Thermodynamic and structural study of DMPC-alkanol systems.

The thermodynamic and structural behaviors of lamellar dimyristoylphosphatidylcholine-alkanol (abbreviation DMPC-CnOH, n = 8-18 is the even number of carbons in the alkyl chain) systems were studied by using DSC and SAXD/WAXD methods at a 0-0.8 CnOH : DMPC molar ratio range. Up to n≤ 10 a significant biphasic effect depending on the main transition temperature tm on the CnOH concentration was observed. Two breakpoints were revealed: turning point (TP), corresponding to the minimum, and threshold concentration (cT), corresponding to the end of the biphasic tendency. These breakpoints were also observed in the alkanol concentration dependent change in the enthalpy of the main transition ΔHm. In the case of CnOHs with n > 10 we propose a marked shift of TP and cT to very low concentrations; consequently, only increase of tm is observed. A partial phase diagram was constructed for a pseudo-binary DMPC-C12OH system. We suggest a fluid-fluid immiscibility of the DMPC-C12OH system above cT with a consequent formation of domains with different C12OH contents. At a constant CnOH concentration, the effects of CnOHs on ΔHm and bilayer repeat distance were found to depend predominantly on the mismatch between CnOH and lipid chain lengths. Observed effects are suggested to be underlined by a counterbalancing effect of interchain van der Waals interactions and headgroup repulsion.

Phase behavior of the DOPE + DOPC + alkanol system.

Small- and wide-angle X-ray diffraction was used to study the effect of 1-alkanols, as simple models of general anesthetics, (abbreviation CnOH, n = 8-18 is the even number of carbons in the aliphatic chain) on the lamellar to hexagonal Lα→ H(II) phase transition in the dioleoylphosphatidylethanolamine-dioleoylphosphatidylcholine = 3 : 1 mol/mol (DOPE + DOPC) system. All studied CnOHs were found to decrease the phase transition temperature of the DOPE + DOPC system in a CnOH chain length and concentration dependent manner and thus promote the formation of the HII phase. Anesthetically active C8OH and C10OH were found to decrease the lattice parameter d of the Lα phase, however longer non-anesthetic CnOHs increased the parameter d; this effect being more pronounced with increasing CnOH concentration. The lattice parameter of the HII phase was decreased in the presence of all CnOHs, even at the lowest concentrations studied. In the scope of the indirect mechanism of general anesthesia observed changes in the lattice parameter d (reflecting changes in the bilayer thickness) due to the intercalation of C8OH and C10OH might induce changes in the activity of integral membrane proteins engaged in neuronal pathways.

Effects of N,N-dimethyl-N-alkylamine-N-oxides on DOPC bilayers in unilamellar vesicles: small-angle neutron scattering study.

Small-angle neutron scattering data were collected from aqueous dispersions of unilamellar vesicles (ULVs) consisting of mixtures of 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine and a homologous series of N,N-dimethyl-N-alkylamine-N-oxides (CnNO, n = 12, 14, 16, and 18, where n is the number of carbon atoms in the alkyl chain). A modeling approach was applied to the neutron scattering curves to obtain the bilayer structural parameters. Particularly, the external (2)H2O/H2O contrast variation technique was carried out on pure dioleoylphosphatidylcholine (DOPC) ULVs to determine the hydrophilic region thickness [Formula: see text] = 9.8 ± 0.6 Å. Consequently, the hydrocarbon region thickness [Formula: see text], the lateral bilayer area per one lipid molecule [Formula: see text], and the number of water molecules located in the hydrophilic region per one lipid molecule [Formula: see text] were obtained from single-contrast neutron scattering curves using the previously determined [Formula: see text]. The structural parameters were extracted as functions of [Formula: see text] (the CnNO:DOPC molar ratio) and n. The dependences [Formula: see text] provided the partial lateral areas of CnNOs ([Formula: see text]) and DOPC ([Formula: see text]) in bilayers. It was observed that the [Formula: see text]'s were constant in the investigated interval of [Formula: see text] and for n = 12, 14, and 16 equal to 36.6 ± 0.4 Å(2), while [Formula: see text] increased to 39.4 ± 0.4 Å(2). The bilayer hydrocarbon region thickness [Formula: see text] decreased with intercalation of each CnNO. This effect increased with [Formula: see text] and decreased with increasing CnNO alkyl chain length. The intercalation of C18NO changed the [Formula: see text] only slightly. To quantify the effect of CnNO intercalation into DOPC bilayers we fit the [Formula: see text] dependences with weighted linear approximations and acquired their slopes [Formula: see text].

