Chain elongation of diacylphosphatidylcholine induces fully bilayer interdigitation under atmospheric pressure

Supramolecular Optimization of Sensory Function of a Hemicurcuminoid through Its Incorporation into Phospholipid and Polymeric Polydiacetylenic Vesicles: Experimental and Computational Insight

This work presents the synthesis of a new representative of hemicurcuminoids with a nonyloxy substituent (HCur) as a fluorescent amphiphilic structural element of vesicular aggregates based on phosphatidylcholine (PC), phosphatidylserine (PS), and 10,12-pentacosadiynoic acid (PCDA). Both X-ray diffraction analysis of the single crystal and ¹H NMR spectra of HCur in organic solvents indicate the predominance of the enol-tautomer of HCur. DFT calculations show the predominance of the enol tautomer HCur in supramolecular assemblies with PC, PS, and PCDA molecules. The results of the molecular modeling show that HCur molecules are surrounded by PC and PS with a rather weak exposure to water molecules, while an exposure of HCur molecules to water is enhanced under its supramolecular assembly with PCDA molecules. This is in good agreement with the higher loading of HCur into PC(PS) vesicles compared to PCDA vesicles converted into polydiacetylene (PDA) ones by photopolymerization. HCur molecules incorporated into HCur-PDA vesicles exhibit greater planarity distortion and hydration effect in comparison with HCur-PC(PS) ones. HCur-PDA is presented as a dual fluorescence-chromatic nanosensor responsive to a change in pH within 7.5-9.5, heavy metal ions and polylysine, and the concentration-dependent fluorescent response is more sensitive than the chromatic one. Thus, the fluorescent response of HCur-PDA allows for the distinguishing between Cd²⁺ and Pb²⁺ ions in the concentration range 0-0.01 mM, while the chromatic response allows for the selective sensing of Pb²⁺ over Cd²⁺ ions at their concentrations above 0.03 mM.

Phase Imaging of Phosphatidylcholine Bilayer Membranes by Prodan Fluorescence

Prodan (6-propiponyl-2-(N,N-dimethylamino)naphthalene) is well known as a polarity-sensitive fluorescent probe and has a high capability of detecting structural changes occurring within phospholipid bilayer membranes. In this study, we carried out the fluorescence spectroscopic observation of bilayer phase behavior for a series of symmetric saturated diacylphosphatidylcholines (CnPCs) with different acyl-chain length n (n = 12-15 and 19-22) using Prodan as a membrane probe to confirm the availability of Prodan along with the previous results for the CnPC bilayer membranes (n = 16-18). The results were discussed by constructing spectral three-dimensional (3D) imaging plots for visualizing the change in bilayer phase states with temperature or pressure to verify the functionality of this 3D imaging plot. It was found that the Prodan fluorescence technique is applicable to the detection of the changes in the bilayer phase states of all CnPCs with a few exceptions. One of the most crucial exceptions was that Prodan cannot be used for the detection of the bilayer-gel state of the C21PC bilayer membrane. It was also found that it is only to the CnPC bilayer membranes with n = 15-18 that the 3D imaging plot is adequately and accurately applicable as a useful graphical tool for visually detecting the bilayer phase states. This is a disadvantageous feature of this technique brought about by the high sensitivity of Prodan as a membrane probe. Further detailed studies on the molecular behavior of Prodan will enable us to find a more useful way of utilizing this membrane probe.

Temperature-sensitive emission of dialkylaminostyrylhetarene dyes and their incorporation into phospholipid aggregates: Applicability for thermal sensing and cellular uptake behavior

A series of dialkylaminostyrylhetarene dyes constructed from electron-rich and electron-deficient moieties of various structures connected via vinylene π-bridges are introduced as temperature-sensitive luminophores. The temperature dependent emission of the dyes in the acidified dichloromethane solutions derives from temperature-induced shift of the equilibrium between neutral and protonated forms of the dyes. The heating-induced blue shift and intensification of emission of neutral form of the dyes make them a promising basis for development of nanoparticles exhibiting temperature-sensitivity in aqueous solutions at pH typical of biological liquids. Hydrophobicity-driven incorporation of the water insoluble dyes into L-α-phosphatidylcholine(PC)-based bilayers allows to obtain water dispersible dye-PC aggregates, and to follow their emission in the aqueous solutions. Structure of the dyes has strong impact on the efficacy of the dyes incorporation into the PC-based bilayers, temperature sensitivity of emission of the dye-PC aggregates and its reversibility under the heating/cooling cycles. This enables structural optimization of the dyes in order to obtain the dye-PC species demonstrating maximal temperature dependence and reversibility of their luminescence in aqueous solutions. The selected leader exhibits low cytotoxicity exemplified for M-HeLa and Chang Liver cell lines, while the efficient cell internalization of the dye, manifested in the staining of the cell cytoplasm, opens further opportunities for biosensing applications.

