Detergent solubilization of phosphatidylcholine bilayers in the fluid state: influence of the acyl chain structure

Beyond the standard model of solubilization: Non-ionic surfactants induce collapse of lipid vesicles into rippled bilamellar nanodiscs

HYPOTHESIS

Although solubilization of lipid membranes has been studied extensively, questions remain regarding the structural pathways and metastable structures involved. This study investigated whether the non-ionic detergent Triton X-100 follows the classical solubilization pathway or if intermediate nanostructures are formed.

EXPERIMENTS

Small angle X-ray and neutron scattering (SAXS/SANS) was used in combination with transmission electron cryo-microscopy and cryo-tomography to deduce the structure of mixtures of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) vesicles and Triton X-100. Time-resolved SAXS and dynamic light scattering were used to investigate the kinetics of the process.

FINDINGS

Upon addition of moderate detergent amounts at low temperatures, the lipid vesicles implode into ordered rippled bilamellar disc structures. The bilayers arrange in a ripple phase to accommodate packing constraints caused by inserted TX-100 molecules. The collapse is suggested to occur through a combination of water structure destabilization by detergents flipping across the membrane and osmotic pressure causing interbilayer attraction internally. The subsequently induced ripples then stabilize the aggregates and prevent solubilization, supported by the observation that negatively charged vesicles undergo a different pathway upon TX-100 addition, forming large bicelles. The findings demonstrate the richness in assembly pathways of simple lipids and detergents and stimulate considerations for the use of certain detergents in membrane solubilization.

Two novel cyanobacterial α-dioxygenases for the biosynthesis of fatty aldehydes

α-Dioxygenases (α-DOXs) are known as plant enzymes involved in the α-oxidation of fatty acids through which fatty aldehydes, with a high commercial value as flavor and fragrance compounds, are synthesized as products. Currently, little is known about α-DOXs from non-plant organisms. The phylogenic analysis reported here identified a substantial number of α-DOX enzymes across various taxa. Here, we report the functional characterization and Escherichia coli whole-cell application of two novel α-DOXs identified from cyanobacteria: CalDOX from Calothrix parietina and LepDOX from Leptolyngbya sp. The catalytic behavior of the recombinantly expressed CalDOX and LepDOX revealed that they are heme-dependent like plant α-DOXs but exhibit activities toward medium carbon fatty acids ranging from C10 to C14 unlike plant α-DOXs. The in-depth molecular investigation of cyanobacterial α-DOXs and their application in an E. coli whole system employed in this study is useful not only for the understanding of the molecular function of α-DOXs, but also for their industrial utilization in fatty aldehyde biosynthesis.

Key points

• Two novel α-dioxygenases from Cyanobacteria are reported

• Both enzymes prefer medium-chain fatty acids

• Both enzymes are useful for fatty aldehyde biosynthesis

Soapwort (Saponaria officinalis L.) Extract vs. Synthetic Surfactants-Effect on Skin-Mimetic Models

Our skin is continuously exposed to different amphiphilic substances capable of interaction with its lipids and proteins. We describe the effect of a saponin-rich soapwort extract and of four commonly employed synthetic surfactants: sodium lauryl sulfate (SLS), sodium laureth sulfate (SLES), ammonium lauryl sulfate (ALS), cocamidopropyl betaine (CAPB) on different human skin models. Two human skin cell lines were employed: normal keratinocytes (HaCaT) and human melanoma cells (A375). The liposomes consisting of a dipalmitoylphosphatidylcholine/cholesterol mixture in a molar ratio of 7:3, mimicking the cell membrane of keratinocytes and melanoma cells were employed as the second model. Using dynamic light scattering (DLS), the particle size distribution of liposomes was analyzed before and after contact with the tested (bio)surfactants. The results, supplemented by the protein solubilization tests (albumin denaturation test, zein test) and oil emulsification capacity (using olive oil and engine oil), showed that the soapwort extract affects the skin models to a clearly different extent than any of the tested synthetic surfactants. Its protein and lipid solubilizing potential are much smaller than for the three anionic surfactants (SLS, ALS, SLES). In terms of protein solubilization potential, the soapwort extract is comparable to CAPB, which, however, is much harsher to lipids.