Molecular and component volumes of N,N-dimethyl-N-alkylamine N-oxides in DOPC bilayers.

The volumetric properties of fluid bilayers formed of dioleoylphosphatidylcholine (DOPC) with incorporated N,N-dimethyl-N-alkylamine N-oxides (CnNO, n=6, 10-18 is the even number of carbons in alkyl chain) were studied by vibrating tube densitometry in the temperature interval from 20°C to 50°C. It was found that the DOPC and CnNO mixed ideally in the investigated composition range and hence the molecular volumes of DOPC (VDOPC) and incorporated CnNO (VCnNO) were constant and additive within error limits. From the temperature dependencies of the molecular volumes of DOPC and CnNO their coefficients of isobaric thermal expansivities in the investigated temperature interval were obtained. The VCnNO volumes of CnNO incorporated into DOPC bilayers showed linear dependencies on the CnNO alkyl chain length at all measured temperatures. This allowed to calculate the component volume of the CnNO methylene group (VCH2) at several temperatures and its coefficient of isobaric thermal expansivity. Using the assumption that the component volume of the CnNO methyl group VCH3 = 2VCH2 we also calculated the component volume and the coefficient of isobaric thermal expansivity of the hydrophilic group of CnNO (VNO). We found that the VCH2 volume increases in the whole temperature interval whereas the VNO volume decreases.

Study of interaction of long-chain n-alcohols with fluid DOPC bilayers by a lateral pressure sensitive fluorescence probe.

The excimer 1,2-dipyrenedecanoyl-sn-glycero-3-phosphatidylcholine (dipy10PC) fluorescence probe was used to determine effects of aliphatic alcohols (CnH2n+1OH, n = 12-18 is the even number of carbons in alkyl chain) on fluid dioleoylphosphatidylcholine (DOPC) +dioleoylphosphatidylserine (DOPS) bilayers in multilamellar vesicles at molar ratio DOPC/DOPS = 24.7. The excimer to monomer fluorescence intensity ratio increases with the increase of CnH2n+1OH/DOPC molar ratio and decreases with the CnH2n+1OH alkyl chain length n at a constant CnH2n+1OH/DOPC = 0.4 molar ratio. These effects indicate changes in the bilayer lateral pressure on the level of pyrenyl moieties location.

Influence of cholesterol and ß-sitosterol on the structure of EYPC bilayers.

The influence of cholesterol and ß-sitosterol on egg yolk phosphatidylcholine (EYPC) bilayers is compared. Different interactions of these sterols with EYPC bilayers were observed using X-ray diffraction. Cholesterol was miscible with EYPC in the studied concentration range (0-50 mol%), but crystallization of ß-sitosterol in EYPC bilayers was observed at X ≥ 41 mol% as detected by X-ray diffraction. Moreover, the repeat distance (d) of the lamellar phase was similar upon addition of the two sterols up to mole fraction 17%, while for X ≥ 17 mol% it became higher in the presence of ß-sitosterol compared to cholesterol. SANS data on suspensions of unilamellar vesicles showed that both cholesterol and ß-sitosterol similarly increase the EYPC bilayer thickness. Cholesterol in amounts above 33 mol% decreased the interlamellar water layer thickness, probably due to "stiffening" of the bilayer. This effect was not manifested by ß-sitosterol, in particular due to the lower solubility of ß-sitosterol in EYPC bilayers. Applying the formalism of partial molecular areas, it is shown that the condensing effect of both sterols on the EYPC area at the lipid-water interface is small, if any. The parameters of ESR spectra of spin labels localized in different regions of the EYPC bilayer did not reveal any differences between the effects of cholesterol and ß-sitosterol in the range of full miscibility.

DNA condensation and its thermal stability influenced by phospholipid bilayer and divalent cations.

