Lipid biophysical characteristics

Despite the fact that many different cationic lipids have been synthesized and tested for transfection (25 34), relatively few systematic structure activity TE-relationship studies have been performed (35 39). As a result, no general relationship between chemical structure and TE could be drawn from these studies. One reason for this is that the chemical structure of a cationic lipid is not directly responsible for TE. TE rather depends on the biophysical characteristics of the cationic lipid aggregate (e.g., liposomes and lipoplexes), which, for its part, is dependent on the chemical structure of the lipids. In a previous study with analogs of the transfection lipid A-[l-(2,3-dioleoyloxy) propyl]-A,A,A-trimethylammoniumchloride (DOTAP) (40) which differ in their nonpolar hydrocarbon chains, it could be shown that the TE strongly depended on the biophysical properties of the resulting liposomes and lipoplexes (35). Minimal alterations of biophysical properties by using lipids with different hydrocarbon chains or by mixing the lipid with different neutral helper lipids could completely allow or prevent transfection. 

A systematic analysis to determine the biophysical characteristics of some cationic lipid/DNA complexes from this series was undertaken (Eastman et al., 1997). Product 1 (Figure 15.7) is currently being used in clinical trials as a carrier of CFTR gene in cystic fibrosis. The synthesis of these cholesteryl derivatives is similar to that of DC-Chol cholesteryl chloroformate is reacted with spermine or spermidine as polycation (Lee et al., 1996). 

The method provides a means of studying biochemical and biophysical characteristics of many lipids, proteins, and amino acids. Many investigations have been devoted to the synthesis of radicals having a suitable reactive functional group to couple as selectively as possible and with minimum perturbation of the biological system. The latter requirement... 

Biophysical analysis of biomolecules like proteins, nucleic acids, or lipids utilizes intrinsic physical properties of the observed molecule itself or of an associated reporter molecule, which reflect information about structural characteristics, interactions, or reactions of the subject observed. In most cases the analysis (and the labels introduced) only interferes slightly with the interaction of interest and does not induce significant changes in the properties of the reactants. 

Lipidated peptides embodying the characteristic linkage region found in the parent lipoproteins and bearing additional functional groups, which could be traced in biological systems or which allowed for their use in biophysical experiments, were used successfully in model studies. However, such model studies only provide a limited amount of information. In order to approximate the situation in a biological system more precisely, experiments with differently lipidated proteins are required. 

Chapman, D. (1973). Phase transitions and fluidity characteristics of lipids and cell membranes. Quarterly Reviews of Biophysics 8,185-235. 

---