Thermotropic Phase Behavior Research Articles

Structure and Thermotropic Phase Behavior of a Homologous Series of Bioactive N-Acyldopamines

N-Acyldopamines (NADAs), which are present in mammalian nervous tissues, exhibit interesting biological and pharmacological properties. In the present study, a homologous series of NADAs with varying acyl chains (n = 12-20) have been synthesized and characterized. Differential scanning calorimetric studies show that in the dry state the transition temperatures, enthalpies, and entropies of NADAs exhibit odd-even alternation with the values corresponding to the even chain length series being slightly higher. Both even and odd chain length NADAs display a linear dependence of the transition enthalpies and entropies on the chain length. However, odd-even alternation was not observed in the calorimetric properties upon hydration, although the transition enthalpies and entropies exhibit linear dependence. Linear least-squares analyses yielded incremental values contributed by each methylene group to the transition enthalpy and entropy and the corresponding end contributions. N-Lauroyldopamine (NLDA) crystallized in the monoclinic space group C2/c with eight symmetry-related molecules in the unit cell. Single-crystal X-ray diffraction studies show that NLDA molecules are organized in the bilayer form, with a head-to-head (and tail-to-tail) arrangement of the molecules. Water-mediated hydrogen bonds between the hydroxyl groups of the dopamine moieties of opposing layers and N-H···O hydrogen bonds between the amide groups of adjacent molecules in the same layer stabilize the crystal packing. These results provide a thermodynamic and structural basis for investigating the interaction of NADAs with other membrane lipids, which are expected to provide clues to understand how they function in vivo, e.g., as signaling molecules in the modulation of pain.

How does acyl chain length affect thermotropic phase behavior of saturated diacylphosphatidylcholine–cholesterol binary bilayers?

Thermotropic phase behavior of diacylphosphatidylcholine (CnPC)–cholesterol binary bilayers (n=14–16) was examined by fluorescence spectroscopy using 6-propionyl-2-(dimethylamino)naphthalene (Prodan) and differential scanning calorimetry. The former technique can detect structural changes of the bilayer in response to the changes in polarity around Prodan molecules partitioned in a relatively hydrophilic region of the bilayer, while the latter is sensitive to the conformational changes of the acyl chains. On the basis of the data from both techniques, we propose possible temperature T–cholesterol composition Xch phase diagrams for these binary bilayers. A notable feature of our phase diagrams, including our previous results for diheptadecanoylphosphatidylcholine (C17PC) and distearoylphosphatidylcholine (C18PC), is that there is a peritectic-like point around Xch=0.15, which can be interpreted as indicating the formation of a 1:6-complex of cholesterol and CnPCs within the binary bilayer irrespective of the acyl chain length. We could give a reasonable explanation for such complex formation using the modified superlattice view. Our results also showed that the Xch value of the abolition of the main transition is almost constant for n=14–17 (ca. 0.33), while it increases to ca. 0.50 for n=18. By contrast, a biphasic n-dependence of Xch was observed for the abolition of the pretransition, suggesting that there are at least two antagonistic n-dependent factors. We speculate that this could be explained by the enhancement of the van der Waals interaction with increases in n and the weakening of the repulsion between the neighboring headgroups with decreases in n.

Chain length mismatch and packing effects on the thermotropic phase behavior of salt-free catanionic surfactants

Thermotropic liquid crystal formation by salt-free catanionic surfactants (alkyltrimethylammonium alkylsulfonates, herein designated as TAmSon) has been investigated as a function of chain length mismatch (asymmetry). Previous studies on these compounds have revealed an unusual and rich asymmetry-dependent lyotropic phase behavior. Herein, phase transition temperatures and transition enthalpies/entropies were determined by differential scanning calorimetry, while mesophases were assigned by polarized light microscopy. Three series of compounds were investigated, namely: the TA16Son series, where n=6–10; the TAmSo8 series, where m=12–16; and a constant m+n series, TAmSon where m+n=22. Typically, several solid phases and two smectic mesophases are found prior to isotropization to the liquid phase. As asymmetry decreases, two somewhat counterintuitive tendencies emerge: a general decrease in enthalpy/entropy for solid–solid and solid–first mesophase transitions, and an increase in solid–first mesophase transition temperatures. Yet, solid phases are seen to be more stable for the most asymmetric compounds, while the second mesophase is more stable for the least asymmetric ones, in what appears to be a more complex behavior than expected. The results are globally interpreted in terms of subtle differences in chain interdigitation and packing, and odd–even chain effects.

