Acylation hydrocarbon cores

Biological membranes are constructed of lipid bilayers 3 nm (30 A) thick, with proteins protruding on each side. The hydrocarbon core of the membrane, made up of the —CH2— and —CH3 of the fatty acyl groups, is about as nonpolar as decane, and liposomes formed in the laboratory from pure lipids are essentially impermeable to polar solutes, as are biological membranes (although the latter, as we shall see, are permeable to solutes for which they have specific transporters). 

B) Amiodarone incorporated into a membrane bilayer, showing the position of the iodine atoms relative to the acyl chains and the size of the molecule relative to the hydrocarbon core region. (Reprinted from Fig. 4 of ref. 126 with permission of the American Chemical Society)... 

Ion conduction is increased when membrane fluidity is increased and molecular packing is decreased. The insertion of polar groups into the hydrocarbon core can increase the fluidity of a membrane by preventing close packing of acyl chains [53]. In contrast, low concentrations of lipophilic molecules such as dolichyl phosphate are oriented in a membrane with their... 

A surpnsing feature of the reactions of hexafluoroacetone, trifluoropyruvates, and their acyl imines is the C-hydroxyalkylation or C-amidoalkylaOon of activated aromatic hydrocarbons or heterocycles even in the presence of unprotected ammo or hydroxyl functions directly attached to the aromatic core Normally, aromatic amines first react reversibly to give N-alkylated products that rearrange thermally to yield C-alkylated products. With aromatic heterocycles, the reaction usually takes place at the site of the maximum n electron density [55] (equaUon 5). 

Fig. 10.5. Cholesterol in the plasma membrane. The polar hydroxyl group of cholesterol is oriented toward the surface. The hydrocarbon tail and the steroid nucleus (blue) lie in the hydrophobic core. A cis double bond in the fatty acyl chain of a phospholipid bends the chain to create a hydrophobic binding site for cholesterol.

Sterols are a class of lipids containing a common steroid core of a fused four-ring structure with a hydrocarbon side chain and an alcohol group. Cholesterol is the primary sterol lipid in mammals and is an important constituent of cellular membranes. Oxidization and/or metabolism of cholesterol yield numerous oxysterols, steroids, bile acids, etc., many of which are important signaling molecules in biological systems. Cholesteryl esters esterified with a variety of fatty acyls are enriched in... 

Possible sites of orientation of the PNPA in the micelles are shown in Fig. 11.16. Either the PNPA is located where specific interactions between the nitro group and the surfactant head groups can occur or alternatively, the aromatic nucleus may interact with the hydrocarbon chains of the surfactant, not far from the polar core while the acyl portion interacts with the head groups. 

Recently, Hoffmann et al. [12] have described the molecular and crystalline solid structure of 2-n-nonyl-1,4-phenylene bis(4-n-octyloxybenzoate), as estimated using X-ray diffraction [12]. The observed structure deviates from the model for the nematic phase in the crystalline solid state, as the lateral chains exist in -trans conformation. The type of link between the lateral hydrocarbon chain and the mesogenic core influences the course of the clearing points. A comparison performed for the 2-alkyl, acyl and alkyloxycarbonyl substituted derivatives 2a-2c in Table 1 showed a break in the clearing-point curve at 2b and 2c for five single units (including the -CO- or -CO-0 groups) within the lateral chains. The lower the transition temperatures the more pronounced the break in the curve [13, 15]. 

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