We studied the effect of divalent alkaline earth metal cations Ca²âº, Mg²âº and transition metals Co²âº, Ni²âº, Cu²âº and Zn²âº on DNA condensation and its protection against thermal denaturation in presence of dioleoylphosphatidylcholine liposomes (DOPC). Experimental results have shown that Ca²âº and Mg²âº as well as Zn²âº mediate DNA condensation. Cu²âº causes DNA double helix destabilization, and does not mediate binding between DNA and DOPC liposomes. Co²âº and Ni²âº can interact with DNA on both ways mentioned above. Static light scattering was use to follow the size of aggregates in DNA condensation process. Phospholipid bilayer and divalent cations protect condensed DNA against thermal destabilization. The highest stabilization effect was found in aggregates with Ca²âº and Zn²âº, whereas in presence of either Co²âº or Ni²âº some volume fraction of DNA is denatured.

The effects of cholesterol and ß-sitosterol on the structure of saturated diacylphosphatidylcholine bilayers.

The structures of DMPC and DPPC bilayers in unilamellar liposomes, in the presence of 33.3 mol% cholesterol or the plant sterol ß-sitosterol, have been studied by small-angle neutron scattering. The bilayer thickness d(L) increases in a similar way for both sterols. The repeat distance in multilamellar liposomes, as determined by small-angle X-ray diffraction, is larger in the presence of ß-sitosterol than in the presence of cholesterol. We observe that each sterol modifies the interlamellar water layer differently, cholesterol reducing its thickness more efficiently than ß-sitosterol, and conclude that cholesterol suppresses bilayer undulations more effectively than ß-sitosterol.

Interaction of long-chain n-alcohols with fluid DOPC bilayers: a neutron diffraction study.

Lamellar phases composed of fluid dioleoylphosphatidylcholine (DOPC) bilayers containing alkan-1-ols (CnOH, n = 8, 10, 14, 16, 18 is the number of carbon atoms) at CnOH : DOPC = 0.3 molar ratio and hydrated with heavy water at 20.2 ≥ D2O : DOPC ≥ 14.4 molar ratio were studied by neutron diffraction. The bilayer thickness d(L) and the bilayer surface area A(L) per DOPC at the bilayer-water interface were obtained from the lamellar repeat period d using molecular volumes of DOPC, CnOH and D2O, and the Luzatti's method. Both the d(L) and A(L) increase with the CnOH chain length n at CnOH : DOPC = 0.3 molar ratio: d(L) = (3.888 ± 0.066) + (0.016 ± 0.005)·n (in nm), A(L) = (0.6711 ± 0.0107) + (0.0012 ± 0.0008)·n (in nm²).

Molecular and component volumes of saturated n-alkanols in DOPC+DOPS bilayers.

The volumetric properties of fluid bilayers consisting of dioleoylphosphatidylcholine (DOPC, 96wt%) and dioleoylphosphatidylserine (DOPS, 4wt%) with incorporated saturated n-alkanols (CnOH, n=10-16 is the even number of carbons in alkyl chain) were studied by vibrating tube densitometry. The mixing of DOPC and DOPS was found to be ideal and the molecular volumes of pure lipids V(DOPC) and V(DOPS) are additive in mixed bilayers. The increase of V(DOPS) with temperature was steeper than that of V(DOPC) as quantified by significantly higher coefficient of isobaric thermal expansivity gamma. This difference is supposed to be related to the high thermal expansivity of the serine headgroup. The molecular volumes of lipids and CnOH (V(CnOH)) in fluid DOPC+DOPS bilayers are constant and additive within error limits for n=12 and 16 and for the CnOH mole fractions in the lipid bilayer x(AL)8.

Areas of monounsaturated diacylphosphatidylcholines.

We have studied the structural properties of monounsaturated diacylphosphatidylcholine lipid bilayers (i.e., diCn:1PC, where n = 14, 16, 18, 20, 22, and 24 is the number of acyl chain carbons). High-resolution x-ray scattering data were analyzed in conjunction with contrast-varied neutron scattering data using a technique we recently developed. Analyses of the data show that the manner by which bilayer thickness increases with increasing n in monounsaturated diacylphosphatidylcholines is dependent on the double bond's position. For commonly available monounsaturated diacylphosphatidylcholines, this results in the nonlinear behavior of both bilayer thickness and lipid area, whereas for diC18:1PC bilayers, lipid area assumes a maximum value. It is worthwhile to note that compared to previous data, our results indicate that lipid areas are smaller by approximately 10%. This observation highlights the need to revisit lipid areas, as they are often used in comparisons with molecular dynamics simulations. Moreover, simulators are encouraged to compare their results not only to x-ray scattering data, but to neutron data as well.

