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Dietel L, Kalie L, Heerklotz H. Lipid Scrambling Induced by Membrane-Active Substances. Biophys J 2020; 119:767-779. [PMID: 32738218 DOI: 10.1016/j.bpj.2020.07.004] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/28/2020] [Revised: 07/03/2020] [Accepted: 07/07/2020] [Indexed: 11/26/2022] Open
Abstract
The functional roles of the lipid asymmetry of biomembranes are attracting increasing attention. This study characterizes the activity of surfactants to induce transmembrane flip-flop of lipids and thus "scramble" this asymmetry. Detergent-induced lipid scrambling of liposomes mimicking the charge asymmetry of bacterial membranes with 20 mol % of 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-rac-glycerol in the outer leaflet only was quantified by ζ-potential measurements for octaethylene glycol dodecyl ether (C12EO8), octyl glucoside (OG), and dodecyl maltoside. Membrane leakage was separately measured by the fluorescence lifetime-based calcein leakage assay and the onset of the membrane-to-micelle transition by isothermal titration calorimetry. Partition coefficients and partial molar areas were obtained as well. For the quickly membrane-permeant C12EO8 and OG, leakage proceeds at a rather sharp threshold content in the membrane, which is well below the onset of solubilization and little dependent on incubation time; it is accompanied by fast lipid scrambling. However, unlike leakage, flip-flop is a relaxation process that speeds up gradually from taking weeks in the detergent-free membrane to minutes or less in the leaking membrane. Hence, after 24 h of incubation, 10 mol % of C12EO8 or 50 mol % of OG in the membrane suffice for virtually complete lipid scrambling, whereas leakage remains below 10% for up to 14 mol % of C12EO8 and 88 mol % of OG. There is thus a concentration window in which lipid scrambling proceeds without leakage. This implies that lipid scrambling must be considered a possible mode of action of antimicrobial peptides and other membrane-active drugs or biomolecules. A related, detergent-based protocol for scrambling the lipid asymmetry of liposomes and maybe cells without compromising their overall integrity would be a very valuable tool to study functions of lipid asymmetry.
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Affiliation(s)
- Lisa Dietel
- Institute of Pharmaceutical Sciences, University of Freiburg, Freiburg, Germany.
| | - Louma Kalie
- Institute of Pharmaceutical Sciences, University of Freiburg, Freiburg, Germany
| | - Heiko Heerklotz
- Institute of Pharmaceutical Sciences, University of Freiburg, Freiburg, Germany; Signaling Research Centers BIOSS and CIBSS, University of Freiburg, Freiburg, Germany; Leslie Dan Faculty of Pharmacy, University of Toronto, Toronto, Ontario, Canada.
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2
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Soto VH, Vázquez-Tato MP, Meijide F, Alvarado MJ, Seijas JA, de Frutos S, Lomonte B, Vázquez Tato J. Aggregation behavior of sodium 3-(octyloxy)-4-nitrobenzoate in aqueous solution. NEW J CHEM 2018. [DOI: 10.1039/c8nj03440k] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
3-(Octyloxy)-4-nitrobenzoate, a PLA2 inhibitor, is a better surfactant than other octyl derivatives and can be used as a model for 3-(octanoyloxy)-4-nitrobenzoic acid.
