accounting for the ramie data and is therefore probably the correct model for both cotton and ramie cellulose. The changes in crystallinity and physical characteristics of microcrystalline cellulose which occur on grinding have been studied. Under one set of grinding conditions, the specific surface area rose... 

The KDF filter was first tested in prototype on a coal mine in northern Germany. It was installed in parallel with existing vacuum filters and it produced filter cakes consistendy lower in moisture content by 5 to 7% than the vacuum filters. Two production models have been installed and operated on a coal mine in Belgium. The filter is controlled by a specially developed computer system this consists of two computers, one monitoring the function of the filter and all of the detection devices installed, and the other controlling the filtration process. The system allows optimization of the performance, automatic start-up or shut-down, and can be integrated into the control system of the whole coal washing plant. 

B) Surface and stick representation of the model, viewed down the fibril axis. (The top coil is shown as sticks, with the remainder showing van derWaals radii.) The glutamine side chains are proposed to form hydrogen-bonded stacks parallel to the fibril axis. The large diameter of the cylinder results in a pore down the center. Both panels were generated with Pymol (DeLano, 2002). 

Fig. 17. Models of A/ (l-40) fibrils, viewed down the fibril axis. (A, B) Cartoon representations of the hairpin model proposed by Petkova et al. (2002). (A) The proposed /1-strands span residues 9-24 and 30-40, with a main-chain bend spanning residues 25-29. Aspartate 23 and lysine 28 are proposed to form a salt bridge (dotted line) based on distance constraints provided by ssNMR. The hairpins stack in-register to form two parallel //-sheets. (B) Two hairpin stacks pack together to form the smallest observed fibrils, or protofilaments, burying the hydrophobic residues of the C-terminal strand. Two or more protofilaments may pack together to form thicker fibrils. (C) Cartoon representation of the parallel /i-helix-like model proposed byj. T. Guo and Y. Xu (unpublished model shown in Fig. 1 of Shivaprasad and Wetzel, 2004). The gray oval highlights residues 17 and 34, proposed to sit in close proximity.

A project produces a subset of models and diagrams that describe the design. For some projects, the order in which these are produced will be mostly top-down for others, more bottom-up. In almost all cases there are multiple development tasks that can proceed in parallel depending on project resources and constraints. In all cases, the relationships among the artifacts are the same, and the most important initial methodology question for any project to answer is... 

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