Small-angle neutron scattering from multilamellar lipid bilayers: Theory, model, and experiment

Lemmich, Jesper; Mortensen, K.; Ipsen, John Hjort; Hønger, Thomas; Bauer, Rogert; Mouritsen, Ole G.

doi:10.1103/physreve.53.5169

Cited by 100 publications

(72 citation statements)

References 45 publications

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“…For the longer tail lengths, this maximum disappears. 48,50 We observe a large increase of the bilayer thickness for all lipid tail lengths, caused by a simultaneously increase of the hydrocarbon thickness and the thickness of the headgroup region. We do not observe a maximum in the curve for the shorter lipids.…”

Section: Discussionmentioning

confidence: 92%

“…2,24 Experimental work and theories suggest that the swelling could be caused by increased interactions between bilayers due to changes in the Helfrich undulation forces. [45][46][47][48][49][50] Approaching the transition temperature T m , the bilayer has a reduced bending rigidity and as a result the fluctuations of the bilayer increase. Due to these increased bilayer fluctuations the steric repulsion between bilayers increases.…”

Section: Discussionmentioning

confidence: 99%

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Phase Behavior of Model Lipid Bilayers

2005

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We investigated the phase behavior of double-tail lipids, as a function of temperature, headgroup interaction and tail length. At low values of the head-head repulsion parameter a hh , the bilayer undergoes with increasing temperature the transitions from the subgel phase L c via the flat gel phase L to the fluid phase L R . For higher values of a hh , the transition from the L c to the L R phase occurs via the tilted gel phase L ′ and the rippled phase P ′ . The occurrence of the L ′ phase depends on tail length. We find that the rippled structure (P ′ ) occurs if the headgroups are sufficiently surrounded by water and that the ripple is a coexistence between the L c or L ′ phase and the L R phase. The anomalous swelling, observed at the P ′ f L R transition, is not directly related to the rippled phase, but a consequence of conformational changes of the tails.

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Section: Discussionmentioning

confidence: 92%

Section: Discussionmentioning

confidence: 99%

Phase Behavior of Model Lipid Bilayers

2005

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Section: Introductionmentioning

confidence: 99%

“…Therefore, quantitative use of the scattering intensity for structure determination requires correction for the fluctuations endemic in such systems [1]. A harmonic theory [2,3] that predicts power law tails in the scattering line shapes fits membrane data very well [4,5], but the anharmonicities that are inherent in realistic potentials have remained a concern for quantitative interpretation [6,7], even though a renormalization group analysis suggested that such effects are small [8].Stacks of bilayers are also much studied because they provide ideal environments to study fundamental interactions between bilayers, especially since the range of interbilayer distances a can be systematically varied by applying osmotic pressure P [9]. The corresponding theory [10] is an approximate first-order perturbation theory that again relies on harmonic assumptions, such as the normality of the probability distribution function of the interbilayer spacing.…”

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confidence: 99%

Simulations of Interacting Membranes in the Soft Confinement Regime

Gouliaev

Nagle

1998

Phys. Rev. Lett.

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The liquid crystalline model biomembrane system consisting of a stack of interacting membranes is studied in the realistic soft confinement regime by the newly developed Fourier Monte Carlo simulation technique. In this regime experiment and simulations show that the functional form of the fluctuation pressure is more nearly exponential rather than the power law valid for the hard confinement regime. The simulations provide quantitative improvement over perturbation theory. It is shown that the harmonic theory that is routinely used to interpret x-ray scattering line shapes is valid. Stacks of lipid bilayers (see Fig. 1) are model systems for biomembranes that are much studied for two reasons. First, such stacks diffract fairly well and this facilitates determination of the structure of individual membranes, which is of primary interest in biophysics. However, these stacks are not crystals with the long-range order that is assumed in traditional biophysical analysis, but smectic liquid crystals with quasi-long-range order. Therefore, quantitative use of the scattering intensity for structure determination requires correction for the fluctuations endemic in such systems [1]. A harmonic theory [2,3] that predicts power law tails in the scattering line shapes fits membrane data very well [4,5], but the anharmonicities that are inherent in realistic potentials have remained a concern for quantitative interpretation [6,7], even though a renormalization group analysis suggested that such effects are small [8].Stacks of bilayers are also much studied because they provide ideal environments to study fundamental interactions between bilayers, especially since the range of interbilayer distances a can be systematically varied by applying osmotic pressure P [9]. The corresponding theory [10] is an approximate first-order perturbation theory that again relies on harmonic assumptions, such as the normality of the probability distribution function of the interbilayer spacing. While this theory has been a valuable guide, it is too inaccurate to extract fundamental interbilayer interactions from P(a) data.Both these issues are addressed using Monte Carlo simulations with realistic intermembrane potentials for the biologically relevant regime where the interbilayer water spacing a is of order 5-30 Å and each membrane is flexible with bending modulus K c ≃ 10-25k B T. In this regime, called the soft confinement regime [10], it is usually supposed that the primary interbilayer interactions for dipolar lipids are the attractive van der Waals (vdW) potential and the mysterious, but easily measured, repulsive hydration potential, where z is the local distance between two membranes [11]. These interactions are significantly anharmonic, to the extent that the potential of a membrane midway between two neighboring membranes (at the dashed positions in Fig. 1) may have a maximum instead of a minimum. The contrasting regime, sometimes called the hard confinement regime, consists of only excluded volume or steric interactions betwe...

