Structural insights on biologically relevant cationic membranes by ESR spectroscopy

Abstract

Cationic bilayers have been used as models to study membrane fusion, templates for polymerization and deposition of materials, carriers of nucleic acids and hydrophobic drugs, microbicidal agents and vaccine adjuvants. The versatility of these membranes depends on their structure. Electron spin resonance (ESR) spectroscopy is a powerful technique that employs hydrophobic spin labels to probe membrane structure and packing. The focus of this review is the extensive structural characterization of cationic membranes prepared with dioctadecyldimethylammonium bromide or diC14-amidine to illustrate how ESR spectroscopy can provide important structural information on bilayer thermotropic behavior, gel and fluid phases, phase coexistence, presence of bilayer interdigitation, membrane fusion and interactions with other biologically relevant molecules.

Exploring the pH-Induced Functional Phase Space of Human Serum Albumin by EPR Spectroscopy

“…The extraction and use of order parameters is well realized in EPR spectroscopy. Most applications in this concern are mainly restricted to physical properties of biological or artificial membrane systems [94,[123][124][125][126][127][128][129][130], but have been also tested, or discussed for albumin [40,131,132]. Here, analyses are restricted to 16-DSA-probed HSA, as the 5-DSA spin probe appears to lack decisive spectral features for order parameter construction, due to the intrinsic strong immobilization, as well as strong, irreducible spectral noise.…”

Structural characterization of cationic DODAB bilayers containing C24:1 β-glucosylceramide

“…The structure of pure DODAB and mixed DODAB + βGlcCer bilayers were compared by means of ESR spectroscopy, which probes the environment around the paramagnetic label attached to the phospholipid acyl chain [27,32]. For instance, 5-PCSL probes the bilayer close to the water interface, whereas 16-PCSL gives information about the bilayer core (Figure 1).…”

“…Empirical parameters obtained from ESR spectra can provide more details on membrane structure. In the gel phase an important parameter is the maximum hyperfine splitting (A max ), which is sensitive to the viscosity and packing of the environment surrounding the label [27]. In the cases where the A max cannot be accurately measured, such as the 16-PCSL spectra shown in Figure 3, the central field linewidth (ΔH 0 ) can be similarly employed.…”

“…At 35°C, the spectra of 5-and 16-PCSL in pure DODAB bilayers and in mixtures with 0.1 mM and 0.2 mM βGlcCer are typical of a tightly packed environment such as the gel phase (Figure 5). On the other hand, in the mixture containing 0.4 mM βGlcCer it is possible to observe two signals in the same spectrum, which are characteristic of the coexistence of rigid and fluid domains within the membranes [27,33] (Figure 5). At 40°C, the spectra of 5-and 16-PCSL in pure DODAB membranes and in mixtures with 0.1 mM and 0.2 mM βGlcCer display two signals in the same spectrum, while the spectra for the mixture containing 0.4 mM βGlcCer are more isotropic and characteristic of a fluid phase (Figure 5).…”

“…DODAB bicelles have been used to solubilize hydrophobic drugs in their hydrophobic edges [27,35]. However, lipid miscibility in DODAB vesicles is dependent on molar fraction: DPPC below 30mol%, for instance, was shown to demix from the cationic gel phase [36].…”