The Behavior of Biological Lipids

Lipids active in biological systems demonstrate a broad range of behavior in water, from hydrocarbons which are insoluble, to molecules such as bile salts that possess potent detergent properties and interact with water rather dynamically. The purpose of this paper is to describe some of the physical properties of lipids with respect to their inter— action with aqueous systems, and to classify lipids based on these inter— actions. The hydrocarbon part of a lipid molecule may be aliphatic or cyclic/aromatic. In predominantly aliphatic lipids, the hydrocarbon part consists of a chain(s), containing eight or more carbons. All such lipids have specific properties related to the hydrocarbon chain packing, while cyclic/aromatic lipids possess unique properties related to the specific hydrocarbon structure. Three basic types of aliphatic chain packing can be found in most predominantly aliphatic chain molecules: 1) a tightly packed aliphatic chain lattice with specific chain—chain interactions and minimum specific volume, 2) an intermediate form of crystalline packing in which specific chain—chain interactions are lost, but the chains are packed in a centered hexagonal lattice, 3) a liquid state in which the —CH2— groups can move more or less freely. Transitions between these states are characterized by abrupt changes in volume and excess specific heat (enthalpy). The partial specific volume of the —CH2— group in the aliphatic chain increases from "233/—CH2— in tightly packed chains to —26A3/—CH2— in hexagonal packing to 293/—CH2— in the liquid state. The enthalpies of the transition are: —1 kcal/—CH2— from the crystalline lattice (specific chain—chain interaction) to liquid and -O.5 kcal/—CH2— from the hexagonal chain packing lattice to liquid. Thus, a change in volume of 1A3/—CH2— requires about 0.17 kcal. The transition from crystalline to liquid chain can occur from either tightly packed or more loosely packed crystalline structures. These transitions are known by a variety of names depending upon the lipid system. For instance, it is called the melting point in alkanes, fatty acids, fatty alcohols, di— and triglycerides, waxes, etc.; the order—disorder transition or gel—liquid crystal transition in soaps, monoglycerides, phospholipids, etc.; the critical micellar temperature or Krafft point in soaps and detergents. This transition temperature is governed by the length of the hydrocarbon chain and the presence of double bonds, cyclic structures, or branches within the chain. The hydrophilic part of the lipid also plays a major part in the chain transition temperatures, as well as in the interaction with aqueous systems. The crystal—liquid chain transition for a given chain length increases in the following order: alkenes < chlorides < alkanes < bromides < aldehydes < alcohols < fatty acids < triglycerides < monoglycerides < Na soaps < phosphatidylcholines < phosphatidylethanolamines < Ca soaps. Lipids in which the aliphatic chain is in the fluid state, may be classified empirically as nonpolar and polar based on their interaction at the air—water interface and in bulk systems. Nonpolar molecules are insoluble in aqueous systems and do not spread at the air—water interface. Nonpolar molecules include hydrocarbons, waxes, and sterol esters. Polar molecules may be divided into three distinct classes: I. Insoluble Non—swelling Amphiphiles. These molecules spread at the air—water interface to form a itable monolayer but are insoluble in the bulk. This class includes long—chain fatty acids, primary amines, alcohols, cholesterol, di— and triglycerides. II. Insoluble Swelling Amphiphiles. These molecules form stable monolayers, but swell in water to form liquid crystalline