Structural changes in dipalmitoylphosphatidylcholine bilayer promoted by Ca2+ ions: a small-angle neutron scattering study.

Small-angle neutron scattering (SANS) curves of unilamellar dipalmitoylphosphatidylcholine (DPPC) vesicles in 1-60mM CaCl2 were analyzed using a strip-function model of the phospholipid bilayer. The fraction of Ca2+ ions bound in the DPPC polar head group region was determined using Langmuir adsorption isotherm. In the gel phase, at 20 degrees C, the lipid bilayer thickness, dL, goes through a maximum as a function of CaCl2 concentration (dL=54.4A at approximately 2.5mM of CaCl2). Simultaneously, both the area per DPPC molecule AL, and the number of water molecules nW located in the polar head group region decrease (DeltaAL=AL(DPPC))-AL(DPPC+Ca)=2.3A2 and Deltan=n(W(DPPC))-n(W(DPPC+Ca))=0.8mol/mol at approximately 2.5mM of CaCl2). In the fluid phase, at 60 degrees C, the structural parameters d(L), A(L), and n(W) show evident changes with increasing Ca2+ up to a concentration C(Ca)(2+) < or = 10mM. DPPC bilayers affected by the calcium binding are compared to unilamellar vesicles prepared by extrusion. The structural parameters of DPPC vesicles prepared in 60mM CaCl2 (at 20 and 60 degrees C) are nearly the same as those for unilamellar vesicles without Ca2+.

Component volumes of unsaturated phosphatidylcholines in fluid bilayers: a densitometric study.

The specific volumes of six 1,2-diacylphosphatidylcholines with monounsaturated acyl chains (diCn:1PC, n=14-24 is the even number of acyl chain carbons) in fluid bilayers in multilamellar vesicles dispersed in H(2)O were determined by the vibrating tube densitometry as a function of temperature. From the data obtained with diCn:1PC (n=14-22) vesicles in combination with the densitometric data from Tristram-Nagle et al. [Tristram-Nagle, S., Petrache, H.I., Nagle, J.F., 1998. Structure and interactions of fully hydrated dioleoylphosphatidylcholine bilayers. Biophys. J. 75, 917-925.] and Koenig and Gawrisch [Koenig, B.W., Gawrisch, K., 2005. Specific volumes of unsaturated phosphatidylcholines in the liquid crystalline lamellar phase. Biochim. Biophys. Acta 1715, 65-70.], the component volumes of phosphatidylcholines in fully hydrated fluid bilayers at 30 degrees C were obtained. The volume of the acyl chain CH and CH(2) group is V(CH)=22.30 A(3) and V(CH2) =A(3), respectively. The volume of the headgroup including the glyceryl and acyl carbonyls, V(H), and the ratio of acyl chain methyl and methylene group volumes, r=V(CH3):V(CH2) are linearly interdependent: V(H)=a-br, where a=434.41 A(3) and b=-55.36 A(3) at 30 degrees C. From the temperature dependencies of component volumes, their isobaric thermal expansivities (alpha(X)=V(X)(-1)(partial differential V(X)/ partial differential T) where X=CH(2), CH, or H were calculated: alpha(CH2)=118.4x10(-5)K(-1), alpha(CH)=71.0x10(-5)K(-1), alpha(H)=7.9x10(-5)K(-1) (for r=2) and alpha(H)=9.6x10(-5)K(-1) (for r=1.9). The specific volume of diC24:1PC changes at the main gel-fluid phase transition temperature, t(m)=26.7 degrees C, by 0.0621 ml/g, its specific volume is 0.9561 and 1.02634 ml/g at 20 and 30 degrees C, respectively, and its isobaric thermal expansivity alpha=68.7x10(-5) and 109.2x10(-5)K(-1) below and above t(m), respectively. The component volumes and thermal expansivities obtained can be used for the interpretation of X-ray and neutron scattering and diffraction experiments and for the guiding and testing molecular dynamics simulations of phosphatidylcholine bilayers in the fluid state.

Synchrotron SAX and WAX diffraction study of a hydrated very long-chain, dendritic amphiphile + DPPC mixture.

