In Silico Design of Robust Bolalipid Membranes

Antivesiculation and Complete Unbinding of Tail-Tethered Lipids

We report the effect of tail-tethering on vesiculation and complete unbinding of bilayered membranes. Amphiphilic molecules of a bolalipid, resembling the tail-tethered molecular structure of archaeal lipids, with two identical zwitterionic phosphatidylcholine headgroups self-assemble into a large flat lamellar membrane, in contrast to the multilamellar vesicles (MLVs) observed in its counterpart, monopolar nontethered zwitterionic lipids. The antivesiculation is confirmed by small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cyro-TEM). With the net charge of zero and higher bending rigidity of the membrane (confirmed by neutron spin echo (NSE) spectroscopy), the current membrane theory would predict that membranes should stack with each other (aka "bind") due to dominant van der Waals attraction, while the outcome of the nonstacking ("unbinding") membrane suggests that the theory needs to include entropic contribution for the nonvesicular structures. This report pioneers an understanding of how the tail-tethering of amphiphiles affects the structure, enabling better control over the final nanoscale morphology.

Synthesis of a photocleavable bola-phosphatidylcholine

Phosphatidylinositol transfer proteins (PITPs) are ubiquitous in eukaryotes and are involved in the regulation of phospholipid metabolism, membrane trafficking, and signal transduction. Sec14 is a yeast PITP that has been shown to transfer phosphatidylinositol (PI) or phosphatidylcholine (PC) from the endoplasmic reticulum to the Golgi. It is now believed that Sec14 may play a greater role than just shuttling PI and PC throughout the cell. Genetic evidence suggests that retrieval of membrane-bound PI by Sec14 also manages to present PI to the phosphatidylinositol-4-kinase, Pik1, to generate phosphatidylinositol-4-phosphate, PI(4)P. To test this hypothetical model, we designed a photocleavable bolalipid to span the entire membrane, having one phosphatidylcholine or phosphatidylinositol headgroup on each leaflet connected by a photocleavable diacid. Sec14 should not be able to present the bola-PI to Pik1 for phosphorylation as the head group will be difficult to lift from the bilayer as it is tethered on the opposite leaflet. After photocleavage the two halves would behave as a normal phospholipid, thus phosphorylation by Pik1 would resume. We report here the synthesis of a photocleavable bola-PC, a precursor to the desired bola-PI. The mono-photocleavable bola-PC lipid was designed to contain two glycerol molecules with choline head groups connected through a phosphodiester bond at the sn3 position. Each glycerol was acylated with palmitic acid at the sn1 position. These two glycerol moieties were then connected through their respective sn2 hydroxyls via a photocleavable dicarboxylic acid containing a nitrophenyl ethyl photolabile protecting group. The bola-PC and its precursors were found to undergo efficient photocleavage when irradiated in solution or in vesicles with 365 nm light for two minutes. Treatment of the bola-PC with a mutant phospholipase D and myo-inositol produced a mono-inositol bola-PC-PI.

Enantioselective Total Synthesis of the Archaeal Lipid Parallel GDGT-0 (Isocaldarchaeol)*

Archaeal glycerol dibiphytanyl glycerol tetraethers (GDGT) are some of the most unusual membrane lipids identified in nature. These amphiphiles are the major constituents of the membranes of numerous Archaea, some of which are extremophilic organisms. Due to their unique structures, there has been significant interest in studying both the biophysical properties and the biosynthesis of these molecules. However, these studies have thus far been hampered by limited access to chemically pure samples. Herein, we report a concise and stereoselective synthesis of the archaeal tetraether lipid parallel GDGT-0 and the synthesis and self-assembly of derivatives bearing different polar groups.

