Lipids and Detergents

Lipids (from the Greek lipos, fat) are constituents of plants or animais that are characterized by their solubility properties. 

In particular, lipids are insoluble in water but are soluble in nonpolar organic solvents, such as diethyl ether. Lipids can be extracted from cells and tissues by organic solvents. This solubility property distinguishes lipids from three other major classes of natural products—carbohydrates, proteins, and nucleic acids—which in general are not soluble in organic solvents. 

Lipids may vary considerably in chemical structure, even though they have similar solubility properties. Some are esters and some are hydrocarbons some are acyclic and others are cyclic, even polycyclic. We will take up each structural type separately. 

Fats and oils are familiar parts of daily life. Common fats include butter, lard, and the fatty portions of meat. Oils come mainly from plants and include corn, cottonseed, olive, peanut, and soybean oils. Although fats are solids and oils are liquids, they have the same basic organic structure. Fats and oils are triesters of glycerol and are called triglycerides. When we boil a fat or oil with alkali and acidify the resulting solution, we obtain glycerol and a mixture of fatty acids. The reaction is called saponification (Sec. 10.13). 

The most common saturated and unsaturated fatty acids obtained in this way are listed in Table 15.1. Although exceptions are known, most fatty acids are unbranched and contain an even number of carbon atoms. If double bonds are present, they usually have the cis (or Z) configuration and are not conjugated. 

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Hydrophobic forces The hydrophobic effect is the name given to those forces that cause nonpolar molecules to minimize their contact with water. This is clearly seen with amphipathic molecules such as lipids and detergents which form micelles in aqueous solution (see Topic El). Proteins, too, find a conformation in which their nonpolar side chains are largely out of contact with the aqueous solvent, and thus hydrophobic forces are an important determinant of protein structure, folding and stability. In proteins, the effects of hydrophobic forces are often termed hydrophobic bonding, to indicate the specific nature of protein folding under the influence of the hydrophobic effect. 

Doige CA, Yu X, Sharom FJ. The effects of lipids and detergents on ATPase-active P-glycoprotein. Biochim Biophys Acta 1993 1146(1) 65—72. 

Preparing lipid suspension. Mix the desired lipids in an aqueous medium, and vortex the mixture until a homogeneous slurry is obtained. Typically, chain-deuterated lipids (and detergents) are used for NMR purposes (see Note 5). 

Describe the behaviour of fats, lipids and detergents in water... 

Special proteins create special problems. It is difficult to incorporate membrane proteins into the crystals. In addition, it is unclear whether the membrane proteins remain associated with lipids and detergents and protein/lipid/detergent complexes jump into the gas (if anything jumps at all). It seems like the membrane protein bacteriorhodopsin was successfully analyzed with MALDI-TOF. With glycoproteins, the sugar residues sometimes shift position or are cut off by the acidic matrix or the photon current. Finally, a part of the matrix molecules disintegrates under the laser, reacts with the proteins, and thereby increases their MW. This becomes noticeable through so-called adduct peaks in the spectrum. 

Appendix Use of Perdeuterated Lipid and Detergent Micelles for Studies of Polypeptide Conformations by NMR... 

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