Spin-Label Electron Spin Resonance Studies on the Dynamics of the Different Phases of N-Biotinylphosphatidylethanolamines

Abstract

The chain dynamics and phase behavior of a homologous series of diacyl-N-biotinylphosphatidylethanolamines of chain lengths from C(12:0) to C(20:0) were investigated by spin-label electron spin resonance (ESR) spectroscopy using both phosphatidylcholine and N-biotinylphosphatidylethanolamine spin-label probes. Chain-melting phase transition temperatures determined from the ESR spectral measurements, for all the five lipids in the presence as well as in the absence of 1 M NaCl, correlate with the endothermic phase transition temperatures determined by differential scanning calorimetry [Swamy, M. J., Angerstein, B., & Marsh, D. (1994) Biophys. J. 66, 31-39], confirming that the latter correspond to chain-melting transitions. ESR spectra obtained in the gel phase from phosphatidylcholine probes with the spin-label near the terminal methyl of the hydrocarbon chain showed a similar degree of immobilization (as reflected by the outer hyperfine splittings) to that for spin-labels positioned close to the glycerol backbone, indicating that the biotin-lipids of chain lengths from 14 to 20 carbon atoms form interdigitated gel phases in the presence of salt, as also do those of chain lengths between 16 and 20 carbon atoms in the absence of salt. For dispersions of the C(16:0) chain length biotin-lipid in 1 M NaCl, continuous monitoring of the central ESR peak intensity as a function of temperature detects a cooperative decrease in mobility in the fluid phase at ca. 65 °C with a spin-label in the lipid head group and an accompanying increase in mobility at the same temperature with spin-labels positioned toward the terminal methyl of the hydrocarbon chain. These changes in mobility correspond to the transition from the novel fluid isotropic (IM) phase, which has been identified also for the biotin-PEs of shorter chain lengths in 1 M NaCl [Swamy, M. J., Würz, U., & Marsh, D. (1993) Biochemistry 32, 9960-9967], to the fluid lamellar (Lα) phase and demonstrate that the surfaces of the lipid aggregates in the IM phase are positively curved. Comparison of the lipid chain flexibility profiles shows that the ESR spectra of spin-labels in the region of the 5 C-atom of the sn-2 chain in dispersions of the different biotin-PEs in the fluid phase are significantly less anisotropic in the absence of salt than they are in the presence of 1 M NaCl, whereas those from labels close to the chain ends were somewhat more motionally hindered. This indicates that the fluid phase in the absence of salt has a chain packing characteristic of (normal) micelles, whereas that in the IM phase is more similar to that of a fluid lamellar phase. © 1994, American Chemical Society. All rights reserved.