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Affiliation(s)
- Victor H. Soto
- Escuela de Química
- Centro de Investigación en Electroquímica y Energía Química (CELEQ)
- Universidad de Costa Rica
- San José
- Costa Rica
| | - M. Pilar Vázquez-Tato
- Departamento de Química Orgánica
- Facultad de Ciencias
- Universidad de Santiago de Compostela
- 27002 Lugo
- Spain
| | - Francisco Meijide
- Departamento de Química Física
- Facultad de Ciencias
- Universidad de Santiago de Compostela
- 27002 Lugo
- Spain
| | - María José Alvarado
- Escuela de Química
- Centro de Investigación en Electroquímica y Energía Química (CELEQ)
- Universidad de Costa Rica
- San José
- Costa Rica
| | - Julio A. Seijas
- Departamento de Química Orgánica
- Facultad de Ciencias
- Universidad de Santiago de Compostela
- 27002 Lugo
- Spain
| | - Santiago de Frutos
- Departamento de Química Física
- Facultad de Ciencias
- Universidad de Santiago de Compostela
- 27002 Lugo
- Spain
| | - Bruno Lomonte
- Instituto Clodomiro Picado
- Facultad de Microbiología
- Universidad de Costa Rica
- San José 11501
- Costa Rica
| | - José Vázquez Tato
- Departamento de Química Física
- Facultad de Ciencias
- Universidad de Santiago de Compostela
- 27002 Lugo
- Spain
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Oluwole AO, Klingler J, Danielczak B, Babalola JO, Vargas C, Pabst G, Keller S. Formation of Lipid-Bilayer Nanodiscs by Diisobutylene/Maleic Acid (DIBMA) Copolymer. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2017; 33:14378-14388. [PMID: 29160078 DOI: 10.1021/acs.langmuir.7b03742] [Citation(s) in RCA: 68] [Impact Index Per Article: 9.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/20/2023]
Abstract
Membrane proteins usually need to be extracted from their native environment and separated from other membrane components for in-depth in vitro characterization. The use of styrene/maleic acid (SMA) copolymers to solubilize membrane proteins and their surrounding lipids into bilayer nanodiscs is an attractive approach toward this goal. We have recently shown that a diisobutylene/maleic acid (DIBMA) copolymer similarly solubilizes model and cellular membranes but, unlike SMA(3:1), has a mild impact on lipid acyl-chain order and thermotropic phase behavior. Here, we used fluorescence spectroscopy, small-angle X-ray scattering, size-exclusion chromatography, dynamic light scattering, and 31P nuclear magnetic resonance spectroscopy to examine the self-association of DIBMA and its membrane-solubilization properties against lipids differing in acyl-chain length and saturation. Although DIBMA is less hydrophobic than commonly used SMA(3:1) and SMA(2:1) copolymers, it efficiently formed lipid-bilayer nanodiscs that decreased in size with increasing polymer/lipid ratio while maintaining the overall thickness of the membrane. DIBMA fractions of different molar masses were similarly efficient in solubilizing a saturated lipid. Coulomb screening at elevated ionic strength or reduced charge density on the polymer at low pH enhanced the solubilization efficiency of DIBMA. The free-energy penalty for transferring phospholipids from vesicular bilayers into nanodiscs became more unfavorable with increasing acyl-chain length and unsaturation. Altogether, these findings provide a rational framework for using DIBMA in membrane-protein research by shedding light on the effects of polymer and lipid properties as well as experimental conditions on membrane solubilization.
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Affiliation(s)
- Abraham Olusegun Oluwole
- Molecular Biophysics, University of Kaiserslautern , Erwin-Schrödinger-Str. 13, 67663 Kaiserslautern, Germany
- Department of Chemistry, University of Ibadan , 200284 Ibadan, Nigeria
| | - Johannes Klingler
- Molecular Biophysics, University of Kaiserslautern , Erwin-Schrödinger-Str. 13, 67663 Kaiserslautern, Germany
| | - Bartholomäus Danielczak
- Molecular Biophysics, University of Kaiserslautern , Erwin-Schrödinger-Str. 13, 67663 Kaiserslautern, Germany
| | | | - Carolyn Vargas
- Molecular Biophysics, University of Kaiserslautern , Erwin-Schrödinger-Str. 13, 67663 Kaiserslautern, Germany
| | - Georg Pabst
- Institute of Molecular Biosciences, Biophysics Division, NAWI Graz, University of Graz , Humboldtstr. 50/III, 8010 Graz, Austria
- BioTechMed-Graz , 8010 Graz, Austria
