Stearic acid unit cell

The mycobacterial cell wall contains the essential lipoglycan structures lipoarabinomannan, lipomannan, and phosphatidyl-my6>-inositol-mannosides (PIMs) [262]. These molecules are attached to the cytoplasmic membrane and extend through the cell wall into the extracellular environment (O Fig. 1). PIMs, LMs, and LAMs are attached to the cytoplasmic membrane by a conserved anchor, which consists of an sn-glycero-3-phospho-(l-D-my6>-inositol), with a single cx-D-Man/> linked at C2 of the my6>-inositol (O Fig. 19c). The mannan core is attached to C6 of the my6>-inositol unit. There are four potential sites of fatty acid attachment on the anchor both free hydroxyls of the glycerol unit, C6 of the Man/ linked to C2 of my6>-inositol and C3 of the my6>-inositol. The fatty acids are most commonly palmitic and tuberculostearic (10-methyl-octadecanoic) acids, and less frequently, stearic acid. Other fatty acids are seen in... 

Figure 6.3 Organization of the interface between a stearic acid monolayer (A), calcium ions (B) and carbonate and calcium layers (C). (C) corresponds to a vaterite ( = CaCOj) subcell. There is no geometric matching between the stearate groups (in A spacing 50 A) and the Ca-Ca distance in vaterite (C spacing 4.13 A). The two-layer sub-unit cell motif of the A-B layer, however, repeats in vaterite (C).

Fig. 4. A Unit cell of stearic acid (E form) while, hydrogen atoms gray, carbon atoms and black, oxygen atoms. Theoretical habit of stearic acid B growth form according to the BFDH law C growth form according to the attachment energy model.

Bromoheptadecanoic acid and 17-iodohepta-decanoic acid exhibit B- and C-forms but no A-form (Larsson, 1964). There is no transition B C on heating, however, and the B-form has the highest melting point (17-bromoheptadecanoic acid B liq. at 74.8 and C liq. at 73.0 °C, 17-iodoheptadecanoic acid B —> liq. at 83.2 °C and C liq. at 82.7 C). The unit cell dimensions of these substituted fatty acids are almost identical to those of stearic acid. 

The fatty acids are long chain carboxylic acids synthesised by the condensation and reduction of acetyl coenzyme-A units by fatty acid synthase. The more important ones have nonsystematic names in wide use. Stearic and palmitic acids are saturated (no double bonds), oleic acid is monounsaturated, and linoleic and linolenic are polyunsaturated (Table 3.1). All these common fatty acids are cis (E) fatty acids. Because of the links in the chain caused by the double bonds, the unsaturated fatty acids tend to be liquids at room temperature (they are less easy to pack together to form a solid). Bacteria and plants (which cannot thermoregulate) will use more unsaturated acids in their cell membranes when they are exposed to cold this helps to maintain membrane fluidity. 

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