The tri-headed anionic dendritic amphiphile, 4-(2-carboxyethyl)-4-[(icosyloxycarbonyl)amino]heptanedioic acid (3CCb20), forms mixed aggregates with dipalmitoylphosphatidylcholine (DPPC) in excess water at 3CCb20:DPPC = 0.91:1 molar ratio. On heating, these mixed aggregates transform into fluid bilayers stacked in the liquid crystalline lamellar L(alpha) phase at about 40 degrees C. This phase transition and the microstructure of 3CCb20 + DPPC aggregates were studied with small- and wide-angle synchrotron X-ray diffraction. The ability of 3CCb20 to solubilize solidlike lipid bilayers could contribute to the antimicrobial activities of 3CCb20, including its anti-HIV activity.

The structural diversity of DNA-neutral phospholipids-divalent metal cations aggregates: a small-angle synchrotron X-ray diffraction study.

We investigate the structure of aggregates formed due to DNA interaction with saturated neutral phosphatidylcholines [dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine] in presence of Ca(2+) and Mg(2+) cations using simultaneous synchrotron small- and wide-angle X-ray diffractions. For DPPC:DNA = 3:1 mol/base and in the range of 1-50 mM Ca(2+), the diffractograms show structural heterogeneity of aggregates. We observe the coexistence of two lamellar phases in aggregates prepared at 1 mM Ca(2+): L(x) phase with the DNA strands (of unknown organization) intercalated in water layers between adjacent lipid bilayers and L(DPPC) phase of DPPC bilayers without any divalent cations and DNA strands. Aggregates prepared in the range 2-50 mM Ca(2+) show a condensed gel lamellar phase L (g) (c) with the lipid bilayer periodicity d approximately 8.0 nm, and the DNA-DNA interhelical distance d (DNA) approximately 5.1 nm. The increase of temperature induces the decrease in the intensity and the increase in the width of the DNA related peak. In the fluid state, the condensed lamellar phase L (alpha) (c) gradually converts into L(x) phase. The aggregates do not exhibit rippled P(beta) phase. The thermal behaviour of aggregates was investigated in the range 20-80 degrees C. Applying heating-cooling cycles, the aggregates converted into energetically more favourable structure: a condensed lamellar phase L(c) (or L(x)) is preserved or we observe lateral segregation of the DNA strands and metal cations (L(x) phase) in coexistence with L(PC) phase of pure phospholipids.

The structure of DNA-DOPC aggregates formed in presence of calcium and magnesium ions: a small-angle synchrotron X-ray diffraction study.

The structure of aggregates formed due to DNA interaction with dioleoylphosphatidylcholine (DOPC) vesicles in presence of Ca(2+) and Mg(2+) cations was investigated using synchrotron small-angle X-ray diffraction. For DOPC/DNA=1:1 mol/base and in the range of concentration of the cation(2+) 0-76.5 mM, the diffractograms show the coexistence of two lamellar phases: L(x) phase with repeat distance d(Lx) approximately 8.26-7.39 nm identified as a phase where the DNA strands are intercalated in water layers between adjacent lipid bilayers, and L(DOPC) phase with repeat distance d(DOPC) approximately 6.45-5.65 nm identified as a phase of partially dehydrated DOPC bilayers without any divalent cations and DNA strands. The coexistence of these phases was investigated as a function of DOPC/DNA molar ratio, length of DNA fragments and temperature. If the amount of lipid increases, the fraction of partially dehydrated L(DOPC) phase is limited, depends on the portion of DNA in the sample and also on the length of DNA fragments. Thermal behaviour of DOPC+DNA+Ca(2+) aggregates was investigated in the range 20-80 degrees C. The transversal thermal expansivities of both phases were evaluated.

Interaction of gemini surfactants butane-1,4-diyl-bis(alkyldimethylammonium bromide) with DNA.