Self-consistent field modeling of mesomorphic phase changes of monoolein and phospholipids in response to additives

Mapping the topological phase behaviour of lipids in aqueous solution is time consuming and finding the ideal lipid system for a desired application is often a matter of trial and error. Modelling techniques that can accurately predict the mesomorphic phase behaviour of lipid systems are therefore of paramount importance. Here, the self-consistent field theory of Scheutjens and Fleer (SF-SCF) in which a lattice refinement has been implemented, is used to scrutinize how various additives modify the self-assembled phase behaviour of monoolein (MO) and 1,2-dioleoyl-phosphatidylcholine (DOPC) lipids in water. The mesomorphic behaviour is inferred from trends in the mechanical properties of equilibrium lipid bilayers with increasing additive content. More specifically, we focus on the Helfrich parameters, that is, the mean and Gaussian bending rigidities (κ and [small kappa, Greek, macron], respectively) supplemented with the spontaneous curvature of the monolayer (Jm0). We use previously established interaction parameters that position the unperturbed DOPC system in the lamellar Lα phase ([small kappa, Greek, macron] < 0, κ > 0 and Jm0 ≈ 0). Similar interaction parameters position the MO system firmly in a bicontinuous cubic phase ([small kappa, Greek, macron] > 0). In line with experimental data, a mixture of MO and DOPC tends to be in one of these two phases, depending on the mixing ratio. Moreover we find good correlations between predicted trends and experimental data concerning the phase changes of MO in response to a wide range of additives. These correlations give credibility to the use of SF-SCF modelling as a valuable tool to quickly explore the mesomorphic phase space of (phospho)lipid bilayer systems including additives.

Modeling the saturation of detergent association in mixed liposome systems

Cells tune the lipid types present in their membranes to adjust for thermal and chemical stability, as well as to promote association and dissociation of small molecules and bound proteins. Understanding the influence of lipid type on molecule association would open doors for targeted cell therapies, in particular when molecular association is observed in the presence of competing membranes. For this reason, we modeled and experimentally observed the association of a small molecule with two membrane types present by measuring the association of the detergent Triton X-100 with two types of liposomes, egg phosphatidylcholine (ePC) liposomes and egg phosphatidic acid (ePA) liposomes, at varying ratios. We called this mixed liposomes, as each liposome population was formed from a different lipid type. Absorbance spectrometry was used to observe the stages of detergent association with mixed liposomes and to determine the detergent concentration at which the liposomes were fully saturated. A saturation model was also derived that predicts the detergent associated with each liposome type when the lipid bilayers are fully saturated with detergent. The techinical input parameters for the model are the detergent to lipid ratio and the relative absorbance intensity for each of the pure liposome species at saturation. With that, the association of detergent with any mixture of those liposome types at saturation can be determined.

Lipid shape determination of detergent solubilization in mixed-lipid liposomes

The effects of lipid charge and head group size on liposome partitioning by detergents is an important consideration for applications such as liposomal drug delivery or proteoliposome formation. Yet, the solubilization of mixed-lipid liposomes, those containing multiple types of lipids, by detergents has received insufficient attention. This study examines the incorporation into and subsequent dissolution of mixed-lipid liposomes comprised of both egg phosphatidylcholine (ePC) and egg phosphatidic acid (ePA) by the detergent Triton-X100 (TX). Liposomes were prepared with mixtures of the two lipids, ePC and ePA, at molar ratios from 0 to 1, then step-wise solubilized with TX. Changes in turbidity, size distribution, and molar heat power at constant temperature throughout the solubilization process were assessed. The data suggest that the difference in lipid shapes (shape factors = 0.74 and 1.4 [1,2]) affects packing in membranes, and hence influences how much TX can be incorporated before disruption. As such, liposomes containing the observed ratios of ePA incorporated higher concentrations of TX before initiating dissolution into detergent and lipid mixed-micelles. The cause was concluded to be increased mismatching in the bilayer from the conical shape of ePA compared to the cylindrical shape of ePC. Additionally, the degree to which ePA is approximated as conical versus cylindrical was modulated with pH. It was confirmed that less conical ePA behaved more similarly to ePC than more conical ePA. The understanding gained here on lipid shape in liposome incorporation of TX enables research to use in vitro liposomes that more closely mimic native membranes.