Structure and Conformational Dynamics of DMPC/Dicationic Surfactant and DMPC/Dicationic Surfactant/DNA Systems

Amphiphilic dicationic surfactants, known as gemini surfactants, are currently studied for gene delivery purposes. The gemini surfactant molecule is composed of two hydrophilic “head” groups attached to hydrophobic chains and connected via molecular linker between them. The influence of different concentrations of 1,5-bis (1-imidazolilo-3- decyloxymethyl) pentane chloride (gemini surfactant) on the thermotropic phase behaviour of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers with and without the presence of DNA was investigated using Fourier transformed infrared (FTIR) and circular dichroism (CD) spectroscopies, small angle scattering of synchrotron radiation and differential scanning calorimetry. With increasing concentration of surfactant in DMPC/DNA systems, a disappearance of pretransition and a decrease in the main phase transition enthalpy and temperature were observed. The increasing intensity of diffraction peaks as a function of surfactant concentration also clearly shows the ability of the surfactant to promote the organisation of lipid bilayers in the multilayer lamellar phase.

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.

Three-arm star compounds composed of 1,3,5-tri(azobenzeneethynyl)benzene cores and flexible PEO arms: synthesis, optical functions, hybrid Ormosil gel glasses

A three-arm star rod-coil block conjugated compound composed of 1,3,5-tri(azobenzeneethynyl)benzene core and flexible poly(ethylene oxide) arms was synthesized. For comparison, two others with the same core but different end groups were also prepared. The compounds show thermotropic discotic liquid crystalline phase behavior and properties of optical limiting and photo-induced trans–cis isomerization. The star-shaped compound with poly(ethylene oxide) arms can be homogeneously dispersed into Ormosil gel glass monoliths up to 12 wt% without any compatibilizer. The glasses are mechanically stable and highly transparent in the visible and near IR region. The nonlinear optical and optical limiting properties of the gel glasses are comparable or superior to those of the corresponding solutions. The photochromic property of the azobenzene units can be maintained in the gel glass state.

Impact of oxidized phospholipids on the structural and dynamic organization of phospholipid membranes: a combined DSC and solid state NMR study

Membranes undergo severe changes under oxidative stress conditions due to the creation of oxidized phospholipid (OxPL) species, which possess molecular properties quite different from their parental lipid components. These OxPLs play crucial roles in various pathological disorders and their occurrence is involved in the onset of intrinsic apoptosis, a fundamental pathway in programmed mammalian cell death. However, the molecular mechanisms by which these lipids can exert their apoptotic action via their host membranes (e.g., altering membrane protein function) are poorly understood. Therefore, we studied the impact of OxPLs on the organization and biophysical properties of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) based lipid membranes by differential scanning calorimetry (DSC) and solid state nuclear magnetic resonance (NMR) spectroscopy. Incorporation of defined OxPLs with either a carboxyl group (1-palmitoyl-2-azelaoyl-sn-glycero-3-phosphocholine (PazePC)) or aldehyde (1-palmitoyl-(9'oxononanoyl)-sn-glycero-3-phosphocholine (PoxnoPC)) at their truncated sn-2-chain ends enabled us to reveal OxPL species-dependent differences. The calorimetric studies revealed significant effects of OxPLs on the thermotropic phase behavior of DMPC bilayers, especially at elevated levels where PazePC induced more pronounced effects than PoxnoPC. Temperature-dependent changes in the solid state 31P NMR spectra, which provided information of the lipid headgroup region in these mixed membrane systems, reflected this complex phase behavior. In the temperature region between 293 K (onset of the Lalpha-phase) and 298 K, two overlapping NMR spectra were visible which reflect the co-existence of two liquid-crystalline lamellar phases with presumably one reflecting OxPL-poor domains and the other OxPL-rich domains. Deconvolution of the DSC profiles also revealed these two partially overlapping thermal events. In addition, a third thermal, non-NMR-visible, event occurred at low temperatures, which can most likely be associated to a solid-phase mixing/demixing process of the OxPL-containing membranes. The observed phase transitions were moved to higher temperatures in the presence of heavy water due its condensing effect, where additional wideline 2H-NMR studies revealed a complex hydration pattern in the presence of OxPLs.