Effect of alcohol on the phase separation in model membranes

The discovery of coexisting liquid-ordered and liquid-disordered phases in multicomponent lipid bilayers has received widespread attention due to its potential relevance for biological systems. One of the many open questions is how the presence of additional components affects the nature of the coexisting phases. Of particular interest is the addition of alcohols because their anesthetic properties may arise from modulating bilayer behavior. We use coarse-grained Molecular Dynamics simulations to gain insight into the partitioning preferences of linear n-alcohols into ordered and disordered bilayers alongside their effects on local membrane structure. We find that alcohols cause only small changes to membrane composition alongside a lack of significant effects on membrane thickness and lipid tail order. Cholesterol and n-alcohol trans-bilayer motion is measured and found to be near or within the range of previous atomistic results. The cholesterol flip-flop rates increase with both n-alcohol length and concentration for octanol, dodecanol, and hexadecanol, indicating a decrease in lipid order. Umbrella sampling simulations of removing cholesterol from tertiary membranes find no significant difference with or without n-alcohols at various concentrations. Simulations of a phase-separated bilayer show that octanol preferentially partitions into the liquid-disordered phase in a ratio of approximately 3:1 over the liquid-ordered phase. Furthermore, partition coefficients of alcohol in single-phase membranes show a preference for longer alcohols (dodecanol and hexadecanol) to partition preferentially into the liquid-ordered phase, while decreasing the length of the alcohol reverses this trend. Our work tests experimental results while also investigating the ability for coarse-grained MARTINI simulations to capture minute differences in model membrane spatial arrangements on the nanoscale level.

Bilayer thickness determines the alignment of model polyproline helices in lipid membranes

Our understanding of protein folds relies fundamentally on the set of secondary structures found in the proteomes. Yet, there also exist intriguing structures and motifs that are underrepresented in natural biopolymeric systems. One example is the polyproline II helix, which is usually considered to have a polar character and therefore does not form membrane spanning sections of membrane proteins. In our work, we have introduced specially designed polyproline II helices into the hydrophobic membrane milieu and used 19F NMR to monitor the helix alignment in oriented lipid bilayers. Our results show that these artificial hydrophobic peptides can adopt several different alignment states. If the helix is shorter than the thickness of the hydrophobic core of the membrane, it is submerged into the bilayer with its long axis parallel to the membrane plane. The polyproline helix adopts a transmembrane alignment when its length exceeds the bilayer thickness. If the peptide length roughly matches the lipid thickness, a coexistence of both states is observed. We thus show that the lipid thickness plays a determining role in the occurrence of a transmembrane polyproline II helix. We also found that the adaptation of polyproline II helices to hydrophobic mismatch is in some notable aspects different from α-helices. Finally, our results prove that the polyproline II helix is a competent structure for the construction of transmembrane peptide segments, despite the fact that no such motif has ever been reported in natural systems.

Role of Transmembrane Proteins for Phase Separation and Domain Registration in Asymmetric Lipid Bilayers

It is well known that the formation and spatial correlation of lipid domains in the two apposed leaflets of a bilayer are influenced by weak lipid–lipid interactions across the bilayer’s midplane. Transmembrane proteins span through both leaflets and thus offer an alternative domain coupling mechanism. Using a mean-field approximation of a simple bilayer-type lattice model, with two two-dimensional lattices stacked one on top of the other, we explore the role of this “structural” inter-leaflet coupling for the ability of a lipid membrane to phase separate and form spatially correlated domains. We present calculated phase diagrams for various effective lipid–lipid and lipid–protein interaction strengths in membranes that contain a binary lipid mixture in each leaflet plus a small amount of added transmembrane proteins. The influence of the transmembrane nature of the proteins is assessed by a comparison with “peripheral” proteins, which result from the separation of one single integral protein into two independent units that are no longer structurally connected across the bilayer. We demonstrate that the ability of membrane-spanning proteins to facilitate domain formation requires sufficiently strong lipid–protein interactions. Weak lipid–protein interactions generally tend to inhibit phase separation in a similar manner for transmembrane as for peripheral proteins.

Computational Modeling of Realistic Cell Membranes

Cell membranes contain a large variety of lipid types and are crowded with proteins, endowing them with the plasticity needed to fulfill their key roles in cell functioning. The compositional complexity of cellular membranes gives rise to a heterogeneous lateral organization, which is still poorly understood. Computational models, in particular molecular dynamics simulations and related techniques, have provided important insight into the organizational principles of cell membranes over the past decades. Now, we are witnessing a transition from simulations of simpler membrane models to multicomponent systems, culminating in realistic models of an increasing variety of cell types and organelles. Here, we review the state of the art in the field of realistic membrane simulations and discuss the current limitations and challenges ahead.