Design of Lipid-Based Nanocarriers via Cation Modulation of Ethanol-Interdigitated Lipid Membranes

Short-chain alcohols (i.e., ethanol) can induce membrane interdigitation in saturated-chain phosphatidylcholines (PCs). In this process, alcohol molecules intercalate between phosphate heads, increasing lateral separation and favoring hydrophobic interactions between opposing acyl chains, which interpenetrate forming an interdigitated phase. Unraveling mechanisms underlying the interactions between ethanol and model lipid membranes has implications for cell biology, biochemistry, and for the formulation of lipid-based nanocarriers. However, investigations of ethanol-lipid membrane systems have been carried out in deionized water, which limits their applicability. Here, using a combination of small- and wide-angle X-ray scattering, small-angle neutron scattering, and all-atom molecular dynamics simulations, we analyzed the effect of varying CaCl₂ and NaCl concentrations on ethanol-induced interdigitation. We observed that while ethanol addition leads to the interdigitation of bulk phase 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers in the presence of CaCl₂ and NaCl regardless of the salt concentration, the ethanol-induced interdigitation of vesicular DPPC depends on the choice of cation and its concentration. These findings unravel a key role for cations in the ethanol-induced interdigitation of lipid membranes in either bulk phase or vesicular form.

Biophysical Mechanisms of Membrane-Thickness-Dependent MscL Gating: An All-Atom Molecular Dynamics Study

The bacterial mechanosensitive channel, MscL, is activated by membrane tension, acting as a safety valve to prevent cell lysis against hypotonic challenge. It has been established that its activation threshold decreases with membrane thickness, while the underlying mechanism remains to be solved. We performed all-atom molecular dynamics (MD) simulations for the initial opening process of MscL embedded in four different types of lipid bilayers with different thicknesses: 1,2-dilauroyl- sn-glycero-3-phosphocholine (DLPC)), 1,2-dimyristoyl-glycero-3-phosphorylcholine (DMPC), 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC), and 1,2-distearoyl- sn-glycero-3-phosphocholine (DSPC). In response to membrane stretching, channel opening occurred only in the thinner membranes (DLPC and DMPC) in a thickness-dependent way. We found that the MscL opening was governed by the rate and degree of membrane thinning and that the channel opening was tightly associated with the tilting of transmembrane (TM) helices of MscL toward the membrane plane. Upon membrane stretching, the order parameter of acyl chains of thinner membranes (DLPC and DMPC) became smaller, whereas other thicker membranes (DPPC and DSPC) showed interdigitation with little changes in the order parameter. The decreased order parameter contributed much more to membrane thinning than did interdigitation. We conclude that the membrane-thickness-dependent MscL opening mainly arises from structural changes in MscL to match the altered membrane thickness by stretching.

Effect of pressure on bilayer phase behavior of N-methylated di-O-hexadecylphosphatidylethanolamines: relevance of head-group modification on the bilayer interdigitation

