Effects of lipid chain length on molecular interactions between paclitaxel and phospholipid within model biomembranes

Molecular interactions between an anticancer drug, paclitaxel, and phosphatidylcholine (PC) of various chain lengths were investigated in the present work by the Langmuir film balance technique and differential scanning calorimetry (DSC). Both the lipid monolayer at the air-water interface and lipid bilayer vesicles (liposomes) were employed as model biological cell membranes. Measurement and analysis of the surface pressure versus molecular area curves of the mixed monolayers of phospholipids and paclitaxel under various molar ratio showed that phospholipids and paclitaxel formed a nonideal miscible system at the interface. Paclitaxel exerted an area-condensing effect on the lipid monolayer at small molecular surface areas and an area-expanding effect at large molecular areas, which could be explained by the intermolecular forces and geometric accommodation between the two components. Paclitaxel and phospholipids could form thermodynamically stable monolayer systems: the stability increased with the chain length in the order DMPC (C14:0)>DPPC (C16:0)>DSPC (C18:0). Investigation of paclitaxel penetration into the pure lipid monolayer showed that DMPC had a higher ability to incorporate paclitaxel and the critical surface pressure for paclitaxel penetration also increased with the chain length in the order DMPC>DPPC>DSPC. A similar trend was testified by DSC studies on vesicles of the mixed paclitaxel/phospholipids bilayer. Paclitaxel showed the greatest interaction with DMPC while little interaction could be measured in the paclitaxel/DSPC liposomes. Paclitaxel caused broadening of the main phase transition without significant change at the peak melting temperature of the phospholipid bilayers, which demonstrated that paclitaxel was localized in the outer hydrophobic cooperative zone of the bilayer. The interaction between paclitaxel and phospholipid was nonspecific and the dominant factor in this interaction was the van der Waals force or hydrophobic force. As the result of the lower net van der Waals interaction between hydrocarbon chains for the shorter acyl chains, paclitaxel interacted more readily with phospholipids of shorter chain length, which also increased the bilayer intermolecular spacing.     

Effects of cholesterol component on molecular interactions between paclitaxel and phospholipid within the lipid monolayer at the air-water interface

Cholesterol is a main component of the cell membrane and could have significant effects on drug-cell membrane interactions and thus the therapeutic efficacy of the drug. It also plays an important role in liposomal formulation of drugs for controlled and targeted delivery. In this research, Langmuir film technique, atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR) are employed for a systematic investigation on the effects of cholesterol component on the molecular interactions between a prototype antineoplastic drug (paclitaxel) and 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) within the cell membrane by using the lipid monolayer at the air-water interface as a model of the lipid bilayer membrane and the biological cell membrane. Analysis of the measured surface pressure (pi) versus molecular area (a) isotherms of the mixed DPPC/paclitaxel/cholesterol monolayers at various molar ratios shows that DPPC, paclitaxel and cholesterol can form a non-ideal miscible system at the air-water interface. Cholesterol enhances the intermolecular forces between paclitaxel and DPPC, produces an area-condensing effect and thus makes the mixed monolayer more stable. Investigation of paclitaxel penetration into the mixed DPPC/cholesterol monolayer shows that the existence of cholesterol in the DPPC monolayer can considerably restrict the drug penetration into the monolayer, which may have clinical significance for diseases of high cholesterol. FTIR and AFM investigation on the mixed monolayer deposited on solid surface confirmed the obtained results.     

