Phosphate-mediated arginine insertion into lipid membranes and pore formation by a cationic membrane peptide from solid-state NMR

The insertion of charged amino acid residues into the hydrophobic part of lipid bilayers is energetically unfavorable yet found in many cationic membrane peptides and protein domains. To understand the mechanism of this translocation, we measured the (13)C-(31)P distances for an Arg-rich beta-hairpin antimicrobial peptide, PG-1, in the lipid membrane using solid-state NMR. Four residues, including two Arg's, scattered through the peptide were chosen for the distance measurements. Surprisingly, all residues show short distances to the lipid (31)P: 4.0-6.5 A in anionic POPE/POPG membranes and 6.5-8.0 A in zwitterionic POPC membranes. The shortest distance of 4.0 A, found for a guanidinium Czeta at the beta-turn, suggests N-H...O-P hydrogen bond formation. Torsion angle measurements of the two Arg's quantitatively confirm that the peptide adopts a beta-hairpin conformation in the lipid bilayer, and gel-phase 1H spin diffusion from water to the peptide indicates that PG-1 remains transmembrane in the gel phase of the membrane. For this transmembrane beta-hairpin peptide to have short (13)C-(31)P distances for multiple residues in the molecule, some phosphate groups must be embedded in the hydrophobic part of the membrane, with the local (31)P plane parallel to the beta-strand. This provides direct evidence for toroidal pores, where some lipid molecules change their orientation to merge the two monolayers. We propose that the driving force for this toroidal pore formation is guanidinium-phosphate complexation, where the cationic Arg residues drag the anionic phosphate groups along as they insert into the hydrophobic part of the membrane. This phosphate-mediated translocation of guanidinium ions may underlie the activity of other Arg-rich antimocrobial peptides and may be common among cationic membrane proteins.     

Interaction of hexadecylbetainate chloride with biological relevant lipids

The present work investigates the interaction of hexadecylbetainate chloride (C(16)BC), a glycine betaine-based ester with palmitoyl-oleoyl-phosphatidylcholine (POPC), sphingomyelin (SM), and cholesterol (CHOL), three biological relevant lipids present in the outer leaflet of the mammalian plasma membrane. The binding affinity and the mixing behavior between the lipids and C(16)BC are discussed based on experimental (isothermal titration calorimetry (ITC) and Langmuir film balance) and molecular modeling studies. The results show that the interaction between C(16)BC and each lipid is thermodynamically favorable and does not affect the integrity of the lipid vesicles. The primary adsorption of C(16)BC into the lipid film is mainly governed by a hydrophobic effect. Once C(16)BC is inserted in the lipid film, the polar component of the interaction energy between C(16)BC and the lipid becomes predominant. Presence of CHOL increases the affinity of C(16)BC for membrane. This result can be explained by the optimal matching between C(16)BC and CHOL within the film rather by a change of membrane fluidity due to the presence of CHOL. The interaction between C(16)BC and SM is also favorable and gives rise to highly stable monolayers probably due to hydrogen bonds between their hydrophilic groups. The interaction of C(16)BC with POPC is less favorable but does not destabilize the mixed monolayer from a thermodynamic point of view. Interestingly, for all the monolayers investigated, the exclusion surface pressures are above the presumed lateral pressure of the plasma membranes suggesting that C(16)BC would be able to penetrate into mammalian plasma membranes in vivo. These results may serve as a useful basis in understanding the interaction of C(16)BC with real membranes.     

Membrane Structure of Substance P. II. Secondary Structure of Substance P, [9-Leucine]substance P,and Shorter Segments in 2,2,2-Trifluoroethanol,Methanol, and on Liposomes Studied by Circular Dichroism

Comparative CD studies with substance P ( 1 ), [Leu⁹]substance P ([Leu⁹]- 1 ), and their shorter peptide segments supported the membrane structures predicted for substance P and [Leu⁹]substance P. They indicated that the C-terminal segments (from residue 3 or 4 onward) can adopt α-helical conformations in hydrophobic environments and on lipid membranes. The N-terminal segment, (residues 1–4) had a poly(proline)-like conformation in aqueous and hydrophobic surroundings. Residues 3 and 4 (Lys-Pro) appeared to belong to both domains and bring about the transition between the two. The estimated free energies of transfer for 1 and [Leu⁹]- 1 from their random conformations in H₂O to their partially helical conformations on an aqueous-hydrophobic interface are too small to allow detectable interaction with neutral lipid membranes at low concentrations. The two peptides should, however, interact detectably with anionic membranes because of favourable Boltzmann distribution factors. This prediction was shown to be correct for liposomes prepared from 1,2-dioleoyl-sn-glycero-3-phosphocholine (neutral) and phosphatidylserine (anionic).     