Elastic property of membranes self-assembled from diblock and triblock copolymers

The elastic property of membranes self-assembled from AB diblock and ABA triblock copolymers, as coarse-grained model of lipids and the bolalipids, are studied using the self-consistent field theory (SCFT). Specifically, solutions of the SCFT equations, corresponding to membranes in different geometries (planar, cylindrical, spherical, and pore) have been obtained for a model system composed of amphiphilic AB diblock copolymers and ABA triblock copolymers dissolved in A homopolymers. The free energy of the membranes with different geometries is then used to extract the bending modulus, Gaussian modulus, and line tension of the membranes. The results reveal that the bending modulus of the triblock membrane is greater than that of the diblock membrane. Furthermore, the Gaussian modulus and line tension of the triblock membrane indicate that the triblock membranes have higher pore formation energy than that of the diblock membranes. The equilibrium bridging and looping fractions of the triblock copolymers are also obtained. Implications of the theoretical results on the elastic properties of biologically equivalent lipid bilayers and the bolalipid membranes are discussed.

Phospholipidic Colchicinoids as Promising Prodrugs Incorporated into Enzyme-Responsive Liposomes: Chemical, Biophysical, and Enzymological Aspects

Enzyme-responsive liposomes release their cargo in response to pathologically increased levels of enzymes at the target site. We report herein an assembly of phospholipase A2-responsive liposomes based on colchicinoid lipid prodrugs incorporated into lipid bilayer of the nanosized vesicles. The liposomes were constructed to addresses two important issues: (i) the lipid prodrugs were designed to fit the structure of the enzyme binding site; and (ii) the concept of lateral pressure profile was used to design lipid prodrugs that introduce almost no distortions into the lipid bilayer packing, thus ensuring that corresponding liposomes are stable. The colchicinoid agents exhibit antiproliferative activity in subnanomolar range of concentrations.

Alcohol Interactions with Lipid Bilayers

We investigate the structural changes to lipid membrane that ensue from the addition of aliphatic alcohols with various alkyl tail lengths. Small angle neutron diffraction from flat lipid bilayers that are hydrated through water vapor has been employed to eliminate possible artefacts of the membrane curvature and the alcohol's membrane-water partitioning. We have observed clear changes to membrane structure in both transversal and lateral directions. Most importantly, our results suggest the alteration of the membrane-water interface. The water encroachment has shifted in the way that alcohol loaded bilayers absorbed more water molecules when compared to the neat lipid bilayers. The experimental results have been corroborated by molecular dynamics simulations to reveal further details. Namely, the order parameter profiles have been fruitful in correlating the mechanical model of structural changes to the effect of anesthesia.

Analytical calculation of the lipid bilayer bending modulus

Bending and Gaussian moduli of a homogenious single-component lipid bilayer are calculated analytically using microscopic model of the lipid hydrocarbon chains. The approach allows for thermodynamic averaging over different chains conformations. Each chain is modeled as a flexible string with finite bending rigidity and an incompressible cross-section area. The interchain steric repulsion is accounted for self-consistently determined single-chain confining parabolic potential. The model provides a simple analytical expression for the membrane bending modulus, which falls within a range of experimental values. An observed dependence of the modulus on hydrocarbon chain length is also reproduced. Correspondence between our microscopic model and the membrane theory of elasticity is established.

Biological Membranes in Extreme Conditions: Anionic Tetraether Lipid Membranes and Their Interactions with Sodium and Potassium

Archaea such as Sulfolobus acidocaldarius tolerate extreme temperatures and high acidity and can grow in the presence of toxic metals and low concentrations of Na⁺ or K⁺. It is believed that their unique tetraether membranes protect them from harsh environments and allow their survival under such conditions. We used molecular dynamics simulations to study membranes comprising glycerol dialkylnonitol tetraether lipids, which are the main component of S. acidocaldarius membranes, in solutions containing different concentrations of NaCl and KCl or with Na⁺ or K⁺ counterions (trace cations, 0 M). Anionic binding sites on the membranes were almost 50% occupied in the presence of counterions. The free energy of cation-phosphate complexation and the residence times of ions near the membranes were found to be both ion- and concentration-dependent. Sodium ions had more favorable interactions with the membranes and a longer residence time, whereas higher cation concentrations led to shorter ion residence times. When only counterions were present in the solutions, large residence times suggested that the membrane may function as a cation-attracting reservoir. The results suggested that the ions can be easily transferred to the cytoplasm as needed, explaining the growth curves of S. acidocaldarius under different salinities and pH.