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“…2. swelling in terms of Helfrich's undulations forces, 22,23 increase in conformational order of the hydrocarbon chains, 24,25 increase in the water thickness, 1 or increase in head-group layer thickness. Moreover, the recent experimental data of Mason et al 8 show that anomalous swelling even occurs in systems that do not form a rippled phase.…”

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Mesoscopic simulations of phase transitions in lipid bilayers

Kranenburg

Laforge

Smit

2004

Phys. Chem. Chem. Phys.

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The formation of the rippled phase in biological membranes and its relation with anomalous swelling are still lacking a molecular explanation. Starting from all-atom simulations we use a mapping to create a mesoscopic model of the lipid dimyristoylphosphatidylcholine (DMPC) in water. We use this model to study the phase behaviour of lipid bilayers. Depending on the lipid structure and head group, our simulations reproduce the experimental phase diagrams. The anomalous swelling is caused by conformational changes of the lipid tails but is not directly related to the rippled phase. A key factor for the rippled phase is a frustration between the surface area of the heads and the lateral density of the tails.Phospholipids can self-assemble in water to form bilayer structures, which are seen as model systems of biological membranes.1 At high temperatures lipids form a flat fluid membrane (L a phase) and at low temperatures a flat gel phase in which the lipid tails are ordered and tilted (L b 0 phase). For certain phospholipids between the L a and L b 0 phase phases a stable supramolecular periodic structure was observed [2][3][4] which is characterized by a long-wavelength rippling of the bilayer (P b 0 phase) and a (anomalous) swelling of the membrane. This rippled phase has attracted the attention of many groups, 2-7 but still lacks a molecular understanding. 1,8 Using state of the art molecular dynamics simulations, it is possible to obtain detailed structural information of a single phase, for example, the gel phase, 9 but these are too time consuming to determine a complete phase diagram. In this work, we use an alternative approach in which realistic all-atom simulations are used to determine the effective intramolecular interaction parameters of a mesoscopic model. At this mesoscopic level simulations are four to five orders of magnitude more efficient, 10,11 allowing us to compute complete phase diagrams. In a dissipative particle dynamics (DPD) simulation a particle represents the centre of mass of a cluster of atoms. The total force on a particle consists of dissipative, random, and conservative forces. 12,13 For the conservative force we use a soft-repulsive interactionwhere r ij is the distance between particles i and j, a ij is the parameter characterising the interaction between two particles, and r c is the cut-off radius. In our mesoscopic model, we distinguish three types of particles, w, h, and t to mimic the water and the head-and tail-atoms of a lipid, respectively. A coarse graining procedure to map the interactions of realistic molecules on DPD interaction parameters has been developed by Groot and Warren. 14 In this procedure the value of the repulsion parameter is taken such that the DPD waterlike particles reproduce the compressibility of water. The interactions of the hydrophilic and hydrophobic particles (a ww ¼ a tt ¼ 25, a ht ¼ a wt ¼ 80, and a hw ¼ 15) are based on the Flory-Huggins solubility parameters. We vary the headhead (a hh ) interaction parameter to study the effect of changin...

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Small-angle neutron scattering from multilamellar lipid bilayers: Theory, model, and experiment

Cited by 100 publications

References 45 publications

Phase Behavior of Model Lipid Bilayers

Phase Behavior of Model Lipid Bilayers

Simulations of Interacting Membranes in the Soft Confinement Regime

Mesoscopic simulations of phase transitions in lipid bilayers

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