Enhanced interfacial properties of novel amino acid-derived surfactants: Effects of headgroup chemistry and of alkyl chain length and unsaturation

Amino acid-derived surfactants have increasingly become a viable biofriendly alternative to petrochemically based amphiphiles as speciality surfactants. Herein, the Krafft temperatures and critical micelle concentrations (cmc) of three series of novel amino acid-derived surfactants have been determined by differential scanning microcalorimetry and surface tension measurements, respectively. The compounds comprise cationic molecules based on serine and tyrosine headgroups and anionic ones based on 4-hydroxyproline headgroups, with varying chain lengths. A linear dependence of the logarithm of cmc on chain length is found for all series, and in comparison to conventional ionic surfactants of equal chain length, the new amphiphiles present lower cmc and lower surface tension at the cmc. These observations highlight their enhanced interfacial performance. For the 18-carbon serine-derived surfactant the effects of counterion change and of the presence of a cis-double bond in the alkyl chain have also been investigated. The overall results are discussed in terms of headgroup and alkyl chain effects on micellization, in the light of available data for conventional surfactants and other types of amino acid-based amphiphiles reported in the literature.     

Force field dependence of phospholipid headgroup and acyl chain properties: Comparative molecular dynamics simulations of DMPC bilayers

The reliability of molecular simulations largely depends on the quality of the empirical force field parameters. Force fields used in lipid simulations continue to be improved to enhance the agreement with experiments for a number of different properties. In this work, we have carried out molecular dynamics simulations of neat DMPC bilayers using united‐atom Berger force field and three versions of all‐atom CHARMM force fields. Three different systems consisting of 48, 72, and 96 lipids were studied. Both particle mesh Ewald (PME) and spherical cut‐off schemes were used to evaluate the long‐range electrostatic interactions. In total, 21 simulations were carried out and analyzed to find out the dependence of lipid properties on force fields, system size, and schemes to calculate long‐range interactions. The acyl chain order parameters calculated from Berger and the recent versions of CHARMM simulations have shown generally good agreement with the experimental results. However, both sets of force fields deviate significantly from the experimentally observed P‐C dipolar coupling values for the carbon atoms that link the choline and glycerol groups with the phosphate groups. Significant differences are also observed in several headgroup parameters between CHARMM and Berger simulations. Our results demonstrate that when changes were introduced to improve CHARMM force field using PME scheme, all the headgroup parameters have not been reoptimized. The headgroup properties are likely to play a significant role in lipid–lipid, protein–lipid, and ligand–lipid interactions and hence headgroup parameters in phospholipids require refinement for both Berger and CHARMM force fields. © 2009 Wiley Periodicals, Inc.J Comput Chem, 2010     

Effect of lipid headgroup composition on the interaction between melittin and lipid bilayers

The effect of the lipid polar headgroup on melittin-phospholipid interaction was investigated by cryo-TEM, fluorescence spectroscopy, ellipsometry, circular dichroism, electrophoresis and photon correlation spectroscopy. In particular, focus was placed on the effect of the lipid polar headgroup on peptide adsorption to, and penetration into, the lipid bilayer, as well as on resulting colloidal stability effects for large unilamellar liposomes. The effect of phospholipid headgroup properties on melittin-bilayer interaction was addressed by comparing liposomes containing phosphatidylcholine, -acid, and -inositol at varying ionic strength. Increasing the bilayer negative charge leads to an increased liposome tolerance toward melittin which is due to an electrostatic arrest of melittin at the membrane interface. Balancing the electrostatic attraction between the melittin positive charges and the phospholipid negative charges through a hydration repulsion, caused by inositol, reduced this surface arrest and increased liposome susceptibility to the disruptive actions of melittin. Furthermore, melittin was demonstrated to induce liposome structural destabilization on a colloidal scale which coincided with leakage induction for both anionic and zwitterionic systems. The latter findings thus clearly show that coalescence, aggregation, and fragmentation contribute to melittin-induced liposome leakage, and that detailed molecular analyses of melittin pore formation are incomplete without considering also these colloidal aspects.     