Biological Membranes in Extreme Conditions: Simulations of Anionic Archaeal Tetraether Lipid Membranes

In contrast to the majority of organisms that have cells bound by di-ester phospholipids, archaeal membranes consist of di- and tetraether phospholipids. Originating from organisms that withstand harsh conditions (e.g., low pH and a wide range of temperatures) such membranes have physical properties that make them attractive materials for biological research and biotechnological applications. We developed force-field parameters based on the widely used Generalized Amber Force Field (GAFF) to enable the study of anionic tetraether membranes of the model archaean Sulfolobus acidocaldarius by computer simulations. The simulations reveal that the physical properties of these unique membranes depend on the number of cyclopentane rings included in each lipid unit, and on the size of cations that are used to ensure charge neutrality. This suggests that the biophysical properties of Sulfolobus acidocaldarius cells depend not only on the compositions of their membranes but also on the media in which they grow.

Elastic deformations of bolalipid membranes

Archaeal membranes have unique mechanical properties that enable these organisms to survive under extremely aggressive environmental conditions. The so-called bolalipids contribute to this exceptional stability. They have two polar heads joined by two hydrocarbon chains. The two headgroups can face different sides of the membrane (O-shape conformation) or the same side (U-shape conformation). We have developed an elasticity theory for bolalipid membranes and show that the energetic contributions of (i) tilt deformations, (ii) area compression/stretching deformations, (iii) as well as those of Gaussian splay from the two membrane surfaces are additive, while splay deformations yield a cross-term. The presence of a small fraction of U-shaped molecules resulted in spontaneous membrane curvature. We estimated the tilt modulus to be approximately equal to that of membranes in eukaryotic cells. In contrast to conventional lipids, the bolalipid membrane possesses two splay moduli, one of which is estimated to be an order of magnitude larger than that of conventional lipids. The projected values of elastic moduli act to hamper pore formation and to decelerate membrane fusion and fission.

LipidBuilder: A Framework To Build Realistic Models for Biological Membranes

The physical and chemical characterization of biological membranes is of fundamental importance for understanding the functional role of lipid bilayers in shaping cells and organelles, steering vesicle trafficking and promoting membrane-protein signaling. Molecular dynamics simulations stand as a powerful tool to probe the properties of membranes at atomistic level. However, the biological membrane is highly complex, and closely mimicking its physiological constitution in silico is not a straightforward task. Here, we present LipidBuilder, a framework for creating and storing models of biologically relevant phospholipid species with acyl tails of heterogeneous composition. LipidBuilder also enables the assembly of these database-stored lipids into realistic bilayers featuring asymmetric distribution on layer leaflets and concentration of given membrane constituents as defined, for example, by lipidomics experiments. The ability of LipidBuilder to assemble robust membrane models was validated by simulating membranes of homogeneous lipid composition for which experimental data are available. Furthermore, taking advantage of the extensive lipid headgroup repertoire, we assembled models of membranes of heterogeneous nature as naturally found in viral (phage PRD1), bacterial (Salmonella enterica, Laurinavicius , S. ; Kakela , R. ; Somerharju , P. ; Bamford , D. H. ; Virology 2004 , 322 , 328 - 336 ) and plant (Chlorella kessleri, Rezanka , T. ; Podojil , M. ; J. Chromatogr. 1989 , 463 , 397 - 408 ) organisms. These realistic membrane models were built using a near-exact lipid composition revealed from analytical chemistry experiments. We suggest LipidBuilder as a useful tool to model biological membranes of near-biological complexity, and as a robust complement to the current efforts to characterize the biophysical properties of biological membrane using molecular simulation.

Depth-Dependent Organization and Dynamics of Archaeal and Eukaryotic Membranes: Development of Membrane Anisotropy Gradient with Natural Evolution

The lipid composition of archaea is unique and has been correlated with increased stability under extreme environmental conditions. In this article, we have focused on the evolution of membrane organization and dynamics with natural evolution. Dynamic anisotropy along the membrane normal (i.e., gradients of mobility, polarity, and heterogeneity) is a hallmark of fluid phase diester or diether phospholipid membranes. We monitored gradients of mobility, polarity, and heterogeneity along the membrane normal in membranes made of a representative archaeal lipid using a series of membrane depth-dependent fluorescent probes, and compared them to membranes prepared from a typical diether lipid from higher organisms (eukaryotes). Our results show that the representative dynamic anisotropy gradient along the membrane normal is absent in membranes made from archaeal lipids. We hypothesize that the dynamic gradient observed in membranes of diester and diether phospholipids is a consequence of natural evolution of membrane lipids in response to the requirement of carrying out complex cellular functions by membrane proteins.