| | - Sandro Keller
- Molecular Biophysics, University of Kaiserslautern , Erwin-Schrödinger-Str. 13, 67663 Kaiserslautern, Germany
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Patel H, Huynh Q, Bärlehner D, Heerklotz H. Additive and synergistic membrane permeabilization by antimicrobial (lipo)peptides and detergents. Biophys J 2014; 106:2115-25. [PMID: 24853740 DOI: 10.1016/j.bpj.2014.04.006] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2013] [Revised: 03/10/2014] [Accepted: 04/01/2014] [Indexed: 12/27/2022] Open
Abstract
Certain antibiotic peptides are thought to permeabilize membranes of pathogens by effects that are also observed for simple detergents, such as membrane thinning and disordering, asymmetric bilayer expansion, toroidal pore formation, and micellization. Here we test the hypothesis that such peptides act additively with detergents when applied in parallel. Additivity is defined analogously to a fractional inhibitory concentration index of unity, and the extent and mechanism of leakage is measured by the fluorescence lifetime-based vesicle leakage assay using calcein-loaded vesicles. Good additivity was found for the concerted action of magainin 2, the fungicidal lipopeptide class of surfactins from Bacillus subtilis QST713, and the detergent octyl glucoside, respectively, with the detergent C12EO8. Synergistic or superadditive action was observed for fengycins from B. subtilis, as well as the detergent CHAPS, when combined with C12EO8. The results illustrate two mechanisms of synergistic action: First, maximal leakage requires an optimum degree of heterogeneity in the system that may be achieved by mixing a graded with an all-or-none permeabilizer. (The optimal perturbation should be focused to certain defect structures, yet not to the extent that some vesicles are not affected at all.) Second, a cosurfactant may enhance the bioavailability of a poorly soluble peptide. The results are important for understanding the concerted action of membrane-permeabilizing compounds in biology as well as for optimizing formulations of such antimicrobials for medical applications or crop protection.
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Affiliation(s)
- Hiren Patel
- Leslie Dan Faculty of Pharmacy, University of Toronto, Toronto, Canada; Department of Biophysics and Chemistry, University of Michigan, Ann Arbor, Michigan
| | - Quang Huynh
- Leslie Dan Faculty of Pharmacy, University of Toronto, Toronto, Canada
| | - Dominik Bärlehner
- Leslie Dan Faculty of Pharmacy, University of Toronto, Toronto, Canada
| | - Heiko Heerklotz
- Leslie Dan Faculty of Pharmacy, University of Toronto, Toronto, Canada; Institut für Medizinische Physik und Biophysik, Universität Leipzig, Leipzig, Germany.
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Sodt AJ, Pastor RW. Molecular modeling of lipid membrane curvature induction by a peptide: more than simply shape. Biophys J 2014; 106:1958-69. [PMID: 24806928 DOI: 10.1016/j.bpj.2014.02.037] [Citation(s) in RCA: 55] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/03/2013] [Revised: 02/06/2014] [Accepted: 02/10/2014] [Indexed: 11/15/2022] Open
Abstract
Molecular dynamics simulations of an amphipathic helix embedded in a lipid bilayer indicate that it will induce substantial positive curvature (e.g., a tube of diameter 20 nm at 16% surface coverage). The induction is twice that of a continuum model prediction that only considers the shape of the inclusion. The discrepancy is explained in terms of the additional presence of specific interactions described only by the molecular model. The conclusion that molecular shape alone is insufficient to quantitatively model curvature is supported by contrasting molecular and continuum models of lipids with large and small headgroups (choline and ethanolamine, respectively), and of the removal of a lipid tail (modeling a lyso-lipid). For the molecular model, curvature propensity is analyzed by computing the derivative of the free energy with respect to bending. The continuum model predicts that the inclusion will soften the bilayer near the headgroup region, an effect that may weaken curvature induction. The all-atom predictions are consistent with experimental observations of the degree of tubulation by amphipathic helices and variation of the free energy of binding to liposomes.
