Carotenoids as raman-active probes of erythrocyte membrane structure

Abstract

1. 1. Erythrocyte ghosts exhibit resonance-enhanced Ramand bands at 1530 cm −1 and 1165 cm −1 attributable to ν(-CC-) and ν(C-C), respectively, of the conjugated polyene chains in carotenoids. In lipid extract of ghosts, these resonance-enhanced bands lie at 1527 and 1158 cm −1. The spectra indicate the presence of membrane-bound β-carotene. 2. 2. The resonance-enhanced Raman spectrum of β-carotene in lecithin liposomes is identical to that obtained with hexane or chloroform solutions. 3. 3. Increasing proportions of cholesterol in cholesterol-lecithin liposomes up to a cholesterol: phospholipid molar ratio of 0.8–0.9 drastically decreases the intensity of both resonance-enhanced bands. 4. 4. In ghosts the carotenoid bands respond to membrane perturbations. Trypsinization, lysolecithin treatment and reduction of pH increase the intensities of the 1530 and 1165 cm −1 bands. In contrast, a decrease in the intensity of both bands follows equilibration of ghosts for 15 min at approx. 50 °C or addition of (0.1%) sodium dodecyl sulfate. 5. 5. We suggest that pertubants known to change lipid-protein interactions in erythrocyte membranes modify the microenvironment and/or configuration of the membrane-bound carotenoid.