Temperature dependence of the organization and molecular interactions within phospholipid/diacetylene Langmuir films

Surface pressure-area isotherms and Brewster angle microscopy images of mixed binary films of dimyristoylphosphatidylcholine (DMPC) and the diacetylene 10,12-tricosadiynoic acid (TRCDA) were recorded at different temperatures and mole ratios to investigate the molecular interactions and cooperative properties of the films. The experiments revealed that segregation, on the one hand, and significant intermolecular interactions, on the other hand, both contribute to the thermodynamic properties of the phospholipids and the diacetylene assemblies. In particular, the data demonstrate that higher temperatures and greater percentage of DMPC promote repulsion between the liquid-condensed phospholipid monolayer and the TRCDA domains. In contrast, at high TRCDA mole ratios, film contraction occurred (lower molecular areas) due to TRCDA multilayer formation (at high temperature) or intermolecular affinities (at low temperature).     

Cholesterol–phospholipid interactions, the liquid-ordered phase and lipid rafts in model and biological membranes

The existence of relatively large and long-lived detergent-insoluble, sphingolipid- and cholesterol-enriched, liquid-ordered lipid raft domains in the plasma membranes of eukaryotic cells has become widely accepted. However, we believe that the evidence for their existence is not compelling despite extensive work on both lipid bilayer model and biological membranes. We review here the results of recent studies, which in our view call into question the existence of lipid rafts in membranes, at least in the form commonly depicted.     

Interfacial interactions between poly[L-lysine]-based branched polypeptides and phospholipid model membranes

The interaction of five poly[L-lysine]-derived branched chain polypeptides of poly[Lys(X(i))] (X(i)K) or poly[Lys(X(i)-DL-Ala(m))] (XAK) with lipid bilayers (DPPC and DPPC/PG, 8:2) was studied by fluorescence polarization techniques. Two fluorescent probes, DPH and TMA-DPH, were utilized to monitor changes of motion in the internal and/or in the polar head regions, respectively. Results indicate that the interaction of polypeptides with neutral (DPPC) bilayers is mainly dependent on the polarity and electrical charge of side chains. The amphoteric E(i)K shows the highest level of interaction. Polycationic polypeptides (H(i)K, P(i)K, TAK) have a relatively small effect on the transition temperature of the lipids, while the polyanionic Succ-EAK has no effect at the alkyl chain region of the bilayer. Data with TMA-DPH indicate the lack of pronounced interaction between the polypeptides and the outer surface of the liposome. Similar tendency was documented for DPPC/PG vesicles. Polypeptides, H(i)K, and P(i)K induce significant changes in the transition temperature, thus indicating their insertion into the hydrophobic core of the bilayer without marked effect on the polar head region. Results suggest that these polypeptides (except E(i)K) have no destabilizing effect on liposomes studied. These properties are considered as beneficial for their use as safe carriers for bioactive molecules.     

Using liquid crystals to report molecular interactions between cationic antimicrobial peptides and lipid membranes

In this study, we develop a novel method for label-free and real-time imaging of molecular interactions between cationic antimicrobial peptides and lipid membranes using liquid crystals (LCs). The optical appearance of LCs changes from bright to dark after transferring phospholipids onto the aqueous/LC interface, representing an orientational transition of LCs from planar to homeotropic state. Bright domains are observed when an antimicrobial agent is in contact with the LC interface decorated with a lipid monolayer of negatively charged phospholipids, while, there is no change in the optical response when the antimicrobial agent is in contact with the LC interface decorated with neutral phospholipids. This approach holds great promise for studying membrane disruption or permeabilization caused by antimicrobial agents.     

Differential scanning calorimetry and Fourier transform infrared spectroscopic studies of phospholipid organization and lipid-peptide interactions in nanoporous substrate-supported lipid model membranes