Unveiling the multi-step solubilization mechanism of sub-micron size vesicles by detergents

The solubilization of membranes by detergents is critical for many technological applications and has become widely used in biochemistry research to induce cell rupture, extract cell constituents, and to purify, reconstitute and crystallize membrane proteins. The thermodynamic details of solubilization have been extensively investigated, but the kinetic aspects remain poorly understood. Here we used a combination of single-vesicle Förster resonance energy transfer (svFRET), fluorescence correlation spectroscopy and quartz-crystal microbalance with dissipation monitoring to access the real-time kinetics and elementary solubilization steps of sub-micron sized vesicles, which are inaccessible by conventional diffraction-limited optical methods. Real-time injection of a non-ionic detergent, Triton X, induced biphasic solubilization kinetics of surface-immobilized vesicles labelled with the Dil/DiD FRET pair. The nanoscale sensitivity accessible by svFRET allowed us to unambiguously assign each kinetic step to distortions of the vesicle structure comprising an initial fast vesicle-swelling event followed by slow lipid loss and micellization. We expect the svFRET platform to be applicable beyond the sub-micron sizes studied here and become a unique tool to unravel the complex kinetics of detergent-lipid interactions.

Lipid- and Cholesterol-Mediated Time-Scale-Specific Modulation of the Outer Membrane Protein X Dynamics in Lipid Bilayers

Membrane protein function fundamentally depends on lipid-bilayer fluidity and the composition of the biological membrane. Although dynamic interdependencies between membrane proteins and the surrounding lipids are suspected, a detailed description is still missing. To uncover lipid-modulated membrane protein backbone dynamics, time-scale-specific NMR relaxation experiments with residue-resolution were recorded. The data revealed that lipid order, modified either biochemically or biophysically, changes the dynamics of the immersed membrane protein in a specific and time-scale-dependent manner. A temperature-dependent dynamics analysis furthermore suggests a direct coupling between lipid and protein dynamics in the picosecond-nanosecond, microsecond, and millisecond time scales, caused by the lipid's trans-gauche isomerization, the segmental and rotational motion of lipids, and the fluidity of the lipid phase, respectively. These observations provide evidence of a direct modulatory capability of the membrane to regulate protein function through lipid dynamics ranging from picoseconds to milliseconds.

Biomembrane solubilization mechanism by Triton X-100: a computational study of the three stage model

The solubilization mechanism of lipid membranes in the presence of Triton X-100 (TX-100) is investigated at molecular resolution using hybrid particle field–self consistence field simulations.

Hepatocyte culture in autologous decellularized spleen matrix

BACKGROUND AND AIMS

Using decellularized scaffold to reengineer liver tissue is a promising alternative therapy for end-stage liver diseases. Though the decellularized human liver matrix is the ideal scaffold for reconstruction of the liver theoretically, the shortage of liver donors is still an obstacle for potential clinical application. Therefore, an appropriate alternative scaffold is needed. In the present study, we used a tissue engineering approach to prepare a rat decellularized spleen matrix (DSM) and evaluate the effectiveness of this DSM for primary rat hepatocytes culture.

