Cellulose physical structure

The terminal complex hypothesis proposes that the cellulose synthesizing enzyme complex can be visualized with electron microscopy. Terminal complex is the name given to collections of plasma membrane particles thought to represent the cellulose synthase. While direct evidence is still not available to support this hypothesis, the amount of indirect supporting evidence has grown dramatically in the past few years. The relationship between terminal complexes, cellulose physical structure and the biochemical events of cellulose biosynthesis will be discussed. 

Cellulose, a polysaccharide consisting of linear 1,4-/ -D-anhydroglucopyra-nose chains laterally associated by hydrogen bonds, is the most abundant and commercially important plant cell wall polymer (1). Consequently, cellulose is also one of the most thoroughly investigated plant cell wall polymers. However, it is enigmatic in the sense that significant elements of cellulose physical structure and the mechanism of cellulose biosynthesis still are not well understood. Since these subjects have been reviewed recently (2-10), this review will update topics covered previously and provide a new analysis of selected topics of contemporary interest. 

Influence of Cellulose Physical Structure on Thermohydrolytic, Hydrolytic, and Enzymatic Degradation of Cellulose... 

Though the major alkaline reactions of cellulose have been relatively well defined, the role of cellulose physical structure in those reactions has not been clearly established. Cellulose molecules have been reported to undergo physical stopping of the peeling reaction when a molecule is peeled back to a crystalline region in the cellulose structure, with the result that the reducing endgroup... 

In the present study, the role of cellulose physical structure in alkaline reactions was investigated by comparing the alkaline degradation of highly crystalline (cellulose I) fibrous hydrocellulose with that of amorphous (noncrystalline) hydrocellulose. The amorphous substrate was taken as a cellulose model the reactivity of which would most closely approximate that of alkali-soluble cellulose. The availablity of such an approximation to the inherent reactivity of cellulose allowed evaluation of the effects of the more highly ordered structure of the fibrous hydrocellulose. 

It has been established that changes can occur in the cellulose physical structure, using x-ray diffraction [132-134] and infrared spectroscopy [135,136] by several workers. Nine allotropes of alkali cellulose have been identified. Sobue [134] has described some of the possible transitions outlined in Figure 10.54 that are related to the alkalinity and temperature. 

Exercise 20-14 Explain how the /3-D-glucoside units of cellulose produce a polymer with a stronger, more compact physical structure than the a-D-glucose units of starch. Models will be helpful. 

Charring is known to preserve aspects of the physical structure of wood, seeds, and fruit (42). Srinivasan and Jakes (43) have shown that in charring some aspects of the physical shape of Indian hemp fiber are retained. In the carbonization of wood, Ercin et al (44) report the loss of cellulose, hemicellulose and lignin infrared absorbance bands in the range of 1300-1000 cm-1 and the appearance of two new bands at 1250 cm 1 attributed to the asymmetric C-O-C and at 1450 cm-1 attributed to aliphatic C-H bending. 

With the heterogeneous hydrolysis of polysaccharides like cellulose, these general considerations are valid, too, of course, but the rate of cleavage is slowed down by one or two orders of magnitude by the limited accessibility of the acetalic O atoms. The rate of reaction depends largely on the physical structure of the original samples and on the state... 

Much less information is available with regard to the action of hydrolyzing reagents in the gaseous state in relation to the physical structure of cellulose. With respect to enzymatic cleavage, the possibility of enhancing the reaction rate by increasing the accessibility is well known (6), but there are still open questions as to the relevance of the different ways to do this. 

A common feature of vapor-phase thermohydrolysis, liquid-phase acid hydrolysis, and enzymatic hydrolysis of cellulose is a significant influence of lateral order on the rate of chain cleavage, the DP finally reached, and weight loss. But with regard to this influence of the physical structure of cellulose, there also exist remarkable differences between these three modes of hydrolytic degradation. 

Further progress in understanding this effect, as well as others, of the physical structure of cellulose on enzymatic degradation may be expected from combining physicochemical and morphological techniques and from kinetic measurements in heterogeneous enzymatic hydrolysis, applying substrates of well-defined physical structure and isolated components of the enzyme systems. 

Lignocellulosic materials have a common basic structure, but vary greatly in chemical composition and physical structure.4 Typically, these materials contain 30 percent to 60 percent cellulose, 10 percent to 30 percent hemicellulose (polyoses), and 10 percent to 20 percent ligmn. Cellulose provides strength and flexibility, while lignin supports and protects the cellulose from biological and chemical attack. Hemicellulose bonds lignin to cellulose. 

Like APIs, pharmaceutical excipients are made by chemical synthesis, fermentation, recovery from natural materials, and so on. Often purification procedures may not be employed in the manufacture of such pharmaceutical excipients as clays, celluloses, starches, and natural gums. In addition, the physical and chemical change of certain excipients during processing is not uncommon. Unlike APIs, many excipients have complicated chemical and physical structures that do not yield easily to modern analytical and chromatographic methods. 

Influence of Physical Structure. The hydrolytic behavior of cellulose is much influenced by its physical structure and lateral order [121-132]. Wood cellulose was hydrolyzed twice as fast as cotton [125]. Hydrolysis rate was significantly increased by physical or chemical pretreatment, with the effect depending on the source of cellulose. Hill and coworkers [127,128] reported that mercerization increased the hydrolysis rate of cotton (by 40%) and of ramie (7%), whereas the opposite effect was observed for linen and a-cellulose samples showing an approximately 30% reduction. Based on kinetic analysis, they concluded that the end-attach model proposed by Sharpies [121] can only be applied to the cellulose II structure and not to the cellulose I crystallite. Thus, the conformation of cellulose is also a significant factor affecting its reactivity and possibly the hydrolytic mechanism as well. 

Cellulose undergoes chemical, physico-chemical and structural modifications on treatment with caustic soda solution of mercerizing strength [5]. Chemical reactions lead to the formation of alkali cellulose, physical reactions, to a change in arrangement of units of cellulose. The optimum modifications in the properties of... 

Stout has written a detailed review on jute and kenaf. X-ray diffraction patterns show the basic cellulose crystal structure, although in jute and kenaf the crystalline orientation is high and the degree of lateral order is lower than in flax. Batra" in a comprehensive review has highlighted the morphological structures and physical, mechanical and chemical properties of other long vegetable fibers. 

In the preparation of ion-exchange celluloses main attention is paid to their physical structure and controlled distribution of exchange groups in their mass. 

There is a wealth of data, both in the scientific and patent literature, on the chemical modification of plant cellulose. All of these methods are equally applicable to bacterial cellulose given that the two types of cellulose are chemically identical. However, it is the physical structure of bacterial cellulose membranes that make them a potential material for PEM fuel cells. Therefore, the aim is to modify bacterial cellulose pellicules in a manner that retains the structure of the cellulose and does not... 

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