The influence of zwitterionic lipids on the electrostatic adsorption of macroions onto mixed lipid membranes

Charged lipid membranes commonly consist of a mixture of charged and zwitterionic lipids. We suggest a model that characterizes the influence of the dipolar nature of the zwitterionic lipid species on the electrostatic adsorption of macroions onto mixed membranes in the fluid state. The model is based on Poisson-Boltzmann theory which we have modified so as to account for the dipolar character of the zwitterionic lipids. In addition the membrane lipids are allowed to adjust their lateral distribution upon macroion adsorption. We consider and compare two experimentally relevant scenarios: cationic macroions adsorbed onto anionic membranes and anionic macroions adsorbed onto cationic membranes. We show that in the former case the adsorption strength is slightly weakened by the presence of the headgroup dipoles of the zwitterionic lipids. Here, macroion-induced lipid demixing is more pronounced and the lipid headgroups tilt away from a cationic macroion upon adsorption. In contrast, for the adsorption of anionic macroions onto a cationic membrane the zwitterionic lipids strongly participate in the electrostatic interaction between membrane and macroion, thus enhancing the adsorption strength significantly (we predict up to 20%). Consistent with that we find less lateral demixing of the charged lipids and a reorientation of the dipoles of the zwitterionic headgroups towards the anionic macroions. Our results may be of importance to understand the differences in the electrostatic adsorption of proteins/peptides onto cellular membranes versus complex formation between cationic membranes and DNA.     

Permeability and thermodynamics study of quaternary ammonium surfactants-phosphocholine vesicle system

Quaternary ammonium compounds (QACs) are recognized as membrane active agents widely used as biocides. The main purpose of this work was to investigate the influence of the QAC head group and acyl chain length on their permeability-perturbing power and on their affinity for lipidic membranes. Permeability perturbations were assessed by measuring the release of calcein entrapped inside vesicles. The affinity of QACs for bilayers was investigated by isothermal titration calorimetry (ITC). QACs bearing C(16) chain were found to be more efficient to decrease the membrane permeability than their C(12) analogues. On the other hand, the chemical nature of the ammonium head group has practically no influence on the permeability perturbations caused by QACs bearing C(16) chains. It was difficult to assess the partitioning of the QACs between the aqueous and lipid phases since the ITC signals could also be associated to morphological changes such as vesicle aggregation. For the systems for which reliable thermodynamic parameters could be obtained, the Gibbs energy of transfer was similar to that for the micellization. The entropy variation represented the main contribution to the Gibbs energy, indicating that the insertion of QACs inside lipidic bilayers is driven by hydrophobic interactions.     

Passive transport of C60 fullerenes through a lipid membrane: a molecular dynamics simulation study

To investigate the implications of the unique properties of fullerenes on their interaction with and passive transport into lipid membranes, atomistic molecular dynamics simulations of a C60 fullerene in a fully hydrated di-myristoyl-phoshatidylcholine lipid membrane have been carried out. In these simulations the free energy and the diffusivity of the fullerene were obtained as a function of its position within the membrane. These properties were utilized to calculate the permeability of fullerenes through the lipid membrane. Simulations reveal that the free energy decreases as the fullerene passes from the aqueous phase, through the head group layer and into the hydrophobic core of the membrane. This decrease in free energy is not due to hydrophobic interactions but rather to stronger van der Waals (dispersion) interactions between the fullerene and the membrane compared to those between the fullerene and (bulk) water. It was found that there is no free energy barrier for transport of a fullerene from the aqueous phase into the lipid core of the membrane. In combination with strong partitioning of the fullerenes into the lipidic core of the membrane, this "barrierless" penetration results in an astonishingly large permeability of fullerenes through the lipid membrane, greater than observed for any other known penetrant. When the strength of the dispersion interactions between the fullerene and its surroundings is reduced in the simulations, thereby emulating a nanometer sized hydrophobic particle, a large free energy barrier for penetration of the head group layer emerges, indicating that the large permeability of fullerenes through lipid membranes is a result of their unique interaction with their surrounding medium.     

