Modeling, cellulose

In one proposed model, cellulose synthase spans the plasma membrane and uses cytosolic UDP-glucose as the precursor for extracellular cellulose synthesis. In another, a membrane-bound form of sucrose synthase forms a complex with cellulose synthase, feeding UDP-glucose from sucrose directly into cell wall synthesis (Fig. 20-32). 

The phase transfer catalyzed alkylation reaction of dodecyl phenyl glycidyl ether (DPGE) with hydroxyethyl cellulose (HEC) was studied as a mechanistic model for the general PTC reaction with cellulose ethers. In this way, the most effective phase transfer catalysts and optimum reaction concentrations could be identified. As a model cellulose ether, CELLOSIZE HEC11 was chosen, and the phase transfer catalysts chosen for evaluation were aqueous solutions of choline hydroxide, tetramethyl-, tetrabutyl-, tetrahexyl-, and benzyltrimethylammonium hydroxides. The molar A/HEC ratio (molar ratio of alkali to HEC) used was 0.50, the diluent to HEC (D/HEC) weight ratio was 7.4, and the reaction diluent was aqueous /-butyl alcohol. Because some of the quaternary ammonium hydroxide charges would be accompanied by large additions of water, the initial water content of the diluent was adjusted so that the final diluent composition would be about 14.4% water in /-butyl alcohol. The results of these experiments are summarized in Table 2. 

This paper is a review of x-ray diffraction work in the authors laboratory to refine the structures of cellulose I and II, and a- and B-chitin, concentrating on the methods used to select between alternate models. Cellulose I is shown to consist of an array of parallel chains, and this conclusion is supported by a separate refinement based on electron diffraction data. In the case of cellulose II, both parallel and antiparallel chain... 

Figure 15.6 Reaction scheme for modeling cellulose hydrolysis [17],...

Meunier-Goddik, L., and Penner, M. H. 1999. Enzyme-Catalyzed Saccharification of Model Celluloses in the Presence of Lignacious Residues. /. Agric. Food Chem., 47, 346-351. 

A start has been made toward solving the structural problems of crystalline cellulose by the same method as that used for proteins. Jones (1046a) gave a set of criteria quite similar to that in Table 10-IV namely, (a) the sugar residues have standard bond lengths and angles, (b) all the O—H groups are H bonded with C—O 0 between 100° and 135°, and, (c) the residues are screw related. This paper also contains a helpful review of previous work on model cellulose structures. 

Only cellulose is the feedstock of pyrolysis process treated in the model. Cellulose mass consunq>tion represents the degree of the pyrolytic conversion. Figures 10 and 11 show the mass losing curves as pyrolysis proceeds for 2 ram and 10 mm particles respectively. As can be seen that cellulose density decreases all the way down to zero. The density in the outside surface layer decreases much faster than the center. It takes about 5s for the 2 mm cellulose particle to be devolatilized completely, and 60s for the 10mm particle. It also shown that devolatilization of cellulose particles proceeds layer by layer, which is more obvious for the outside layers and for the large particle. 

Enzymatic action can be defined on three levels operational kinetics, molecular architecture, and chemical mechanism. Operational kinetic data have given indirect information about cellulolytic enzyme mode of action along with important information useful for modeling cellulose hydrolysis by specific cellulolytic enzyme systems. These data are based on measurement of initial rates of enzyme hydrolysis with respect to purified celluloses and their water soluble derivatives over a range of concentrations of both substrate and products. The resulting kinetic patterns facilitate definition of the enzyme s mode of action, kinetic equations, and concentration based binding constants. Since these enable the enzymes action to be defined with little direct knowledge of its mechanistic basis, the rate equations obtained are referred to as operational kinetics. The rate patterns have enabled mechanisms to be inferred, and these have often coincided with more direct observations of the enzyme s action on a molecular level [2-4]. 

In two closely related studies Wendler and Frazier examined, by NMR, the interaction between both model cellulose compounds [22] and wood with enriched polymeric diphenylmethane diisocyanate (pMDI) [23]. The resin formed is used commercially as a wood adhesive. Previous work [17] had shown that this reaction is sensitive to moisture, the formation of different products depending upon the degree of moisture present (Fig. 15.2.22). 

FIGURE 9.23 Contours of iso-n and iso-h values in —ip space for model cellulose helices composed of the nonreducing residue of crystalline cellobiose [ 179] and a value of t = 116. Also shown as dots are the experimentally determined values of

from crystal structures of small molecules related to cellulose. With one exception at the bottom of the map, all of the dots correspond to extended helices with the numbers of residues per turn between 2 and 3. Geometries found in complexes of cellulose fragments and proteins are shown as triangles. They have a similar distribution but the range is expanded. Left-handed helices are indicated with negative values of n. 

Joshi SC, Lam YC (2006) Modeling cellulose heat and degree of gelation for methyl hydrogels with NaQ additives. J Appl Polym Sci 101 1620-1629... 

TG-DTA-FTIR curves of model cellulose compounds, such as glucose and sucrose, can be found in Ref. 32. 

A series of assumptions were made to build the model. Cellulose has been added in the compound data bank considering that only density properties were necessary, since the deinking process is almost isothermal. It was assumed that waste paper enters the pulper at 15°C, that the temperature of fresh water and white water entering the system is set at 50°C, that there is no heat loss from the process piping and equipment, and that output... 

Wickholm, K., Hull, E.-L., Larsson, P T, Iversen, T, and Lennholm, E. (2001). Quantification of cellulose forms in complex cellulose materials A chemometric model. Cellulose, 8,139-148. 

Eriksson, J Malmsten, M Tiberg, F CaUisen, TH Damhus, T Johansen, KS. Model cellulose films exposed to H. insolens glucoside hydrolase family 45 endo-cellulase— the effect of the carbohydrate-binding module. Journal of Colloid and Interface Science, 2005,285, 94-9... 

---