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Affiliation(s)
- Alexander J Sodt
- National Heart, Lung, and Blood Institute, National Institutes of Health, Rockville, Maryland.
| | - Richard W Pastor
- National Heart, Lung, and Blood Institute, National Institutes of Health, Rockville, Maryland
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Comparative study of the interaction of CHAPS and Triton X-100 with the erythrocyte membrane. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2013; 1838:859-66. [PMID: 24239862 DOI: 10.1016/j.bbamem.2013.11.006] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/17/2013] [Revised: 10/22/2013] [Accepted: 11/07/2013] [Indexed: 11/21/2022]
Abstract
The zwitterionic detergent CHAPS, a derivative of the bile salts, is widely used in membrane protein solubilization. It is a "facial" detergent, having a hydrophilic side and a hydrophobic back. The objective of this work is to characterize the interaction of CHAPS with a cell membrane. To this aim, erythrocytes were incubated with a wide range of detergent concentrations in order to determine CHAPS partition behavior, and its effects on membrane lipid order, hemolytic effects, and the solubilization of membrane phospholipids and cholesterol. The results were compared with those obtained with the nonionic detergent Triton X-100. It was found that CHAPS has a low affinity for the erythrocyte membrane (partition coefficient K=0.06mM(-1)), and at sub-hemolytic concentrations it causes little effect on membrane lipid order. CHAPS hemolysis and phospholipid solubilization are closely correlated. On the other side, binding of Triton X-100 disorders the membrane at all levels, and has independent mechanisms for hemolysis and solubilization. Differential behavior was observed in the solubilization of phospholipids and cholesterol. Thus, the detergent resistant membranes (DRM) obtained with the two detergents will have different composition. The behaviors of the two detergents are related to the differences in their molecular structures, suggesting that CHAPS does not penetrate the lipid bilayer but binds in a flat position on the erythrocyte surface, both in intact and cholesterol depleted erythrocytes. A relevant result for Triton X-100 is that hemolysis is not directly correlated with the solubilization of membrane lipids, as it is usually assumed.
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Yokoyama H, Ikeda K, Wakabayashi M, Ishihama Y, Nakano M. Effects of lipid membrane curvature on lipid packing state evaluated by isothermal titration calorimetry. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2013; 29:857-860. [PMID: 23270307 DOI: 10.1021/la304532k] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
In this report, we present a novel approach for the elucidation of the physicochemical properties of lipid membranes by isothermal titration calorimetry (ITC) to quantify the heat absorbed during the solubilization of vesicles into TritonX-100 micelles. By using large and small unilamellar vesicles for comparison, this method provides calorimetric data on the gel-to-liquid-crystalline phase transition and its curvature effects and, in particular, the enthalpy change upon membrane deformation from a planar to a curved shape, which cannot be obtained by the conventional approach using differential scanning calorimetry. The results showed quantitatively that the increase in membrane curvature increases the enthalpy of 1,2-dimyristoyl-sn-glycero-3-phosphocholine membranes both below and above the phase-transition temperature, and that the effect is more significant for the former condition. The calorimetric data obtained are further discussed in relation to the elastic bending energy of the membranes and membrane-peptide interaction.
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Affiliation(s)
- Hirokazu Yokoyama
- Graduate School of Pharmaceutical Sciences, Kyoto University, Sakyo-ku, Kyoto, Japan
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A general path for large-scale solubilization of cellular proteins: from membrane receptors to multiprotein complexes. Protein Expr Purif 2012; 87:111-9. [PMID: 23137940 DOI: 10.1016/j.pep.2012.10.007] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/13/2012] [Revised: 10/05/2012] [Accepted: 10/20/2012] [Indexed: 11/20/2022]
Abstract
Expression of recombinant proteins in bacterial or eukaryotic systems often results in aggregation rendering them unavailable for biochemical or structural studies. Protein aggregation is a costly problem for biomedical research. It forces research laboratories and the biomedical industry to search for alternative, more soluble, non-human proteins and limits the number of potential "druggable" targets. In this study we present a highly reproducible protocol that introduces the systematic use of an extensive number of detergents to solubilize aggregated proteins expressed in bacterial and eukaryotic systems. We validate the usefulness of this protocol by solubilizing traditionally difficult human protein targets to milligram quantities and confirm their biological activity. We use this method to solubilize monomeric or multimeric components of multi-protein complexes and demonstrate its efficacy to reconstitute large cellular machines. This protocol works equally well on cytosolic, nuclear and membrane proteins and can be easily adapted to a high throughput format.