METHODS

Rat decellularized spleen matrix (DSM) was prepared by perfusion of a series of detergents through spleen vasculature. DSM was characterized by residual DNA and specific extracellular matrix distribution. Thereafter, primary rat hepatocytes were cultured in the DSM in a 3-dimensional dynamic culture system, and liver cell survival and biological functions were evaluated by comparison with 3-dimensional sandwich culture and also with cultured in decellularized liver matrix (DLM).

RESULTS

Our research found that DSM did not exhibit any cellular components, but preserved the main extracellular matrix and the intact vasculature evaluated by DNA detection, histology, immunohistochemical staining, vessel corrosion cast and upright metallurgical microscope. Moreover, the method of DSM preparation procedure was relatively simple with high success rate (100%). After seeding primary hepatocytes in DSM, the cultured hepatocytes survived inside DSM with albumin synthesis and urea secretion within 10 d. Additionally, hepatocytes in dynamic culture medium had better biological functions at day 10 than that in sandwich culture. Albumin synthesis was 85.67 ± 6.34 μg/10(7) cell/24h in dynamic culture in DSM compared to 62.43 ± 4.59 μg/10(7) cell/24h in sandwich culture (P < 0.01) and to 87.54 ± 5.25 μg/10(7) cell/24h in DLM culture (P > 0.05); urea release was 32.14 ± 8.62 μg/10(7) cell/24h in dynamic culture in DSM compared to 20.47 ± 4.98 μg/10(7) cell/24h in sandwich culture (P < 0.05) and to 37.38 ± 7.29 μg/10(7) cell/24h cultured in DLM (P > 0.05).

CONCLUSION

The present study demonstrates that DSM can be prepared successfully using a tissue engineering approach. The DSM is an appropriate scaffold for primary hepatocytes culture.

Lipid bilayers in the gel phase become saturated by triton X-100 at lower surfactant concentrations than those in the fluid phase

It has been repeatedly observed that lipid bilayers in the gel phase are solubilized by lower concentrations of Triton X-100, at least within certain temperature ranges, or other nonionic detergents than bilayers in the fluid phase. In a previous study, we showed that detergent partition coefficients into the lipid bilayer were the same for the gel and the fluid phases. In this contribution, turbidity, calorimetry, and 31P-NMR concur in showing that bilayers in the gel state (at least down to 13-20°C below the gel-fluid transition temperature) become saturated with detergent at lower detergent concentrations than those in the fluid state, irrespective of temperature. The different saturation may explain the observed differences in solubilization.

Lipids of plant membrane rafts

Lipids tend to organize in mono or bilayer phases in a hydrophilic environment. While they have long been thought to be incapable of coherent lateral segregation, it is now clear that spontaneous assembly of these compounds can confer microdomain organization beyond spontaneous fluidity. Membrane raft microdomains have the ability to influence spatiotemporal organization of protein complexes, thereby allowing regulation of cellular processes. In this review, we aim at summarizing briefly: (i) the history of raft discovery in animals and plants, (ii) the main findings about structural and signalling plant lipids involved in raft segregation, (iii) imaging of plant membrane domains, and their biochemical purification through detergent-insoluble membranes, as well as the existing debate on the topic. We also discuss the potential involvement of rafts in the regulation of plant physiological processes, and further discuss the prospects of future research into plant membrane rafts.

Coexistence of immiscible mixtures of palmitoylsphingomyelin and palmitoylceramide in monolayers and bilayers