Impact of Oxidized Phospholipids on Membrane Organization

Lipids have often been seen as basic structural membrane subunits with proteins doing the actual work. This view has changed in recent years where it has been shown that lipids are also directly involved in numerous physiological processes and are often required for specific membrane protein functions. However, how a membrane and its function becomes modified under intracellular oxidative conditions, which e.g. trigger mitochondria-mediated apoptotic cell death, is still not really known. Oxidative stress can generate oxidized phospholipids (OxPls), which have a great impact on mitochondrial membrane integrity. Therefore, we studied the impact of OxPls on DMPC based bilayers membranes by differential scanning calorimetry (DSC) and solid state nuclear magnetic resonance (NMR) spectroscopy. Incorporation of OxPls with functional groups (carboxyl or aldehyde) at their truncated sn-2-chain ends generated information about the effect which OxPl species exert on the basic structural and physico-chemical properties of DMPC bilayers. DSC experiments revealed significant changes in the thermotropic phase behavior of these vesicles in the presence of OxPls as a function of their concentration. In addition, solid state 31P NMR provided molecular information of the behavior of the DMPC headgroups when OxPls were present. In addition changes could also be monitored during temperature induced phase transitions, where OxPls induced a very complex phase behavior. Between 293 K (onset of Lα-phase) and 298 K two overlapping NMR subspectra occured which indicated the co-existence of two liquid-crystalline lamellar phases. Most likely one phase reflected an OxPls poor domain and the other an OxPl-rich domain. In summary, the presence of OxPls seems to alter the mitochondrial membrane organization, which has serious implication for the role of this membrane and its Bcl-2 proteins involved in mitochondrial apoptosis.

Nanostructures and Thermal Properties of the Binary Mixture of DNA and a Zwitterionic Phospholipid in the Bulk

The mixtures of polyanionic DNA and zwitterionic phospholipids (ZL) have recently received much attention because of their potential for use as vectors in gene therapy or as a template for nanostructure construction. The past few studies on DNA-ZL system were carried out in their aqueous solutions and it was reported that a small fraction of DNA was capable of binding to zwitterionic lipids. However, it is still not known whether, as in the aqueous state, the DNA will be still intercalated between lipid bilayers in the bulk state. In the present study, we examined the DNA-ZL interactions in the bulk state by investigating a binary system composing of DNA and a zwitterionic lipid, 1,2-di(cis-9-octadecenoyl)-sn-glycero-3-phosphocholine (DOPC). The nanostructures and thermotropic phase behavior of this system were investigated using small angle X-ray scattering (SAXS) and differential scanning calorimeter (DSC), respectively. Our SAXS with one-dimensional correlation function results revealed that, as in the aqueous state, the DNA/DOPC bulk mixture forms a multilamellar phase, where the DNA was confined between the DOPC lipid bilayers. The thickness of the hydrophobic layers composed of lipids tails was greater in the DNA-DOPC mixture than in the pure DOPC. However, interestingly, the thickness of the hydrophilic layer composed of lipid headgroups in the DNA-DOPC mixture was same as in the pure DOPC even though in the former DNA was intercalated in this layer. Furthermore, according to the DSC endotherms we also observe that DNA induced a significant depression of gel-to-liquid crystalline phase transition temperature of DOPC bilayer. A thermodynamic model was presented that described the experimentally observed morphological and thermotropic phase behavior.

