Conformational order of phospholipids incorporated into human erythrocytes: An FTIR spectroscopy study

Acyl chain perdeuterated dimyristoylphosphatidylcholine (DMPC-d₅₄) and dimyristoylphos-phatidylserine (DMPS-d₅₄) were incorporated by incubation into human erythrocytes. Light microscopic analysis demonstrated that erythrocytes incubated with DMPC-d₅₄ became echinocytic while those incubated with DMPS-d₅₄ became stomatocytic. This indicates that DMPC- d₅₄ was incorporated preferentially into the outer monolayer whereas DMPS- d₅₄ was selectively incorporated into the inner monolayer. Fourier transform infrared (FTIR) spectroscopy was used to monitor the conformational order of the incorporated phospholipids. The asymmetric CD₂ stretching frequency of the inserted perdeuterated acyl chains was measured in both isolated membranes and intact erythrocytes as a function of temperature. DMPC-d₅₄ incorporated into erythrocytes exhibited a cooperative phase transition at ~19 °C, i.e., at the same temperature as pure vesicles. In contrast, DMPS-d₅₄ incorporated into red cells exhibited no phase transition, but possessed conformational order similar to that of the liquid- crystalline state. These results suggest that DMPC-d₅₄ persists in domains in the outer monolayer while DMPS-d₅₄ is dispersed in the inner monolayer. These experiments are the first to demonstrate that FTIR spectroscopy can be utilized to monitor directly a specific species of lipid molecule from the entire phospholipid population.