A combination of lipid monolayer- and bilayer-based model systems has been applied to explore in detail the interactions between and organization of palmitoylsphingomyelin (pSM) and the related lipid palmitoylceramide (pCer). Langmuir balance measurements of the binary mixture reveal favorable interactions between the lipid molecules. A thermodynamically stable point is observed in the range approximately 30-40 mol % pCer. The pSM monolayer undergoes hyperpolarization and condensation with small concentrations of pCer, narrowing the liquid-expanded (LE) to liquid-condensed (LC) pSM main phase transition by inducing intermolecular interactions and chain ordering. Beyond this point, the phase diagram no longer reveals the presence of the pSM-enriched phase. Differential scanning calorimetry (DSC) of multilamellar vesicles reveals a widening of the pSM main gel-fluid phase transition (41 degrees C) upon pCer incorporation, with formation of a further endotherm at higher temperatures that can be deconvoluted into two components. DSC data reflect the presence of pCer-enriched domains coexisting, in different proportions, with a pSM-enriched phase. The pSM-enriched phase is no longer detected in DSC thermograms containing >30 mol % pCer. Direct domain visualization has been carried out by fluorescence techniques on both lipid model systems. Epifluorescence microscopy of mixed monolayers at low pCer content shows concentration-dependent, morphologically different pCer-enriched LC domain formation over a pSM-enriched LE phase, in which pCer content close to 5 and 30 mol % can be determined for the LE and LC phases, respectively. In addition, fluorescence confocal microscopy of giant vesicles further confirms the formation of segregated pCer-enriched lipid domains. Vesicles cannot form at >40 mol % pCer content. Altogether, the presence of at least two immiscible phase-segregated pSM-pCer mixtures of different compositions is proposed at high pSM content. A condensed phase (with domains segregated from the liquid-expanded phase) showing enhanced thermodynamic stability occurs near a compositional ratio of 2:1 (pSM/pCer). These observations become significant on the basis of the ceramide-induced microdomain aggregation and platform formation upon sphingomyelinase enzymatic activity on cellular membranes.

Nuclear envelope formation: mind the gaps

During mitosis in metazoans, the nuclear envelope (NE) breaks down at prophase and reassembles at telophase. The regulation of NE assembly is essential to correct cell functioning. The complex issue of the regulation of NE formation remains to be solved. It is still uncertain that a single mechanism depicts NE formation during mitosis. The aim of this review is to address some of the cytological, biophysical, and molecular aspects of models of NE formation. Our emphasis is on the role of lipids and their modifying enzymes in envelope assembly. We consider how the NE can be used as a model in characterizing membrane dynamics during membrane fusion. Fusion mechanisms that give insight into the formation of the double membrane of the envelope are summarized. We speculate on the possible roles of phosphoinositides in membrane fusion and NE formation.

Visualization and analysis of lipopolysaccharide distribution in binary phospholipid bilayers

Lipopolysaccharide (LPS) is an endotoxin released from the outer membrane of Gram-negative bacteria during infections. It have been reported that LPS may play a role in the outer membrane of bacteria similar to that of cholesterol in eukaryotic plasma membranes. In this article we compare the effect of introducing LPS or cholesterol in liposomes made of dipalmitoylphosphatidylcholine/dioleoylphosphatidylcholine on the solubilization process by Triton X-100. The results show that liposomes containing LPS or cholesterol are more resistant to solubilization by Triton X-100 than the binary phospholipid mixtures at 4 degrees C. The LPS distribution was analyzed on GUVs of DPPC:DOPC using FITC-LPS. Solid and liquid-crystalline domains were visualized labeling the GUVs with LAURDAN and GP images were acquired using a two-photon microscope. The images show a selective distribution of LPS in gel domains. Our results support the hypothesis that LPS could aggregate and concentrate selectively in biological membranes providing a mechanism to bring together several components of the LPS-sensing machinery.

