Structure Long Hydrocarbon Chains

Let us first consider the lipid molecular structures required. First is the hydrophobic matching. The length of the hydrophobic chain determines the thickness of the hydrophobic part of the lipid bilayer, this should correspond closely to the dimension of the native membrane. As most biological membranes contain diacylglycerol lipids with hydrophobic chain lengths of 16 18 carbon atoms. Thus, synthetic lipids should possess relatively long hydrocarbon chain length, e.g., 16-18 carbon atoms. 

In Chapter 9, the displacement of an iodide ion from methyl iodide by a hydroxide ion (Eq. 9-76) was considered. Can we similarly displace a methyl group from ethane, CH3-CH3, to break the C-C bond and form CH3OH The answer is no. Ethane is perfectly stable in sodium hydroxide and is not cleaved by a simple displacement process within our bodies. Likewise, long hydrocarbon chains such as those in the fatty acids cannot be broken by a corresponding process during metabolism of fatty acids. Not every structure will allow a nucleophilic displacement reaction to occur and not every anion or neutral base can act to displace another group. 

Lxotmpii liquid crystals possess at least two components. One of these is water and the other is amphihle (a polar head group attached to one or more long hydrocarbon chains). In the lamellar form, water molecules are sandwiched between the polar heads of adjacent layers while the hydrocarbon tails lie in a nonpolar environment. Lyotropic liquid crystals have very complex structures, but occur abundantly in nature, particularly in living systems. See Fig. 3. 

The ring structure has long hydrocarbon chains attached at the corners so that they stand up on one side. These chains provide the hydrophobic component and the polarisable ring structure provides the hydrophilic moiety. 

The cleaning properties of the soap depend on its structure and bonding. Sodium stearate consists of a long hydrocarbon chain which is hydrophobic (water hating) attached to an ionic head which is hydrophilic (water loving) (Figure 15.16). 

Table 1.6 The classification of phospholipids according to the nature of their Ri group. R groups have long hydrocarbon chains, the same as those found in fatty acids. The R groups in a particular molecule do not have to have the same structures...

Structure and Fatty acids have a long hydrocarbon chain with a terminal carboxylic acid... 

When water is added to certain dry phospholipids with long hydrocarbon chains, the phospholipids swell, and when they are dispersed in more water, structures known as liposomes are formed. Liposomes are vesicles with multilayers of phospholipid. See Fig. 6-5. When subjected to ultrasonic vibration (sonication), liposomes are transformed into vesicles that have only a single bilayer of phospholipid. 

Both of these structures can be extended indefinitely into very long hydrocarbon chains. 

In most cases, groups in parentheses in the foimulas are side chains attached to the main polymer backbone. Some exceptions to this rule ate phenyl groups in the main chain, shown by (C H4), and long hydrocarbon chains, shown by ((THa). Keeping track of the number of bonds to each atom can allow the structure to be determined unambiguously. 

The three-dimensional structures of the fatty acids in Figure 10.6 illustrate how Z double bonds introduce kinks in the long hydrocarbon chain, decreasing the ability of the fatty acid to pack well in a crystalline lattice. The latter the numher of Z double bonds, the more kinks in the hydrocarbon chain, and the lower the melting point. 

Higher intrapellet residence times increase the contribution of chain initiation by a-olefins to chain growth pathways. This intrapellet delay, caused by the slow diffusion of large hydrocarbons, leads to non-Flory carbon number distributions and to increasingly paraffinic long hydrocarbon chains during FT synthesis. But intrapellet residence time also depends on the effective diameter and on the physical structure (porosity and tortuosity) of the support pellets. The severity of transport restrictions and the probability that a-olefins initiate a surface chain as they diffuse out of a pellet also de-... 

Some of the important organic acids occurring in nature are those in which there is a carboxyl group at the end of a long hydrocarbon chain. Palmitic acid, CjgHgiCOOH, and stearic add, CjyHgrCOOH, have structures of this sort. Oleic acid, C17H33COOH, is similar to stearic acid except that it contains a double bond between two of the carbon atoms in the chain. 

The stationary phase in partition chromatography is usually a long hydrocarbon chain covalently bound to silica gel. Silica gel has the structure shown in Fig. 1. The siloxane bonds ( Si—O—Si ) are stable to water and the other solvents used in HPLC between pH 2 and 9. The silanol ( Si—OH) groups can be bound to long hydrocarbon chains through reactions such as the following ... 

---