Synthesis and photoresponsive behavior of azobenzene‐containing side‐chain liquid crystalline diblock polymers with polypeptide block

AbstractWell‐defined azobenzene‐containing side‐chain liquid crystalline diblock copolymers composed of poly[6‐(4‐methoxy‐azobenzene‐4′‐oxy) hexyl methacrylate] (PMMAZO) and poly(γ‐benzyl‐L‐glutamate) (PBLG) were synthesized by click reaction from alkyne‐ and azide‐functionalized homopolymers. The alkyne‐terminated PMMAZO homopolymers were synthesized by copper‐mediated atom transfer radical polymerization with a bromine‐containing alkyne bifunctional initiator, and the azido‐terminated PBLG homopolymers were synthesized by ring‐opening polymerization of γ‐benzyl‐L‐glutamate‐N‐carboxyanhydride in DMF at room temperature using an amine‐containing azide initiator. The thermotropic phase behavior of PMMAZO‐b‐PBLG diblock copolymers in bulk were investigated using differential scanning calorimetry and polarized light microscopy. The PMMAZO‐b‐PBLG diblock copolymers exhibited a smectic phase and a nematic phase when the weight fraction of PMMAZO block was more than 50%. Photoisomerization behavior of PMMAZO‐b‐PBLG diblock copolymers and the corresponding PMMAZO homopolymers in solid film and in solution were investigated using UV–vis. In solution, trans–cis isomerization of diblock copolymers was slower than that of the corresponding PMMAZO homopolymers. These results may provide guidelines for the design of effective photoresponsive anisotropic materials. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Thermotropic Phase Behavior of Binary Bilayer Membrane of Dipalmitoylphosphatidylcholine and Ergosterol

We investigated the phase behavior of a binary bilayer membrane of dipalmitoylphosphatidylcholine (DPPC) and ergosterol, using Prodan fluorescence spectroscopy and differential scanning calorimetry to construct a temperature–composition phase diagram. We found that several important features of the phase diagram can be explained by the superlattice view, on the basis of the hexagonal lattice, which suggests the presence of a molecular cluster composed of an ergosterol molecule and its six surrounding DPPC molecules above an ergosterol composition of ca. 0.15.

Competitive molecular interaction among paeonol-loaded liposomes: Differential scanning calorimetry and synchrotron X-ray diffraction studies

Thermotropic phase behavior of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes containing 5 mol% cholesterol, or 5 mol% stigmasterol, or 5 mol% paeonol have been investigated by differential scanning calorimetry (DSC) and synchrotron X-ray diffraction (XRD) techniques, to investigate the competitive molecular interaction among paeonol-loaded liposomes. The results show that both sterol and paeonol can incorporate into hydrophobic region and interact with acyl chains of DPPC. Both 5 mol% sterols and 5 mol% paeonol can promote the formation of rippled gel phase of DPPC liposomes at room temperature. 5 mol% paeonol can induce the occurrence of phase separation in DPPC liposomes, but 5 mol% cholesterol or 5 mol% stigmasterol cannot induce this phenomenon. Both the repeat distance and the correlation length of paeonol-poor domain are larger than those of coexisted paeonol-rich domain. Both calorimetric data and SAXS patterns show that sterols have more favorable, stabilizing interactions with DPPC than paeonol, implying that high concentrations of sterols will have a negative effect on the loading of paeonol. In addition, calorimetric data show that cholesterol have a little more favorable, stabilizing interactions with DPPC than stigmasterol. The results of this study will play an important role in optimizing the formulation of paeonol-loaded liposomes.