Membrane protein frustration: protein incorporation into hydrophobic mismatched binary lipid mixtures

Bacteriophage M13 major coat protein was reconstituted in different nonmatching binary lipid mixtures composed of 14:1PC and 22:1PC lipid bilayers. Challenged by this lose-lose situation of hydrophobic mismatch, the protein-lipid interactions are monitored by CD and site-directed spin-label electron spin resonance spectroscopy of spin-labeled site-specific single cysteine mutants located in the C-terminal protein domain embedded in the hydrophobic core of the membrane (I39C) and at the lipid-water interface (T46C). The CD spectra indicate an overall alpha-helical conformation irrespective of the composition of the binary lipid mixture. Spin-labeled protein mutant I39C senses the phase transition in 22:1PC, in contrast to spin-labeled protein mutant T46C, which is not affected by the transition. The results of both CD and electron spin resonance spectroscopy clearly indicate that the protein preferentially partitions into the shorter 14:1PC both above and below the gel-to-liquid crystalline phase transition temperature of 22:1PC. This preference is related to the protein tilt angle and energy penalty the protein has to pay in the thicker 22:1PC. Given the fact that in Escherichia coli, which is the host for M13 bacteriophage, it is easier to find shorter 14 carbon acyl chains than longer 22 carbon acyl chains, the choice the M13 coat protein makes seems to be evolutionary justified.

Interactions of surfactants with lipid membranes

Surfactants are surface-active, amphiphilic compounds that are water-soluble in the micro- to millimolar range, and self-assemble to form micelles or other aggregates above a critical concentration. This definition comprises synthetic detergents as well as amphiphilic peptides and lipopeptides, bile salts and many other compounds. This paper reviews the biophysics of the interactions of surfactants with membranes of insoluble, naturally occurring lipids. It discusses structural, thermodynamic and kinetic aspects of membrane-water partitioning, changes in membrane properties induced by surfactants, membrane solubilisation to micelles and other phases formed by lipid-surfactant systems. Each section defines and derives key parameters, mentions experimental methods for their measurement and compiles and discusses published data. Additionally, a brief overview is given of surfactant-like effects in biological systems, technical applications of surfactants that involve membrane interactions, and surfactant-based protocols to study biological membranes.

Phase diagrams of lipid mixtures relevant to the study of membrane rafts

The present paper reviews the phase properties of phosphatidylcholine-sphingomyelin-cholesterol mixtures, that are often used as models for membrane "raft" microdomains. The available data based on X-ray, microscopic and spectroscopic observations, surface pressure and calorimetric measurements, and detergent solubilization assays, are critically evaluated and rationalized in terms of triangular phase diagrams. The remaining uncertainties are discussed specifically and separately from the data on which a consensus appears to exist.

Surfactants as microbicides and contraceptive agents: a systematic in vitro study

Background

The urgent need for cheap and easy-to-use protection against both unwanted pregnancies and sexually transmitted diseases has stimulated considerable interest in the use of surfactants as microbicides, anti-viral, and contraceptive agents in recent years. In the present study we report a systematic in vitro evaluation of the microbicidal, anti-viral and contraceptive potential of cationic, anionic, zwitterionic, and non-ionic surfactants.

Methodology/Principal Findings

Toxicity was evaluated in mammalian columnar epithelial (MDCK) cells, human sperm cells, Candida albicans, Escherichia coli, Pseudomonas aeruginosa, Neisseria gonorrhoeae, Streptococcus agalactiae and Enterococcus faecalis. The inhibition of adenovirus and lentivirus infection of MDCK cells was also tested. A homologous series of cationic surfactants, alkyl-N,N,N-trimethylammonium bromides (C_nTAB), with varying alkyl chains were shown to be bactericidal and fungicidal at doses that were related to the surfactant critical micelle concentrations (CMC), all of them at concentrations significantly below the CMC. In general, bacteria were more susceptible to this surfactant group than C. albicans and this organism, in turn, was more susceptible than MDCK cells. This suggests that the C_nTAB may be useful as vaginal disinfectants only in so far as bacterial and fungal infections are concerned. None of the surfactants examined, including those that have been used in pre-clinical studies, showed inhibition of adenovirus or lentivirus infection of MDCK cells or spermicidal activity at doses that were sub-toxic to MDCK cells.

Conclusions/Significance

The results of this study lead us to propose that systematic analysis of surfactant toxicity, such as we report in the present work, be made a mandatory pre-condition for the use of these substances in pre-clinical animal and/or human studies.