High-sensitivity differential scanning calorimetry was utilized to examine whether lipids capable of forming an inverted nonlamellar hexagonal II (HII) phase can be deposited into nanoporous substrate-supported arrays. Particularly, we compare the thermotropic phase properties of nanoconfined unsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine lipid bilayers with unsupported dispersions to assess nanoconfinement effects, focusing on the lamellar fluid (Lalpha) to HII phase transition. Experimental results provide direct and clear evidence for the formation of an HII phase upon both heating and cooling. However, a small shift in the Lalpha/HII phase transition temperature, as well as an increase in the magnitude of the associated temperature hysteresis, was observed in the nanoporous substrate-supported system. Additionally, nanoconfinement effects on the interaction and location of the antimicrobial peptide gramicidin S (GS) with nanoporous substrate-supported cardiolipin bilayers were examined by Fourier transform infrared spectroscopy as a function of temperature and phospholipid phase state. Upon heating, GS molecules began to insert into nanoconfined, substrate-supported cardiolipin bilayers at lower temperatures relative to the gel/liquid-crystalline phase transition temperature than into unsupported bilayers. The reduction in the polarity and hydrogen-bonding potential environment of GS in the Lalpha state suggests that GS is located at the polar/apolar interfacial region in both supported and unsupported cardiolipin bilayers and that the capacity of GS to interact with nanoporous substrate-supported cardiolipin bilayers was not significantly hindered by nanoconfinement. These studies further demonstrate the usefulness of supported lipid bilayers inside nanoporous substrates.     

Understanding the Hofmeister effect in interactions between chaotropic anions and lipid bilayers: molecular dynamics simulations

A set of all-atom molecular dynamics simulations have been performed to better understand critical phenomena regarding a Hofmeister series of anions and lipid bilayers. The simulations isolate the effect of anion size and show clear differences in the interactions with the dipolar phoshpatidylcholine headgroup. Cl- anions penetrate into the headgroup region of the bilayer, but the simulations confirm theories which predict that larger anions penetrate more deeply, into a more heterogeneous and hydrophobic molecular region. That anion size leads to such differences in partitioning in the bilayer provides atomic-level support to hypotheses inspired by several experimental studies. The ability of larger anions to bury deep within the bilayer is correlated with a less well-structured hydration shell, shedding of which upon penetration incurs a smaller penalty for the larger anions than for Cl-.     

Effects of unsaturation on film structure and friction of fatty acids in a model base oil

The normal and friction forces between layers of three fatty acids (stearic, oleic, and linoleic acid) and a rosin acid (dehydroabietic acid) have been measured in n-hexadecane with a surface forces apparatus. Stearic, oleic, and dehydroabietic acid form loose-packed monolayers on mica surfaces when adsorbed from dry n-hexadecane. Linoleic acid forms an additional dimer layer between monolayer-covered surfaces, where it is stabilized by interactions between the double-bond-rich regions of the molecules. The monolayers formed by linoleic and dehydroabietic acid are thinner than the ones formed by stearic and oleic acid, but are not as easily removed from between the mica surfaces when the load or pressure is increased. The friction force increased linearly with load in all systems, and the friction coefficient increased with increasing unsaturation. Linoleic acid showed two regimes of linear friction with increasing load, corresponding to two different film thicknesses. Its friction was sensitive to sliding speed and adsorption time, and the thinner film observed at higher load had a lower friction coefficient. Such features were not observed for stearic and oleic acid, where the monolayers were removed and the friction coefficient changed to that of pure n-hexadecane at a pressure of 3.5 MPa.     

A monolayer study on interactions of docetaxel with model lipid membranes

Docetaxel (DCT) is an antineoplastic drug for the treatment of a wide spectrum of cancers. DCT surface properties as well as miscibility studies with l-alpha-dipalmitoyl phosphatidylcholine (DPPC), which constitutes the main component of biological membranes, are comprehensively described in this contribution. Penetration studies have revealed that when DCT is injected under DPPC monolayers compressed to different surface pressures, it penetrates into the lipid monolayer promoting an increase in the surface pressure. DCT is a surface active molecule able to decrease the surface tension of water and to form insoluble films when spread on aqueous subphases. The maximum surface pressure reached after compression of a DCT Langmuir film was 13 mN/m. Miscibility of DPPC and DCT in Langmuir films has been studied by means of thermodynamic properties as well as by Brewster angle microscopy (BAM) analysis of the mixed films at the air-water interface, concluding that DPPC and DCT are miscible and they form non-ideally mixed monolayers at the air-water interface. Helmholtz energies of mixing revealed that no phase separation occurs. In addition, Helmholtz energies of mixing become more negative with decreasing areas per molecule, which suggests that the stability of the mixed monolayers increases as the monolayers become more condensed. Compressibility values together with BAM images indicate that DCT has a fluidizing effect on DPPC monolayers.     

