An official website of the United States government
Nanoplastics, small plastic particles smaller than microplastics, have been suggested to have a wide-range of unique interactions when they encounter lipid membranes. Recent studies have demonstrated that the smaller size of nanoplastic particles may allow them to penetrate and dissolve in lipid membranes. Following this penetration, however, there is not yet a clear picture of how such particles impact the local lipid environment. A recent study by the present authors found that when lipid vesicles that included laurdan, a fluorescent dye molecule typically thought to report on the membrane phase, were exposed to polystyrene nanoparticles, they exhibited a concentration-dependent blue shift consistent with a fluid-to-gel phase transition. However, coarse-grained simulations suggested that no such transition was taking place; instead, the simulations observed that polymer chains from the polystyrene nanoparticles penetrated into the liposome membrane. In the present work, we use all-atom molecular dynamics simulations to demonstrate that the inclusion of polystyrene within a lipid membrane causes significant changes to the local hydration and structure of that membrane while maintaining the membrane phase. Specifically, through the explicit incorporation of laurdan within the present simulations, we demonstrate that the local hydration environment of the dye molecule changes significantly but continuously as membranes are exposed to polystyrene, thus suggesting a possible explanation for the previously reported experimental observation. The present results provide a picture of the complex heterogeneity generated within polymer-containing membranes.
more »
« less
Mechanical properties of lipid bilayers: a note on the Poisson ratio
We investigate the Poisson ratio ν of fluid lipid bilayers, i.e. , the question how area strains compare to the changes in membrane thickness (or, equivalently, volume) that accompany them. We first examine existing experimental results on the area- and volume compressibility of lipid membranes. Analyzing them within the framework of linear elasticity theory for homogeneous thin fluid sheets leads us to conclude that lipid membrane deformations are to a very good approximation volume-preserving, with a Poisson ratio that is likely about 3% smaller than the common soft matter limit . These results are fully consistent with atomistic simulations of a DOPC membrane at varying amount of applied lateral stress, for which we instead deduce ν by directly comparing area- and volume strains. To assess the problematic assumption of transverse homogeneity, we also define a depth-resolved Poisson ratio ν ( z ) and determine it through a refined analysis of the same set of simulations. We find that throughout the membrane's thickness, ν ( z ) is close to the value derived assuming homogeneity, with only minor variations of borderline statistical significance.
more »
« less
- Award ID(s):
- 1764257
- PAR ID:
- 10175637
- Date Published:
- Journal Name:
- Soft Matter
- Volume:
- 15
- Issue:
- 44
- ISSN:
- 1744-683X
- Page Range / eLocation ID:
- 9085 to 9092
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Liposomal carriers provide a flexible and effective strategy for delivering therapeutics across a broad spectrum of diseases. Cholesterol is frequently included in these systems to improve membrane rigidity and limit permeability. Despite its widespread use, the optimal cholesterol-to-lipid proportion for achieving stable and efficient liposome performance remains to be fully determined. In this work, we apply all-atom molecular dynamics simulations to explore how different cholesterol concentrations influence the structural and dynamic characteristics of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) bilayers, considering both planar and curved membrane geometries. Bilayers with cholesterol molar ratios of 0%, 10%, 20%, 30%, 40%, and 50% were simulated, and key biophysical parameters including area per lipid (APL), membrane thickness, leaflet interdigitation, and deuterium order parameters (SCD) were analyzed. In planar bilayers, increasing cholesterol concentration led to a progressive decrease in APL from approximately 60°A2to 40°A2, accompanied by increased membrane thickness and lipid ordering, consistent with cholesterol’s classical condensing effect. In contrast, curved bilayers exhibited a cholesterol-induced expansion effect, particularly in the inner leaflet, where APL increased from approximately 60°A2to 90°A2with rising cholesterol levels. SCD profiles showed that cholesterol enhanced tail ordering up to 40% concentration, beyond