Controlling the morphology of cross beta-sheet assemblies by rational design

Low molecular weight peptidomimetics with simple amphiphilic sequences can help to elucidate the structures of cross beta-sheet assemblies, such as amyloid fibrils. The peptidomimetics described herein comprise a dibenzofuran template, two peptide strands made up of alternating hydrophilic and hydrophobic residues, and carboxyl termini, each of which can be varied to probe the structural requirements for beta-sheet self-assembly processes. The dibenzofuran template positions the strands approximately 10 A apart, allowing corresponding hydrophobic side chains in the strands to pack into a collapsed U-shaped structure. This conformation is stabilized by hydrophobic interactions, not intramolecular hydrogen bonds. Intermolecular stacking of the collapsed peptidomimetics, enabled by intermolecular hydrogen bonding and hydrophobic interactions, affords 25-27 A wide protofilaments having a cross beta-sheet structure. Association of protofilaments, mediated by the dibenzofuran substructures and driven by the hydrophobic effect, affords 50-60 A wide filaments. These widths can be controlled by changing the length of the peptide strands. Further assembly of the filaments into fibrils or ribbons can be controlled by modification of the template, C-terminus, and buffer ion composition.     

Effect of peptide length on the interaction between consensus peptides and DOPC/DOPA bilayers

The effect of peptide length and electrostatics on the interaction between Cardin motif peptides and lipid membranes was investigated for (AKKARA)(n) (n = 1-4) and (ARKAAKKA)(n) (n = 1-3) peptides (A, K, and R refer to alanine, lysine, and arginine, respectively) by fluorescence spectroscopy, circular dichroism, ellipsometry, z potential, and photon correlation spectroscopy measurements. The effect of the peptides regarding leakage induction of both zwitterionic and anionic liposomes increased with increasing peptide length, as did the peptide-induced killing of Enterococcus faecalis and Bacillus subtilis bacteria. The peptides, characterized by a random coil conformation both in buffer and when attached to the liposomes (helix content less than 20%), displayed an increased adsorption with increasing peptide length, and plateau adsorption for the longest peptides corresponded to 1 peptide per 65 and 17 lipid molecules for zwitterionic and anionic membranes, respectively. Control experiments with uncharged peptide analogues as well as experiments at high excess electrolyte concentration showed that peptide charges are important both for peptide adsorption and leakage induction. These observations, together with observations of the liposome z potential at different peptide additions as well as a comparison between the results for zwitterionic and anionic liposomes, suggest that electrostatically affected local packing effects are crucial for the action of these peptides, although pore formation such as that observed for many AMPs cannot be excluded at present.     

Interaction of lipoplexes with anionic lipids resulting in DNA release is a two-stage process

We propose a mechanism for DNA release from lipoplexes in cells that accounts for various observations of lipoplex-anionic lipid interactions. We examined the structural evolution of lipoplexes upon interaction with cellular lipids by synchrotron small-angle X-ray diffraction (SAXD), and the extent of DNA release from lipoplexes was determined by gel electrophoresis. We find that the interaction of lipoplexes with anionic cellular lipids is a two-stage process. In the first step, anionic lipids laterally diffuse into the complex and neutralize the cationic lipids. As a result, the membrane charge density of lipoplexes decreases and interactions between cationic lipids and DNA become weaker, but DNA is extremely poorly released. Only after the cationic charge of lipoplex membranes is completely neutralized by anionic lipids does DNA starts to be released significantly.     

Endocytosis-like uptake of surface-modified drug nanocarriers into giant unilamellar vesicles

We had previously developed surface-modified poly (D,L-lactide-co-glycolide) (PLGA) nanoparticles (NPs) for use as a cellular drug delivery system. The cellular uptake of PLGA-NPs was mediated predominantly by endocytosis, and this uptake was increased by surface modifications with polymers, such as chitosan (CS) and polysorbate 80 (P80). In the present study, we prepared a cell-sized giant unilamellar vesicle (GUV) that mimics a cell membrane to investigate the interaction between cell membranes and NPs. Endocytosis-like uptake of NPs into a GUV was observed when the NPs were modified with nonionic surfactant P80 probably due to change in viscoelasticity and enhanced fusion activity of the membrane induced by P80. In contrast, unmodified NPs and those modified with CS were not internalized into a GUV. These results suggest that surface properties of PLGA-NPs are an important formulation parameter for their interaction with lipid membranes.     