The size and structure of aggregates formed by interaction of DNA with homologous series of cationic gemini surfactants butane-1,4-diyl-bis(alkyldimethylammonium bromide) (CnGS, n=10-16 is the number of alkyl carbons) were investigated using UV-vis turbidity, dynamic light scattering and small-angle synchrotron X-ray (SAX) diffraction. The detailed analysis of turbidity in the range of lambda=450-600 nm indicates an anomaly in the growth of CnGS+DNA aggregates with increasing concentration of CnGS, possibly involving changes of structure and size of aggregates. Using dynamic light scattering, changes of the effective diameter of CnGS+DNA (n=12 and 16) aggregates formed in the CnGS concentration range 0.002-0.140 mmol/l were observed. SAX diffractograms show the presence of long-range organization of CnGS+DNA (n=12, 13, 14 and 16) aggregates due to DNA interaction with CnGS above the critical micellar concentration. The CnGS+DNA (n=12, 13 and 14) aggregates at 25 degrees C are packed in a lattice of two-dimensional hexagonal symmetry. With increasing C14GS:DNA molar ratio the changes of the lattice parameter in the range of 4.80-5.27 nm are observed at 25 degrees C. The aggregates undergo structural changes induced by temperature in the range 60-95 degrees C, which are accompanied by changes of the diffraction patterns, namely in the region of reciprocal spacing s=0.15-0.30 nm(-1).

Bilayer thickness in unilamellar extruded 1,2-dimyristoleoyl and 1,2-dierucoyl phosphatidylcholine vesicles: SANS contrast variation study of cholesterol effect.

Small-angle neutron scattering on extruded unilamellar vesicles in water was used to study bilayer thickness when cholesterol (CHOL) was added at 44.4 mol% to 1,2-dimyristoleoylphosphatidylcholine (diC14:1PC) and 1,2-dierucoylphosphatidylcholine (diC22:1PC) bilayers. Using the (1)H(2)O/(2)H(2)O contrast variation and the small-angle form of Kratky-Porod approximation, the bilayer gyration radii at infinite contrast R(g,infinity) and the bilayer thickness parameters d(g,infinity) = 12(0.5)R(g,infinity) were obtained at 30 degrees C. Addition of cholesterol to diC14:1PC increased the d(g,infinity) from 3.72 +/- 0.02 to 4.26 +/- 0.01 nm, while in the diC22:1PC bilayers the d(g,infinity) change observed was within the experimental error: +0.23 +/- 0.23 nm.

The structure of DNA-DLPC-cationic gemini surfactant aggregates: a small angle synchrotron X-ray diffraction study.

The structure of aggregates formed by interaction of DNA with unilamellar dilauroylphosphatidylcholine (DLPC) vesicles (DNA:DLPC=1:1 base/mol) in the presence of gemini surfactant butane-1,4-diyl-bis(dodecyldimethylammonium bromide) (C12GS) was investigated using synchrotron small angle X-ray diffraction. In the concentration range C12GS+:DLPC< or =1 mol/mol, a condensed lamellar Lalphac phase was found with a repeat period of lipid bilayer stacking in the range d approximately 5.70-6.53 nm and the DNA interhelical distance d(DNA) approximately 3.52-3.99 nm, depending on the concentration of C12GS. At molar ratio C12GS+:DLPC> or =0.35:1, the diffractograms have shown the presence of a second lamellar phase with the repeat period d approximately 5.31 nm which slightly decreases with increasing concentration of C12GS+. The increasing fraction of this phase in the aggregates with increasing concentration of C12GS supports the association of this phase with microscopic domains enriched by surfactant molecules. The temperature behaviour of aggregates was investigated in the range 25-60 degrees C and the transversal thermal expansivities of the observed phases were determined.

Models to analyze small-angle neutron scattering from unilamellar lipid vesicles.

Small-angle scattering has been employed to study the structure of lipid bilayers in unilamellar vesicles. This paper evaluates the use of a model approach for the analysis of such data. A long molecular dynamics simulation of a dipalmitoylphosphatidylcholine bilayer in the L(alpha) phase provides detailed structural information from which scattering length density profiles and scattering intensity are obtained. A sequence of increasingly realistic models are defined and then fit to the simulated scattering intensity data for values of q that are experimentally accessible. The models are evaluated by how well they fit the intensity data and the structural parameters of the simulation. Although the conventional approach that extracts only the radius of gyration from a Kratky-Porod plot provides a reasonable fit to much of the data, the available experimental q range supports refined models with two independent parameters. Of the many two-parameter models, we propose that particular choices should be inspired by the functional form of the scattering length density profile of simulations. Constraints that limit realistic models to two independent parameters are described in detail. The analysis supports the proposition that reliable results for area/lipid and hydrocarbon thickness can be obtained from small-angle neutron scattering of unilamellar vesicles.