Macrofibril

Cellulose is insoluble in water because of the high affinity of the polymer chains for one another. Its individual polymeric chains have molecular weights of 50,000 or greater. The molecular chains of cellulose interact in parallel bundles of about 2,000 chains. Each bundle constitutes a single microfibril. Many microfibrils arranged in parallel constitute a macrofibril, which can be seen under the light microscope. Figure 12.10 shows the inner cell walls of the plant Valonia the fibrils in the wall are almost pure cellulose. 

In macrofibrils of the hair, KAPs comprise the amorphous matrix that is believed to crosslink keratin bundles in the mature hair (Powell and Rogers, 1997). How KAPs contribute to this crosslinking phenomenon remains controversial. Disulphide bonds between keratin tails and matrix proteins, as well as within and in between different matrix proteins, may play a role. Alternatively, apolar interactions between glycine loops in glycine-tyrosine-rich KAPs type II keratins may take place (Powell and Rogers, 1997). 

Dale, B. A., Holbrook, K. A., and Steinert, P. M. (1978). Assembly of stratum corneum basic protein and keratin filaments in macrofibrils. Nature 276, 729-731. 

From the nano scale to the macroscopic aspect, wood is structured in a hierarchical way. The molecules of cellulose (40-50%) are arranged first in microfibrils, then in macrofibrils and finally they constitute the layers of the wall of every cell in the wood (Figure 5.4). 

Figure 3-2. Schematic drawing of a wool fiber, showing alpha-helix, protofibril, microfibril, and macrofibril (Drawing courtesy of Stephen Tee-garden). 

The final fiber cell wall structure is essentially layers of the microfibrils or macrofibrils aligned in several different directions, as shown in Fig. 28.6. The microfibrils that make up the wood fiber are visible under the scanning electron microscope, which has a greater magnification than the light microscope. 

Cotton Cellulose To prepare a fibrillar or macrofibrillar suspension of native cellulosic fibrils, approximately 1% of cotton fiber was blended with water for 3 h in an ordinary blender and then filtered through a polyester screen having openings of 240 X 240 /un supported by a Buechner funnel to which a vacuum was applied. The filtrate was a cloudy suspension that contained fibrils and macrofibrils ranging in size from 100A to several micrometers in width. 

In the structure of a single hair, microfi- brils are embedded in an amphorous protein matrix and hundreds of such microfibrils are cemented into an irregular fibrous bundle called a macrofibril. These macrofibrils are 1 grouped together to form the cortex (or the main body) layers of the hair fiber, as illustrated in Figure 9-29. 

Figure 15. TEM of cellulose micr ibrils (Mi) and macrofibrils (Ma) uMhin the secondary fiber wall of balsam fir direct carbon replica.

The annealed surface layer may be stabilized by lateral association with those hemicelluloses that bond strongly to the microfibrils. Also in the G-layer, which is mostly cellulose, the presence of any (uncertain) hemicelluloses and the layer s strongly hydrated nature (gelatinous implies a gel) must prevent strong lateral bonding of the individual microfibrils to form larger macrofibrils. 

In order to study the interactions of side chains in the rod domains within the macrofibrils consisting of KIF/filaggrin, NMR experiments were per-... 

Fig. 23.20. A series of NMR speetra taken at 76.77 MHz of mouse KIF and mouse macrofibrils where, in both creases, the keratin chains have been labeled with l-[4,4,5,5- H4]lysine. Each spectrum was acquired using the quadrupole-echo sequence and is an average of 50000 acquisitions. The temperatures at which these equilibrium spectra were taken are (a,f) 25, (b,g) -10, (c,h) -20, (d,i) -35, (e,j) -45°C. Even at the lowest temperature, variations in the lineshapes indicate that the onset of the quenching of side chain motions is occurring more significantly for the lysines of the macrofibrils than for those of the KIF.

The a-helical rodlike domains of two keratin polypeptides form a coiled coil. Two staggered antiparallel rows of these dimers form a supercoiled protofilament. Hundreds of filaments, each containing four protofilaments, form a macrofibril. 

The cell walls are organised in layers, of which the secondary wall is the thickest (several microns), possessing the greatest degree of crystallinity and making the dominant contribution to the physical properties of the fibre. Within these walls the macrofibrils are wound around the fibre axis, with a sense and angle characteristic of the fibre species - for flax, the secondary cell wall is S (i.e. anticlockwise) wound at about 6.5°. 

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