Quantitative determination of conformational order in biomembranes: From aqueous monolayers to living cells

Two regions of the IR spectrum of phospholipid molecules have been utilized to monitor acyl chain conformational order in model biomembranes of increasing complexity from one component systems to living cells. The rocking modes of CD₂ groups synthetically incorporated into the acyl chains provide a measure of % gauche rotamer formation as a function of chain position from the interfacial region to the bilayer center. In addition, the CH₂ wagging vibrations in disordered phases provide a means of identifying the particular two- and three-bond conformational states that dominate the disordering process, while the band progressions that result from the wagging of coupled CH₂ oscillators in ordered states monitor the fraction of all-trans conformations. The latter spectral parameter has been used to characterize the "liquidordered" phase that occurs in some regions of the cholesterol/phosphatidylcholine phase diagram as well as to demonstrate the existence of residual all-trans order in living cell membranes of Acholeplasma laidlawii B. Finally, the feasibility of studying both the lipid and protein components of insoluble monolayer films at the A/W interface is demonstrated. The coupling of a miniaturized surface balance to an IR interferometer in a "dual channel" design permits reduction of the water vapor signals to the point where protein spectra can be readily acquired.