Effects of lipid chain length on the surface properties of alkylaminomethyl rutin and of its mixture with model lecithin membrane

Three model flavonoid-based bioactive molecules with different lipid chain lengths (RuCn: n=8, 12, 18) were newly synthesized. The surface properties [surface pressure (π)-area (A), surface potential (ΔV)-surface pressure (π) and dipole moment (u(⊥))-surface pressure (π)] of pure RuCn and the lecithin membrane compounds had been investigated by using the Langmuir monolayer technology. The results suggested that the distinctive monolayer behavior of RuCn is strongly dependent on the lipid chain length. The great differences in the monolayer properties brought by the lipid chain length could be attributed to two major factors: (i) the ionization degree of the bulky hydrophilic head group (including hydroxyl and NH groups) alters its local field solely via the surface potential; (ii) tring molecular (or dipole) packing density within monolayers. The excess Gibbs energy (ΔG((ex))) calculated for the RuCn-lecithin mixed monolayers infers that higher stability of the mixed monolayer can be strengthened as the lipid chain length decreases. And the addition of RuCn into lecithin membrane may increase the total u(⊥) of the binary mixed monolayers, which could inhibit the hydration of the lecithin's hydrophilic head groups. The shorter the lipid chain length of RuCn (e.g., RuC8) is, the higher the surface activity can be. Our findings provide a molecular basis for the application of such class of biomolecules in the functional food, cosmetics and medicine.     

高效液相色谱法测定紫杉醇及其脂质纳米粒包封率的方法学研究

建立了一种对本实验室研制的新型载带紫杉醇脂质纳米载体(nanostructured lipid carriers,NLC)进行定性定量分析的新方法,并计算紫杉醇的包封率.用反相高效液相色谱法进行测定.采用C18柱对未纯化的紫杉醇脂质纳米粒进行分离检测,梯度法分离,流动相为水和乙腈,紫外检测器波长为227 nm,荧光检测器激发波长为428 nm,发射波长为515 nm.结果表明梯度法对紫杉醇脂质纳米粒和游离紫杉醇的分离效果良好,辅料和试剂无干扰,紫杉醇在3.0～80.0 mg/L之间有良好的线性关系,线性方程为:Y=42100ρ 8250,r=0.9999,日内RSD为0.15%(n=5),日间RSD为0.25%(n=4),平均回收率为109.09%,RSD为0.12%,通过本方法,测定的紫杉醇纳米粒平均包封率为68.78%.本法可对紫杉醇进行定量分析,也可对紫杉醇脂质纳米粒制剂进行定性和定量分析,可以直接测量计算其包封率.     

Dynamics imaging of lipid phases and lipid-marker interactions in model biomembranes

Biomembranes are complex systems that regulate numerous biological processes. Lipid phases that constitute these membranes influence their properties and transport characteristics. Here, we demonstrate the potential of short-range dynamics imaging (excited-state lifetime, rotational diffusion, and order parameter) as a sensitive probe of lipid phases in giant unilamellar vesicles (GUVs). Liquid-disordered and gel phases were labeled with Bodipy-PC at room temperature. Two-photon fluorescence lifetime imaging microscopy of single-phase GUVs reveals more heterogeneity in fluorescence lifetimes of Bodipy in the gel phase (DPPC: 3.8+/-0.6 ns) as compared with the fluid phase (DOPC: 5.2+/-0.2 ns). The phase-specificity of excited-state lifetime of Bodipy-PC is attributed to the stacking of ordered lipid molecules that possibly enhances homo-FRET. Fluorescence polarization anisotropy imaging also reveals distinctive molecular order that is phase specific. The results are compared with DiI-C12-labeled fluid GUVs to investigate the sensitivity of our fluorescence dynamics assay to different lipid-marker interactions. Our results provide a molecular perspective of lipid phase dynamics and the nature of their microenvironments that will ultimately help our understanding of the structure-function relationship of biomembranes in vivo. Furthermore, these ultrafast excited-state dynamics will be used for molecular dynamics simulation of lipid-lipid, lipid-marker and lipid-protein interactions.     