Erratum. Computational insight in the role of fusogenic lipopeptides at the onset of liposome fusion

We performed an extensive computational study to obtain insight in the molecular mechanisms that take place prior to membrane fusion. We focused on membrane-anchored hybrid macromolecules (lipid–polymer–oligopeptide) that mimic biological SNARE proteins in terms of liposome fusion characteristics [H. Robson Marsden et al., 2009]; efficient micro-second simulation was enabled by combining validated MARTINI force fields for the molecular building blocks in coarse-grained molecular dynamics (CGMD). We find that individual peptide domains in the hybrid macromolecules bind and partially integrate parallel to the membrane surface, in agreement with experimental findings. By varying several experimental design parameters, we observe that peptide domains remain in the solvent phase only in two cases: (1) for solitary lipopeptides (low concentration), below a threshold area per lipid in the membrane, and (2) when the lipopeptide concentration is high enough for the peptide domains to self-assemble into tetrameric homo-complexes. The peptide-membrane binding is not affected by solvent-induced peptide unfolding, which we mimicked by relaxing the usual MARTINI helix constraints. Remarkably, in this case, a reverse transition to a helical secondary structure is observed after binding, highlighting the role of the membrane as a template (partitioning-folding coupling). Our findings undermine the current view of the initial stages towards fusion, in which membranes are thought to be kept in close apposition via dimerization of individual complementary peptides in the solvent phase. Although we did not study actual fusion, our simulations show that the formation of homomers, which is suppressed in experimental peptide pair design and therefore believed to be insignificant for fusion, by peptides anchored to the same membrane does play a key role in this locking mechanism and potentially also in membrane destabilization that precedes fusion.

Biomembranes in atomistic and coarse-grained simulations

The architecture of biological membranes is tightly coupled to the localization, organization, and function of membrane proteins. The organelle-specific distribution of lipids allows for the formation of functional microdomains (also called rafts) that facilitate the segregation and aggregation of membrane proteins and thus shape their function. Molecular dynamics simulations enable to directly access the formation, structure, and dynamics of membrane microdomains at the molecular scale and the specific interactions among lipids and proteins on timescales from picoseconds to microseconds. This review focuses on the latest developments of biomembrane force fields for both atomistic and coarse-grained molecular dynamics (MD) simulations, and the different levels of coarsening of biomolecular structures. It also briefly introduces scale-bridging methods applicable to biomembrane studies, and highlights selected recent applications.

Characterization of thylakoid lipid membranes from cyanobacteria and higher plants by molecular dynamics simulations

The thylakoid membrane is mainly composed of non-common lipids, so called galactolipids. Despite the importance of these lipids for the function of the photosynthetic reaction centers, the molecular organization of these membranes is largely unexplored. Here we use multiscale molecular dynamics simulations to characterize the thylakoid membrane of both cyanobacteria and higher plants. We consider mixtures of up to five different galactolipids plus phosphatidylglycerol to represent these complex membranes. We find that the different lipids generally mix well, although nanoscale heterogeneities are observed especially in case of the plant membrane. The fluidity of the cyanobacterial membrane is markedly reduced compared to the plant membrane, even considering elevated temperatures at which thermophilic cyanobacteria are found. We also find that the plant membrane more readily undergoes a phase transformation to an inverted hexagonal phase. We furthermore characterized the conformation and dynamics of the cofactors plastoquinone and plastoquinol, revealing of the fast flip-flop rates for the non-reduced form. Together, our results provide a molecular view on the dynamical organization of the thylakoid membrane.