Permeabilization of baker's yeast with N-lauroyl sarcosine

N-Lauroyl sarcosine (LS), a cationic, non-toxic and biodegradable detergent readily permeabilized whole cells of baker's yeast (Saccharomyces cerevisiae). Permeabilization was carried out to increase assayable cellular catalase activity, an enzyme of great physiological and industrial importance, and to release 5'-nucleotides which find food/nutritional applications. The event of permeabilization was concentration, time and temperature dependent. Maximum permeabilization of yeast cells were observed when 1 g wet weight (0.2 g dry wt) of cells were permeabilized with 1.0 ml of 2% LS at 45 degrees C for 15 min. LS-permeabilized cells showed 350-fold increase in catalase activity and the supernatant obtained after permeabilization was rich in 5'-nucleotides. LS-permeabilized baker's yeast cells can be used as a source of biocatalyst and to isolate valuable by-products.

Factors determining detergent resistance of erythrocyte membranes

The degree of detergent insolubility of cell membranes is a useful parameter to test the strength of lipid-lipid interactions relative to lipid-detergent interactions. Thus, solubility studies could give insights about lipid-lipid interactions relevant in domain formation. In this work we perform a detailed study of the solubilization of four different erythrocyte membrane systems: intact human and bovine erythrocytes, and human and bovine erythrocytes depleted in cholesterol with methyl-beta-cyclodextrin. Each system was incubated with different concentrations of the non-ionic detergent Triton X-100, and the insoluble fraction was characterized by determining cholesterol and phosphorus content. A distinct solubilization behavior was obtained for the four systems, which was quantified by a "detergent resistance parameter" obtained from the fit of the solubility curves. In order to correlate these findings with membrane structural parameters, we quantify the degree of acyl chain order/rigidity of the original membranes by EPR spectroscopy, finding that detergent resistance is higher when acyl chains are more rigid. Regarding compositional properties, we found a good correlation between detergent resistance parameters and the total amount of cholesterol plus sphingomyelin in the original membranes. Our results suggest that a high degree of acyl chain packing is the determinant membrane factor for resistance to the action of Triton X-100 in erythrocytes.

Triton X-100 partitioning into sphingomyelin bilayers at subsolubilizing detergent concentrations: effect of lipid phase and a comparison with dipalmitoylphosphatidylcholine

We examined the partitioning of the nonionic detergent Triton X-100 at subsolubilizing concentrations into bilayers of either egg sphingomyelin (SM), palmitoyl SM, or dipalmitoylphosphatidylcholine. SM is known to require less detergent than phosphatidylcholine to achieve the same extent of solubilization, and for all three phospholipids solubilization is temperature dependent. In addition, the three lipids exhibit a gel-fluid phase transition in the 38-41 degrees C temperature range. Experiments have been performed at Triton X-100 concentrations well below the critical micellar concentration, so that only detergent monomers have to be considered. Lipid/detergent mol ratios were never <10:1, thus ensuring that the solubilization stage was never reached. Isothermal titration calorimetry, DSC, and infrared, fluorescence, and (31)P-NMR spectroscopies were applied in the 5-55 degrees C temperature range. The results show that, irrespective of the chemical nature of the lipid, DeltaG degrees of partitioning remained in the range of -27 kJ/mol lipid in the gel phase and of -30 kJ/mol lipid in the fluid phase. This small difference cannot account for the observed phase-dependent differences in solubilization. Such virtually constant DeltaG degrees occurred as a result of the compensation of enthalpic and entropic components, which varied with both temperature and lipid composition. Consequently, the observed different susceptibilities to solubilization cannot be attributed to differential binding but to further events in the solubilization process, e.g., bilayer saturability by detergent or propensity to form lipid-detergent mixed micelles. The data here shed light on the relatively unexplored early stages of membrane solubilization and open new ways to understand the phenomenon of membrane resistance toward detergent solubilization.