The effects of molecular and lattice structures on thermotropic phase behaviour of zinc(II) undecanoate and isomeric zinc(II) undecynoates

Molecular structures, hydrocarbon chain packing, in the crystal lattice and their effects on the thermal behaviour of saturated and isomeric zinc(II) undecynoates have been investigated by a variety of physical methods. All the compounds crystallize in a monoclinic crystal system with a being the long axis. The hydrocarbon chains adopt the fully extended all-trans conformation and are arranged as methyl–methyl overlapping bilayers within a lamellar. Furthermore, in order to enhance lattice stability, hydrocarbon chains, from different layers in the lamellar are not in the same plane but are packed in an alternating spatial arrangement and are tilted at ca. 60° to the metal basal plane. In a molecule, four carbonyl groups bind to a zinc atom, in a bridging bidentate mode, to form a three dimensional polymeric network. At elevated temperatures a highly viscous phase, a polymeric ionic mesophase, possibly a smectic C phase, is observed for 10-undecynoate only, whereas two crystal–crystal and crystal-isotropic liquid transitions are observed for the undecanoate and 9-undecynoate, respectively. Though head group coordination is nearly iso-structural, differences in molecular symmetry and lattice packing are evident. These arise from differences in the spatial orientation of the high electron density hydrocarbon chains in the crystal lattice. Indeed, it is the relative balance between head group electrostatic and van der Waals interactions at elevated temperatures that accounts for formation of the mesophase. Surprisingly, all the freshly synthesized compounds are meta-stable, only achieving complete stability over several days. Furthermore, on heating and cooling the compounds, thermotropic behaviour is altered so that on re-heating, subtle changes in phase textures are observed.

The T′ phase and its ‘sandwich-like layer’ structure as shown by ionic liquid crystals containing a biphenyl ester-based rigid-core modified by 3-alkylimidazolium salts

Eight ionic liquid crystals containing a biphenyl ester-based rigid core modified by a 3-alkyl-imidazolium cation were synthesised and characterised. The length of the alkyl chain varied between n = 4–18 carbon atoms. Their thermotropic phase behaviour was investigated using X-ray diffraction (XRD) measurements, variable temperature small-angle X-ray scattering (SAXS), polarising optical microscopy (POM) and differential scanning calorimetry (DSC). The compounds show rich mesomorphism, such as SmA2 phase (for compound C18) and SmC2 phases (for compounds C12–C16). For the molecules containing long alkyl chains (n ≥ 12), a novel mesophase, the T′ phase, was seen which has a unique ‘sandwich-like layer’ structure. The T′ phase is different from the known T phase and is formed at room temperature. Based on XRD, SAXS, DSC and POM data, models for the formation of the T′ phase are proposed.

Photoresponsive Poly(S-(o-nitrobenzyl)-l-cysteine)-b-PEO from a l-Cysteine N-Carboxyanhydride Monomer: Synthesis, Self-Assembly, and Phototriggered Drug Release

A photoresponsive S-(o-nitrobenzyl)-l-cysteine N-carboxyanhydride (NBC-NCA) monomer was for the first time designed, and the related poly(S-(o-nitrobenzyl)-l-cysteine)-b-poly(ethylene glycol) (PNBC-b-PEO) block copolymers were synthesized from the ring-opening polymerization (ROP) of NBC-NCA in DMF solution at 25 °C. Their molecular structures, physical properties, photoresponsive self-assembly, and drug release of PNBC-b-PEO were thoroughly investigated. The β-sheet conformational PNBC block within copolymers presented a thermotropic liquid crystal phase behavior, and the crystallinity of PEO block was progressively suppressed over the PNBC composition. The characteristic absorption peaks of these copolymers at about 310 and 350 nm increased over UV irradiation time and then leveled off, indicating that the o-nitrobenzyl groups were gradually photocleaved from copolymers until the completion of photocleavage. The PNBC-b-PEO copolymers self-assembled into spherical nanoparticles in aqueous solution, presenting a photoresponsive self-assembly behavior, together with a size reduction of nanoparticles after irradiation. The anticancer drug doxorubicin can be released in a controlled manner by changing the light irradiation time, which was induced by gradually photocleaving the PNBC core of nanoparticles. This work provides a facile strategy not only for the synthesis of photoresponsive polypeptide-based block copolymers but also for the fabrication of photoresponsive nanomedicine potential for anticancer therapy.