Computational insight in the role of fusogenic lipopeptides at the onset of liposome fusion

We performed an extensive computational study to obtain insight in the molecular mechanisms that take place prior to membrane fusion. We focused on membrane-anchored hybrid macromolecules (lipid-polymer-oligopeptide) that mimic biological SNARE proteins in terms of liposome fusion characteristics [H. Robson Marsden et al., 2009]; efficient micro-second simulation was enabled by combining validated MARTINI force fields for the molecular building blocks in coarse-grained molecular dynamics (CGMD). We find that individual peptide domains in the hybrid macromolecules bind and partially integrate parallel to the membrane surface, in agreement with experimental findings. By varying several experimental design parameters, we observe that peptide domains remain in the solvent phase only in two cases: (1) for solitary lipopeptides (low concentration), below a threshold area per lipid in the membrane, and (2) when the lipopeptide concentration is high enough for the peptide domains to self-assemble into tetrameric homo-complexes. The peptide-membrane binding is not affected by solvent-induced peptide unfolding, which we mimicked by relaxing the usual MARTINI helix constraints. Remarkably, in this case, a reverse transition to a helical secondary structure is observed after binding, highlighting the role of the membrane as a template (partitioning-folding coupling). Our findings undermine the current view of the initial stages towards fusion, in which membranes are thought to be kept in close apposition via dimerization of individual complementary peptides in the solvent phase. Although we did not study actual fusion, our simulations show that the formation of homomers, which is suppressed in experimental peptide-pair design and therefore believed to be insignificant for fusion, by peptides anchored to the same membrane does play a key role in this locking mechanism and potentially also in membrane destabilization that precede fusion.

Liquid but durable: molecular dynamics simulations explain the unique properties of archaeal-like membranes

Archaeal plasma membranes appear to be extremely durable and almost impermeable to water and ions, in contrast to the membranes of Bacteria and Eucaryota. Additionally, they remain liquid within a temperature range of 0-100°C. These are the properties that have most likely determined the evolutionary fate of Archaea, and it may be possible for bionanotechnology to adopt these from nature. In this work, we use molecular dynamics simulations to assess at the atomistic level the structure and dynamics of a series of model archaeal membranes with lipids that have tetraether chemical nature and "branched" hydrophobic tails. We conclude that the branched structure defines dense packing and low water permeability of archaeal-like membranes, while at the same time ensuring a liquid-crystalline state, which is vital for living cells. This makes tetraether lipid systems promising in bionanotechnology and material science, namely for design of new and unique membrane nanosystems.

The power of coarse graining in biomolecular simulations

Computational modeling of biological systems is challenging because of the multitude of spatial and temporal scales involved. Replacing atomistic detail with lower resolution, coarse grained (CG), beads has opened the way to simulate large-scale biomolecular processes on time scales inaccessible to all-atom models. We provide an overview of some of the more popular CG models used in biomolecular applications to date, focusing on models that retain chemical specificity. A few state-of-the-art examples of protein folding, membrane protein gating and self-assembly, DNA hybridization, and modeling of carbohydrate fibers are used to illustrate the power and diversity of current CG modeling.

A bola-phospholipid bearing tetrafluorophenylazido chromophore as a promising lipid probe for biomembrane photolabeling studies

A bola-phospholipid probe, carrying a tetrafluorophenylazido chromophore in the middle of the transmembrane diacyl chain, was synthesized and characterized with a view to studying biomembranes by a photolabeling approach. This probe shows the advantageous stability of bola-lipids in giant vesicle formation alongside excellent photochemical properties conferred by the tetrafluorophenylazido chromophore, and thus constitutes a promising probe for biomembrane photolabeling studies.

The Martini coarse-grained force field

The Martini force field is a coarse-grained force field suited for molecular dynamics simulations of biomolecular systems. The force field has been parameterized in a systematic way, based on the reproduction of partitioning free energies between polar and apolar phases of a large number of chemical compounds. In this chapter the methodology underlying the force field is presented together with details of its parameterization and limitations. Then currently available topologies are described with a short overview of the key elements of their parameterization. These include the new polarizable Martini water model. A set of three selected ongoing studies using the Martini force field is presented. Finally the latest lines of development are discussed.

On physical properties of tetraether lipid membranes: effects of cyclopentane rings

This paper reviews the recent findings related to the physical properties of tetraether lipid membranes, with special attention to the effects of the number, position, and configuration of cyclopentane rings on membrane properties. We discuss the findings obtained from liposomes and monolayers, composed of naturally occurring archaeal tetraether lipids and synthetic tetraethers as well as the results from computer simulations. It appears that the number, position, and stereochemistry of cyclopentane rings in the dibiphytanyl chains of tetraether lipids have significant influence on packing tightness, lipid conformation, membrane thickness and organization, and headgroup hydration/orientation.