Journal of Colloid and Interface Science 256, 100–105 (2002) doi:10.1006/jcis.2002.8470 Electron Paramagnetic Resonance Study of the Structure of Lipid Bilayers in the Presence of Sodium Dodecyl Sulfate Namita Deo, ∗ P. Somasundaran, ∗,1 K. Subramanyan,† and K. P. Ananthapadmanabhan† ∗ NSF IUCR Center for Advanced Studies in Novel Surfactants, Langmuir Center for Colloid and Interfaces, Columbia University, New York, New York 10027; and †Unilever Research USA, 45 River Road, Edgewater, New Jersey 07020 Received August 7, 2001; accepted May 9, 2002 Harshness (skin irritation) of personal cleansing products is re- lated to surfactant interactions with proteins and lipids in the upper layers of skin (stratum corneum). Cleanser surfactants can damage stratum corneum lipids either by their solubilzation in surfactant micelles or by fluidization of the lipid bilayers by surfactant pen- etration. The mechanism of interaction of sodium dodecyl sulfate (SDS) with a model phospholipid membrane is investigated in this work by studying the vesicle-to-micelle structural transition, which occurs due to the interaction of a phospholipid bilayer membrane with SDS. It was observed that the optical density as well as the hydrodynamic diameter increased upon the addition of SDS up to 2 mM due to surfactant adsorption on the liposomes and then de- creased gradually upon further addition of SDS due to transition of the vesicle to a micelle. Two inflection points were observed on both the surface tension as well as SDS monomer activity plotted vs solubilization, corresponding to the onset and complete solubiliza- tion of the liposome, respectively. The electron paramagnetic reso- nance (EPR) spectrum of 5-doxyl stearic acid (5-DSA), a lipid probe molecule, indicates immobilization of the probe molecule in the lipid bilayer in SDS-free solution. The mobility of 5-doxyl molecules in the liposome changes slightly with SDS concentration up to 2 mM, supporting the hypothesis that SDS molecules adsorb on the lipo- some without any structural disruption. Upon further addition of SDS, the mobility of the lipid probe increases sharply, indicating the disruption of the bilayer, ultimately resulting in complete solubiliza- tion of the liposome into a mixed micelle with SDS. The hyperfine coupling constant value of 5-doxyl molecules in a mixed micelle is observed to be higher than that in a pure SDS micelle, suggesting the core of the mixed micelle is more hydrophobic than that of the SDS micelle. C  2002 Elsevier Science (USA) INTRODUCTION Biological membranes composed of regularly packed am- phiphilic molecules with a polar head group and a hydropho- bic tail are of fundamental importance in the biochemistry of living systems as they provide suitable barriers and microenvi- ronments for controlled transport of solutes. In aqueous envi- ronments these molecules form bilayers where the packing of 1 To whom correspondence should be addressed. the constituent molecules is determined by the lipid head group– water interactions, the inter- and intramolecular interactions of the alkyl chains, and the shape of the molecules. In human skin, lipid bilayers are known to provide the water barrier function of the outermost layers of skin. Surfactants are commonly used as solubilizing agents in the isolation, purification, and reconstitution or crystallization of membrane proteins. For the efficient use of surfactants, it is im- portant to have an accurate knowledge of how they interact with integral membrane proteins and membrane lipid, under both solubilizing and nonsolubilizing conditions. Harshness (skin ir- ritation) of personal cleansing products is related to surfactant interactions with proteins and lipids in the upper layers of skin (stratum corneum). Cleanser surfactants can damage stratum corneum lipids either by their solubilzation in surfactant mi- celles or by fluidization of the lipid bilayers by surfactant pen- etration. The sublytic action of surfactants on the phospholipid bilayers leads to the incorporation of surfactant molecules into the bilayer structures as governed by the equilibrium between bilayers and the aqueous phase (1). This incorporation involves complex perturbations in the physical properties of vesicle mem- branes, which depend on the type and amount of the surfactants partitioned (2–4). Several spectroscopic tools have been used in the past to inves- tigate the microstructure of biological membranes (5–7). Elec- tron paramagnetic resonance (EPR) has been used to investigate the microenvironment in micelles and vesicles by measuring the nitrogen-coupling constant and EPR spectral linewidths (8) of nitroxide spin probes. The coupling constant is affected by the local polarity in which the nitroxide moiety resides. A more polar environment gives larger coupling constants because of the greater electron density on the nitrogen. The linewidths are controlled by rotational and lateral diffusion of the spin probes, which in turn is affected by the viscosity and temperature of the local environment (9, 10). Larger ordering parameters of doxyl stearic acids at the interfacial regions of micelles or vesicles compared with that of the hydrocarbon region, for example, in- dicates oriented DSA with its polar head group at the interface. Broader linewidth results from the slower tumbling of the spin probe with a longer relaxation time, which can be controlled by changing the local viscosity around the nitroxide probe. 100 0021-9797/02 $35.00 C  2002 Elsevier Science (USA) All rights reserved.