A DSC study of paeonol-encapsulated liposomes, comparison the effect of cholesterol and stigmasterol on the thermotropic phase behavior of liposomes

Thermotropic phase behaviors of paeonol-encapsulated liposomes containing stigmasterol or cholesterol have been investigated by differential scanning calorimetry. We compared the thermotropic phase behavior of pure dipalmitoylphosphatidylcholine (DPPC) liposomes, sterol/DPPC liposomes, and paeonol/sterol/DPPC liposomes increasing the ratio of paeonol to sterol from 0 to 1, by analyzing the calorimetric parameters of main phase transition of liposomes including phase transition temperature (onset temperature and peak temperature) and phase transition cooperativity. The results showed that paeonol could incorporate into the hydrophobic region of DPPC, thus, decrease phase transition temperature of DPPC. Though stigmasterol interacts with DPPC less favorably than cholesterol, thermotropic phase behavior of paeonol/cholesterol/DPPC liposomes and that of paeonol/stigmasterol/DPPC liposomes are very similar. A phase separation occurred when the molar ratio of paeonol to sterol reached 1:1 in paeonol-encapsulated liposomes, where a paeonol-rich domain coexisted with a sterol-rich domain. The packing order of acyl chains of DPPC in sterol-rich domain is a little higher than that in paeonol-rich domain.

Induction of Thermotropic Bicontinuous Cubic Phases in Liquid-Crystalline Ammonium and Phosphonium Salts

Two series of wedge-shaped onium salts, one ammonium and the other phosphonium, having 3,4,5-tris(alkyloxy)benzyl moieties, exhibit thermotropic bicontinuous "gyroid" cubic (Cub(bi)) and hexagonal columnar liquid-crystalline (LC) phases by nanosegregation between ionophilic and ionophobic parts. The alkyl chain lengths on the cationic moieties, anion species, and alkyl chain lengths on the benzyl moieties have crucial effects on their thermotropic phase behavior. For example, triethyl-[3,4,5-tris(dodecyloxy)benzyl]ammonium hexafluorophosphate forms the thermotropic Ia3d Cub(bi) LC phase, whereas an analogous compound with trifluoromethanesulfonate anion shows no LC properties. Synchrotron small-angle diffraction intensities from the Ia3d Cub(bi) LC materials provide electron density maps in the bulk state. The resulting maps show convincingly that the Ia3d Cub(bi) structure is composed of three-dimensionally interconnected ion nanochannel networks surrounded by aliphatic domains. A novel differential mapping technique has been applied successfully. The map of triethyl-[3,4,5-tris(decyloxy)benzyl]ammonium tetrafluoroborate has been subtracted from that of the analogous ammonium salt with hexafluorophosphate anion in the Ia3d Cub(bi) phases. The differential map shows that the counteranions are located in the core of the three-dimensionally interconnected nanochannel networks. Changing from trimethyl- via triethyl- to tripropylammonium cation changes the phase from columnar to Cub(bi) to no mesophase, respectively. This sensitivity to the widened shape for the narrow end of the molecule is explained successfully by the previously proposed semiquantitative geometric model based on the radial distribution of volume in wedge-shaped molecules. The LC onium salts dissolve lithium tetrafluoroborate without losing the Ia3d Cub(bi) LC phase. The Cub(bi) LC materials exhibit efficient ion-transporting behavior as a result of their 3D interconnected ion nanochannel networks. The Ia3d Cub(bi) LC material formed by triethyl-[3,4,5-tris(decyloxy)benzyl]phosphonium tetrafluoroborate shows ionic conductivities higher than the analogous Ia3d Cub(bi) material based on ammonium salts. The present study indicates great potential of Cub(bi) LC nanostructures consisting of ionic molecules for development of transportation nanochannel materials.