Effect of polymer size and chain length on depletion interactions between two colloids

A density functional theory based on the weighted density has been developed to investigate the depletion interactions between two colloids immersed in a bath of the binary polymer mixtures, where the colloids are modeled as hard spheres and the polymers as freely jointed tangent hard-sphere chain mixtures. The theoretical calculations for the depletion forces between two colloids induced by the polymer are in good agreement with the computer simulations. The effects of polymer packing fraction, degree of polymerization, polymer/polymer size ratio, colloid/polymer size ratio on the depletion interactions, and colloid-colloid second virial coefficient B2 due to polymer-mediated interactions have been studied. With increasing the polymer packing fraction, the depletion interaction becomes more long ranged and the attractive interaction near the colloid becomes deeper. The effect of degree polymerization shows that the long chain gives a more stable dispersion for colloids rather than the short chain. The strong effective colloid-colloid attraction appears for the large colloid/polymer and polymer/polymer size ratio. The location of maximum repulsion Rmax is found to appear Rmax approximately sigmac+Rg2 for the low polymer packing fraction and this is shifted to smaller separation Rmax approximately sigmac+sigmap2 with increasing the polymer packing fraction, where sigmap2 and Rg2 are the small-particle diameter and the radius of gyration of the polymer with the small-particle diameter, respectively.     

Synthesis, characterization and crystal structure of mesogenic compounds with an alkyloxy and a semi-perfluorinated chain. Influence of the alkyloxy chain length on molecular arrangement and molecular interactions

This paper describes the synthesis and study of the mesomorphic properties (by DSC, microscopic observation and X-ray diffraction) of three new series of semi-perfluorinated liquid crystals: 4-(2,2,3,3,4,4,4-heptafluorobutyloxycarbonyl)phenyl, 4-(2,2,3,3,4,4,5,5-octafluoropentyloxycarbonyl)phenyl, and 4-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-octyloxy-carbonyl)phenyl 4-(n-alkyloxy)benzoates. One compound of the second family crystallizes in the P1 space group with Z ' 2. The molecule adopts a slightly bent conformation with a fully extended alkyloxy and semi-perfluorinated chain in a smectic C-like structure. The comparison of the present structure is made with that of a similar compound of the first family with a longer alkyloxy chain (this latter crystallizes in the P1 space group with two independent molecules in the unit cell). Comparison of the molecular packing shows the importance of the chain lengths in the molecular arrangement and molecular interactions in both structures.     

A breathing wormlike chain model on DNA denaturation and bubble: effects of stacking interactions

DNA stably exists as a double-stranded structure due to hydrogen-bonding and stacking interactions between bases. The stacking interactions are strengthened when DNA is paired, which results in great enhancement of bending rigidity. We study the effects of this stacking-induced stiffness difference on DNA denaturation and bubble formations. To this end, we model double-stranded DNA as a duplex of two semiflexible chains whose persistence length varies depending on the base-pair distance. Using this model, we perform the Langevin dynamics simulation to examine the characteristics of the denaturation transition and the statistics of the bubbles. We find that the inclusion of the stacking interactions causes the denaturation transition to be much sharper than otherwise. At physiological temperature, the stacking interactions prohibit the initiation of bubble formation but promote bubbles, once grown, to retain the large size.