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Oftedal L, Myhren L, Jokela J, Gausdal G, Sivonen K, Døskeland SO, Herfindal L. The lipopeptide toxins anabaenolysin A and B target biological membranes in a cholesterol-dependent manner. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2012; 1818:3000-9. [PMID: 22842546 DOI: 10.1016/j.bbamem.2012.07.015] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/25/2012] [Revised: 06/29/2012] [Accepted: 07/19/2012] [Indexed: 11/26/2022]
Abstract
The two novel cyanobacterial cyclic lipopeptides, anabaenolysin (Abl) A and B permeabilised mammalian cells, leading to necrotic death. Abl A was a more potent haemolysin than other known biodetergents, including digitonin, and induced discocyte-echinocyte transformation in erythrocytes. The mitochondria of the dead cells appeared intact with regard to both ultrastructure and membrane potential. Also isolated rat liver mitochondria were resistant to Abl, judged by their ultrastructure and lack of cytochrome c release. The sparing of the mitochondria could be related to the low cholesterol content of their outer membrane. In fact, a supplement of cholesterol in liposomes sensitised them to Abl. In contrast, the prokaryote-directed cyclic lipopeptide surfactin lysed preferentially non-cholesterol-containing membranes. In silico comparison of the positions of relevant functional chemical structures revealed that Abl A matched poorly with surfactin in spite of the common cyclic lipopeptide structure. Abl A and the plant-derived glycolipid digitonin had, however, predicted overlaps of functional groups, particularly in the cholesterol-binding tail of digitonin. This may suggest independent evolution of Abl and digitonin to target eukaryotic cholesterol-containing membranes. Sub-lytic concentrations of Abl A or B allowed influx of propidium iodide into cells without interfering with their long-term cell viability. The transient permeability increase allowed the influx of enough of the cyanobacterial cyclic peptide toxin nodularin to induce apoptosis. The anabaenolysins might therefore not only act solely as lysins, but also as cofactors for the internalisation of other toxins. They represent a potent alternative to digitonin to selectively disrupt cholesterol-containing biological membranes.
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Affiliation(s)
- Linn Oftedal
- Department of Biomedicine, University of Bergen, Norway
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Nazari M, Kurdi M, Heerklotz H. Classifying surfactants with respect to their effect on lipid membrane order. Biophys J 2012; 102:498-506. [PMID: 22325272 DOI: 10.1016/j.bpj.2011.12.029] [Citation(s) in RCA: 111] [Impact Index Per Article: 9.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/19/2011] [Revised: 10/04/2011] [Accepted: 12/09/2011] [Indexed: 11/19/2022] Open
Abstract
We propose classifying surfactants with respect to their effect on membrane order, which is derived from the time-resolved fluorescence anisotropy of DPH. This may help in understanding why certain surfactants, including biosurfactants such as antimicrobial lipopeptides and saponins, often show a superior performance to permeabilize and lyse membranes and/or a better suitability for membrane protein solubilization. Micelle-forming surfactants induce curvature stress in membranes that causes disordering and, finally, lysis. Typical detergents such as C(12)EO(8), octyl glucoside, SDS, and lauryl maltoside initiate membrane lysis after reaching a substantial, apparently critical extent of disordering. In contrast, the fungicidal lipopeptides surfactin, fengycin, and iturin from Bacillus subtilis QST713 as well as digitonin, CHAPS, and lysophosphatidylcholine solubilize membranes without substantial, overall disordering. We hypothesize they disrupt the membrane locally due to a spontaneous segregation from the lipid and/or packing defects and refer to them as heterogeneously perturbing. This may account for enhanced activity, selectivity, and mutual synergism of antimicrobial biosurfactants and reduced destabilization of membrane proteins by CHAPS or digitonin. Triton shows the pattern of a segregating surfactant in the presence of cholesterol.