Probing molecular interactions of poly(styrene-co-maleic acid) with lipid matrix models to interpret the therapeutic potential of the co-polymer

To understand and maximize the therapeutic potential of poly(styrene-co-maleic acid) (SMA), a synthetic, pharmacologically-active co-polymer, its effect on conformation, phase behavior and stability of lipid matrix models of cell membranes were investigated. The modes of interaction between SMA and lipid molecules were also studied. While, attenuated total reflection-Fourier-transform infrared (ATR-FTIR) and static 31P nuclear magnetic resonance (NMR) experiments detected SMA-induced conformational changes in the headgroup region, differential scanning calorimetry (DSC) studies revealed thermotropic phase behavior changes of the membranes. 1H NMR results indicated weak immobilization of SMA within the bilayers. Molecular interpretation of the results indicated the role of hydrogen-bond formation and hydrophobic forces between SMA and zwitterionic phospholipid bilayers. The extent of membrane fluidization and generation of isotropic phases were affected by the surface charge of the liposomes, and hence suggested the role of electrostatic interactions between SMA and charged lipid headgroups. SMA was thus found to directly affect the structural integrity of model membranes.

Structural and biophysical characterization of an antimicrobial peptide chimera comprised of lactoferricin and lactoferrampin

Lactoferricin and lactoferrampin are two antimicrobial peptides found in the N-terminal lobe of bovine lactoferrin with broad spectrum antimicrobial activity against a range of Gram-positive and Gram-negative bacteria as well as Candida albicans. A heterodimer comprised of lactoferrampin joined to a fragment of lactoferricin was recently reported in which these two peptides were joined at their C-termini through the two amino groups of a single Lys residue (Bolscher et al., 2009, Biochimie 91(1):123–132). This hybrid peptide, termed LFchimera, has significantly higher antimicrobial activity compared to the individual peptides or an equimolar mixture of the two. In this work, the underlying mechanism behind the increased antibacterial activity of LFchimera was investigated. Differential scanning calorimetry studies demonstrated that all the peptides influenced the thermotropic phase behaviour of anionic phospholipid suspensions. Calcein leakage and vesicle fusion experiments with anionic liposomes revealed that LFchimera had enhanced membrane perturbing properties compared to the individual peptides. Peptide structures were evaluated using circular dichroism and NMR spectroscopy to gain insight into the structural features of LFchimera that contribute to the increased antimicrobial activity. The NMR solution structure, determined in a miscible co-solvent mixture of chloroform, methanol and water, revealed that the Lys linkage increased the helical content in LFchimera compared to the individual peptides, but it did not fix the relative orientations of lactoferricin and lactoferrampin with respect to each other. The structure of LFchimera provides insight into the conformation of this peptide in a membranous environment and improves our understanding of its antimicrobial mechanism of action.

Evaluation of the influence of ionization states and spacers in the thermotropic phase behaviour of amino acid-based cationic lipids and the transfection efficiency of their assemblies

The influence of both the ionization states and the hydrocarbon chain spacer of a series of amino acid-based cationic lipids was evaluated in terms of gene delivery efficiency and cytotoxicity to the COS-7 cell line and compared with that of Lipofectamine™ 2000. We synthesized a series of amino acid-based cationic lipids with different ionization states (i.e., –NH 2, –NH 3 +Cl − or –NH 3 +TFA −) in the lysine head group and different hydrocarbon chain spacers (i.e., 0, 3, 5 or 7 carbon atoms) between the hydrophilic head group and hydrophobic moieties. In the 3-carbon series, the cationic assemblies formed a micellar structure in the presence of –NH 3 +Cl − and a vesicular structure both in the presence of –NH 2 and –NH 3 +TFA −. Differential scanning calorimetry (DSC) data revealed a significantly lower (8.1 °C) gel-to-liquid crystalline phase transition temperature for cationic assemblies bearing –NH 3 +TFA − when compared to their –NH 2 counterparts. Furthermore, the zeta potential of cationic assemblies having –NH 3 +TFA − in the hydrophilic head group was maximum followed by –NH 3 +Cl − and –NH 2 irrespective of their hydrocarbon chain spacer length. The gene delivery efficiency in relation to the ionization states of the hydrophilic head group was as follows: –NH 3 +TFA − > –NH 3 +Cl − > –NH 2.