which the effect plateaued or slightly declined, suggesting structural saturation or packing frustration. Membrane thickness displayed a monotonic increase in planar bilayers but followed a nonlinear trend in curved systems due to curvature-induced stress. These findings highlight that cholesterol’s influence on membrane properties is highly dependent on bilayer geometry and asymmetry. While planar bilayers exhibit predictable responses, curved systems reveal nonclassical behaviors that challenge traditional models of cholesterol-lipid interactions. This work provides molecular-level insights and establishes a computational framework for the rational design of liposomal systems, emphasizing the need to account for curvature and asymmetry in membrane engineering.more » « less
-
Cell membranes are responsible for a range of biological processes that require interactions between lipids and proteins. While the effects of lipids on proteins are becoming better understood, our knowledge of how protein conformational changes influence membrane dynamics remains rudimentary. Here, we performed experiments and computer simulations to study the dynamic response of a lipid membrane to changes in the conformational state of pH-low insertion peptide (pHLIP), which transitions from a surface-associated (SA) state at neutral or basic pH to a transmembrane (TM) α-helix under acidic conditions. Our results show that TM-pHLIP significantly slows down membrane thickness fluctuations due to an increase in effective membrane viscosity. Our findings suggest a possible membrane regulatory mechanism, where the TM helix affects lipid chain conformations, and subsequently alters membrane fluctuations and viscosity.more » « less
-
null (Ed.)The plasma membrane of eukaryotic cells is known to be compositionally asymmetric. Certain phospholipids, such as sphingomyelin and phosphatidylcholine species, are predominantly localized in the outer leaflet, while phosphatidylethanolamine and phosphatidylserine species primarily reside in the inner leaflet. While phospholipid asymmetry between the membrane leaflets is well established, there is no consensus about cholesterol distribution between the two leaflets. We have performed a systematic study, via molecular simulations, of how the spatial distribution of cholesterol molecules in different “asymmetric” lipid bilayers are affected by the lipids’ backbone, head-type, unsaturation, and chain-length by considering an asymmetric bilayer mimicking the plasma membrane lipids of red blood cells, as well as seventeen other asymmetric bilayers comprising of different lipid types. Our results reveal that the distribution of cholesterol in the leaflets is solely a function of the extent of ordering of the lipids within the leaflets. The ratio of the amount of cholesterol matches the ratio of lipid order in the two leaflets, thus providing a quantitative relationship between the two. These results are understood by the observation that asymmetric bilayers with equimolar amount of lipids in the two leaflets develop tensile and compressive stresses due to differences in the extent of lipid order. These stresses are alleviated by the transfer of cholesterol from the leaflet in compressive stress to the one in tensile stress. These findings are important in understanding the biology of the cell membrane, especially with regard to the composition of the membrane leaflets.more » « less
-
We show in the companion paper that the free membrane shape of lipid bilayer vesicles containing the mechanosensitive ion channel Piezo can be predicted, with no free parameters, from membrane elasticity theory together with measurements of the protein geometry and vesicle size [C. A. Haselwandter, Y. R. Guo, Z. Fu, R. MacKinnon, Proc. Natl. Acad. Sci. U.S.A. , 10.1073/pnas.2208027119 (2022)]. Here we use these results to determine the force that the Piezo dome exerts on the free membrane and hence, that the free membrane exerts on the Piezo dome, for a range of vesicle sizes. From vesicle shape measurements alone, we thus obtain a force–distortion relationship for the Piezo dome, from which we deduce the Piezo dome’s intrinsic radius of curvature, 42 ± 12 nm, and bending stiffness, 18 ± 2.1 k B T , in freestanding lipid bilayer membranes mimicking cell membranes. Applying these estimates to a spherical cap model of Piezo embedded in a lipid bilayer, we suggest that Piezo’s intrinsic curvature, surrounding membrane footprint, small stiffness, and large area are the key properties of Piezo that give rise to low-threshold, high-sensitivity mechanical gating.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C9SM01290G
