Chitin unit cell, proposed

The most acceptable structure for a-chitin is the unit cell proposed by Carlstrom, which is shown in Fig. 13. It is orthorhombic, and the cell dimensions are listed in Table I (see p. 422). Some 62 separate reflections are present in the fiber diagram, permitting a relatively high degree of confidence in the final result. The fiber repeat observed is identical with that of cellulose, and, in the early studies, this led Meyer and Mark to postulate that chitobiose is the fiber repeat, before it was isolated as a... 

Fig. 13.—The Unit Cell Proposed by Carlstrom for a-Chitin (Chitin 2). > (The cell is orthorhombic, and has two chains, one up, and the other down. Chains having the same polarity are hydrogen-bonded through planes of C=0-- H—N running perpendicular to the fiber axis.)...

The most acceptable structure for a-chitin is the unit cell proposed by Carlstrom( ) shown in Figure 8. The fiber repeat is identical with that of cellulose, the space group is P2i2 2j and the unit cell contains two antiparallel chains. 

Figure 8. The unit cell proposed by Carlstrdm (29) for a-chitin. The cell is orthorhombic and has two antiparallel chains. Chains navii...

Electron-diffraction patterns for a-chitin from the grasping spines of the marine worm Sagitta led to an orthorhombic unit-cell, with a = 0.474 nm, b = 1.886 nm, and c = 1.032 nm. The appearance of the 001 (1, odd) and OkO (k, odd) reflections cast doubt on the P21212, space group proposed by Blackwell and coworkers.60,61... 

The dihedral angles about the glycosidic bonds in a-chitin were the same as those in p-chitin, " but in a-chitin there are two chitin chains in the unit cell. To account for an X-ray reflection which should not have been diffracted by crystals of the derived space group, it was proposed that the hydroxymethyl group was disordered. Subsequently, however, it was shown that this reflection was a double diffraction artefact. The detailed structure of a-chitin is therefore uncertain. 

Chltln.—In a re-examination of X-ray diffraction data for chitin-protein systems, the complex from the ovipositor of the ichneumon fly Megarhssa has been used. It is proposed that the crystalline cr-chitin fibril is surrounded by a protein sheath of higher electron density to give a total fibril radius of 725 nm. Fibrils of this type are then packed on a hexagonal network vnth one fibril per unit cell. 

Possible conformations of the disaccharide-peptlde sub-units of the peptido-glycans of Staphylococcus aureus and Micrococcus luteus have been studied by an energy-minimization procedure. " Contrary to earlier reports the favoured conformation of the disaccharide 2-acetamido-2-deoxy-/8-D-glucosyl-(l->4)-.A -acetylmuramic acid is different from that of cellulose or chitin. Three types of conformation, two compact and one extended, are postulated with all three being stabilized by intramolecular hydrogen bonds. Two different models were proposed for the three-dimensional arrangement of peptidoglycan in the cell wall. 

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