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Affiliation(s)
- Mozhgan Nazari
- Leslie Dan Faculty of Pharmacy, University of Toronto, Toronto, Ontario, Canada
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Krylova OO, Jahnke N, Keller S. Membrane solubilisation and reconstitution by octylglucoside: comparison of synthetic lipid and natural lipid extract by isothermal titration calorimetry. Biophys Chem 2010; 150:105-11. [PMID: 20392557 DOI: 10.1016/j.bpc.2010.03.013] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/29/2010] [Revised: 03/12/2010] [Accepted: 03/13/2010] [Indexed: 11/28/2022]
Abstract
We have studied the solubilisation and reconstitution of lipid membranes composed of either synthetic phosphatidylcholine or Escherichia. coli polar lipid extract by the non-ionic detergent octylglucoside. For both lipid systems, composition-dependent transformations of unilamellar vesicles into micelles or vice versa were followed by high-sensitivity isothermal titration calorimetry. Data obtained over a range of detergent and lipid concentrations could be rationalised in terms of a three-stage phase separation model involving bilayer, bilayer/micelle coexistence, and micellar ranges, yielding the detergent/lipid phase diagrams and the bilayer-to-micelle partition coefficients of both detergent and lipid. The most notable difference between the lipids investigated was a substantial widening of the bilayer/micelle coexistence range for E. coli lipid, which was due to an increased preference of the detergent and a decreased affinity of the lipid for the micellar phase as compared with the bilayer phase. These effects on the bilayer-to-micelle partition coefficients could be explained by the high proportion in E. coli membranes of lipids possessing negative spontaneous curvature, which hampers both their transfer into strongly curved micellar structures as well as the insertion of detergent into condensed bilayers.
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Affiliation(s)
- Oxana O Krylova
- Leibniz Institute of Molecular Pharmacology (FMP), Robert-Rössle-Str. 10, 13125 Berlin, Germany
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Survey of the year 2008: applications of isothermal titration calorimetry. J Mol Recognit 2010; 23:395-413. [DOI: 10.1002/jmr.1025] [Citation(s) in RCA: 53] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
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Inhomogeneities in sodium decylsulfate doped 1,2-dipalmitoylphosphatidylcholine bilayer. J Colloid Interface Sci 2010; 343:401-7. [DOI: 10.1016/j.jcis.2009.11.054] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2009] [Revised: 11/19/2009] [Accepted: 11/20/2009] [Indexed: 11/17/2022]
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Abstract
We describe an intuitive and rapid procedure for analyzing experimental data by nonlinear least-squares fitting (NLSF) in the most widely used spreadsheet program. Experimental data in x/y form and data calculated from a regression equation are inputted and plotted in a Microsoft Excel worksheet, and the sum of squared residuals is computed and minimized using the Solver add-in to obtain the set of parameter values that best describes the experimental data. The confidence of best-fit values is then visualized and assessed in a generally applicable and easily comprehensible way. Every user familiar with the most basic functions of Excel will be able to implement this protocol, without previous experience in data fitting or programming and without additional costs for specialist software. The application of this tool is exemplified using the well-known Michaelis-Menten equation characterizing simple enzyme kinetics. Only slight modifications are required to adapt the protocol to virtually any other kind of dataset or regression equation. The entire protocol takes approximately 1 h.
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Monitoring detergent-mediated solubilization and reconstitution of lipid membranes by isothermal titration calorimetry. Nat Protoc 2009; 4:686-97. [PMID: 19373233 DOI: 10.1038/nprot.2009.35] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
The solubilization and reconstitution of biological or liposomal membranes by detergents and biomolecules with detergent-like properties play a major role for technical applications (e.g., the isolation of membrane proteins) and biological phenomena (of, e.g., amphiphilic peptides). It is therefore important to know and understand the amounts of a given detergent required for the onset and completion of membrane solubilization and the detergent-lipid interactions in general. Lipid-detergent systems can form a variety of aggregate structures, which can be grouped into two pseudophases (lamellae and micelles) so that solubilization can be approximately described as a phase transition. Here we present a protocol for establishing the phase diagram and a detailed thermodynamic description of a lipid-detergent system based on isothermal titration calorimetry (ITC). The protocol can also be used to detect additive-induced membrane destabilization, permeabilization, domain formation and lipid-dependent transitions between rod-like and spherical micelles. A minimal protocol consisting of all sample preparation procedures and a single solubilization experiment can be accomplished within 2 days; a more extensive series comprising both solubilization and reconstitution experiments requires several days to a few weeks, depending on the number of titrations performed.
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Abstract
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.
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