The phase transitions of N-methylated di-O-hexadecylphosphatidylethanolamines (DHPE, DH-N-methyl-PE (DHMePE) and DH-N,N-dimethyl-PE (DHMe₂PE)) were observed by differential scanning calorimetry (DSC) and fluorometry under atmospheric pressure and by light-transmittance measurements under high pressure. The DSC thermograms showed that the N-methylated DHPE bilayers underwent the phase transition from the gel phase to the liquid crystalline (L_α) phase under atmospheric pressure. The gel phase was identified by fluorometry as the lamellar gel (L_β) phase, and not interdigitated gel (L_βI) phase. The gel/L_α transition temperature increased with pressure while decreased stepwise with increasing polar head-group size. This stepwise depression of the transition temperature may be caused by the inverse-proportional hydrogen-bonding capabilities of the head-group to the head-group size. The thermodynamic quantities of the gel/L_α transition were comparable for the N-methylated DHPE bilayers. The pressure-induced L_βI phase was not found in these bilayers although the bilayer of di-O-hexadecylphosphatidylcholine (DHPC), which is a kind of N-methylated DHPEs, forms the L_βI phase only by hydration under atmospheric pressure. Taking into account that the bilayers of diacyl-homologs of N-methylated DHPEs, N-methylated dipalmitoyl-PEs except for dipalmitoylphosphatidylcholine (DPPC), do not form the L_βI phase in the whole pressure range investigated but the DPPC bilayer forms the L_βI phase under high pressure, we can say that the interdigitation requires weaker interaction between large-sized head groups like the bulky choline group.

How Do Membranes Respond to Pressure?

Bilayers formed by phospholipids are fundamental structures of biological membranes. The mechanical perturbation brought about by pressure significantly affects the membrane states of phospholipid bilayers. In this chapter, we focus our attention on the pressure responsivity for bilayers of some major phospholipids contained in biological membranes. At first, the membrane states and phase transitions of phospholipid bilayers depending on water content, temperature and pressure are explained by using the bilayer phase diagrams of dipalmitoylphosphatidylcholine (DPPC), which is the most familiar phospholipid in model membrane studies. Subsequently, the thermotropic and barotropic bilayer phase behavior of various kinds of phospholipids with different molecular structures is discussed from the comparison of their temperature--pressure phase diagrams to that of the DPPC bilayer. It turns out that a slight change in the molecular structure of the phospholipids produces a significant difference in the bilayer phase behavior. The systematic pressure studies on the phase behavior of the phospholipid bilayers reveal not only the pressure responsivity for the bilayers but also the role and meaning of several important phospholipids existing in real biological membranes.

Thermotropic and barotropic phase behavior of phosphatidylcholine bilayers

Bilayers formed by phospholipids are frequently used as model biological membranes in various life science studies. A characteristic feature of phospholipid bilayers is to undergo a structural change called a phase transition in response to environmental changes of their surroundings. In this review, we focus our attention on phase transitions of some major phospholipids contained in biological membranes, phosphatidylcholines (PCs), depending on temperature and pressure. Bilayers of dipalmitoylphosphatidylcholine (DPPC), which is the most representative lipid in model membrane studies, will first be explained. Then, the bilayer phase behavior of various kinds of PCs with different molecular structures is revealed from the temperature-pressure phase diagrams, and the difference in phase stability among these PC bilayers is discussed in connection with the molecular structure of the PC molecules. Furthermore, the solvent effect on the phase behavior is also described briefly.

Study on the subgel-phase formation using an asymmetric phospholipid bilayer membrane by high-pressure fluorometry

The myristoylpalmitoylphosphatidylcholine (MPPC) bilayer membrane shows a complicated temperature-pressure phase diagram. The large portion of the lamellar gel (L(β)'), ripple gel (P(β)'), and pressure-induced gel (L(β)I) phases exist as metastable phases due to the extremely stable subgel (L(c)) phase. The stable L(c) phase enables us to examine the properties of the L(c) phase. The phases of the MPPC bilayers under atmospheric and high pressures were studied by small-angle neutron scattering (SANS) and fluorescence spectroscopy using a polarity-sensitive fluorescent probe Prodan. The SANS measurements clearly demonstrated the existence of the metastable L(β)I phase with the smallest lamellar repeat distance. From a second-derivative analysis of the fluorescence data, the line shape for the L(c) phase under high pressure was characterized by a broad peak with a minimum of ca. 460 nm. The line shapes and the minimum intensity wavelength (λ″(min)) values changed with pressure, indicating that the L(c) phase has highly pressure-sensible structure. The λ″(min) values of the L(c) phase spectra were split into ca. 430 and 500 nm in the L(β)I phase region, which corresponds to the formation of a interdigitated subgel L(c) (L(c)I) phase. Moreover, the phase transitions related to the L(c) phase were reversible transitions under high pressure. Taking into account the fluorescence behavior of Prodan for the L(c) phase, we concluded that the structure of the L(c) phase is highly probably a staggered structure, which can transform into the L(c)I phase easily.