Immobilization of acetylcholinesterase in lipid membranes deposited on self-assembled monolayers

Human red blood cell acetylcholinesterase was incorporated into planar lipid membranes deposited on alkanethiol self-assembled monolayers (SAMs) on gold substrates. Activity of the protein in the membrane was detected with a standard photometric assay and was determined to be similar to the protein in detergent solution or incorporated in lipid vesicles. Monolayer and bilayer lipid membranes were generated by fusing liposomes to hydrophobic and hydrophilic SAMs, respectively. Liposomes were formed by the injection method using the lipid dimyristoylphosphatidylcholine (DMPC). The formation of alkanethiol SAMs and lipid monolayers on SAMs was confirmed by sessile drop goniometry, ellipsometry, and electrochemical impedance spectroscopy. In this work, we report acetylcholinesterase immobilization in lipid membranes deposited on SAMs formed on the gold surface and compare its activity to enzyme in solution.     

Inverse colloidal crystal membranes for hydrophobic interaction membrane chromatography

Hydrophobic interaction membrane chromatography has gained interest due to its excellent performance in the purification of humanized monoclonal antibodies. The membrane material used in hydrophobic interaction membrane chromatography has typically been commercially available polyvinylidene fluoride. In this contribution, newly developed inverse colloidal crystal membranes that have uniform pores, high porosity and, therefore, high surface area for protein binding are used as hydrophobic interaction membrane chromatography membranes for humanized monoclonal antibody immunoglobulin G purification. The capacity of the inverse colloidal crystal membranes developed here is up to ten times greater than commercially available polyvinylidene fluoride membranes with a similar pore size. This work highlights the importance of developing uniform pore size high porosity membranes in order to maximize the capacity of hydrophobic interaction membrane chromatography.     

Membrane Structure of Substance P. I. Prediction of Preferred Conformation,Orientation, and Accumulation of Substance P on Lipid Membranes

Preferred conformation, orientation, and accumulation of substance P on a neutral hydrophilic-hydrophobic interface was estimated and extrapolated to interactions with neutral and anionic lipid bilayer membranes according to our general procedure. Nine residues at the C-terminus were predicted to be transferred to the hydrophobic phase as an α-helical domain, oriented quite perpendicularly on the membrane surface. The N-terminal residues remained in the aqueous phase with their charges exposed to H₂O. The molecular amphiphilic moment vector was strong (338 arbitrary units) and pointed its hydrophilic end towards the N-terminus, only 15° away from the helix axis. The molecular electric dipole moment vector was also strong (124 debye) and pointed its positive end towards the N-terminus, only 9° away from the helix axis. Thus, it reinforced the effect of the amphiphilic moment of a peptide intruding into the membrane dipole layer. The estimated dissociation constant for the equilibrium between membrane-bound and free substance P was K_d ≈? 46 mM for neutral membranes, and K_d ≈?0.43 mM for anionic membranes with a Gouy-Chapman surface potential of ?40 mV. Thus, substance P behaved similarly to dynorphin A and adrenocorticotropin peptides which insert their N-terminal message segments as perpendicularly oriented helical domains into membranes, whereas their C-terminal address segments remain in the aqueous phase as random coils. Substance P is the first instance of a neuropeptide which is expected to insert a C-terminal message into lipid membranes.     

Monitoring lipid membrane translocation of sodium dodecyl sulfate by isothermal titration calorimetry

We establish high-sensitivity isothermal titration calorimetry (ITC) as a fast, reliable, and versatile tool for assessing membrane translocation of charged compounds. A combination of ITC uptake and release titrations can discriminate between the two extreme cases of half-sided binding and complete transbilayer equilibration on the experimental time scale. To this end, we derive a general fit function for both assays that allows for incorporation of different membrane partitioning models. Electrostatic effects are taken into account with the aid of Gouy-Chapman theory, thus rendering uptake and release experiments amenable to the investigation of charged solutes. This is exemplified for the flip-flop of the anionic detergent sodium dodecyl sulfate (SDS) across the membranes of 100-nm-diameter unilamellar vesicles composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) in aqueous solution (10 mM phosphate buffer, 154 mM NaCl, pH 7.4). If repulsive electrostatic forces are accounted for adequately, SDS binding to POPC membranes can be evaluated on the basis of ideal mixing in all phases. At 25 degrees C, the intrinsic partition coefficient between the interfacial aqueous phase and the membrane amounts to 3.5 x 10(6); however, detergent flip-flop is negligibly slow under these conditions. Raising the temperature to 65 degrees C lowers the intrinsic partition coefficient to 1.4 x 10(6) but enables rapid transbilayer distribution of the detergent and, therefore, binding to or desorption from both membrane leaflets. Thus, combining a surface partition equilibrium with simple electrostatic theory appears highly useful in monitoring transmembrane movement of ionic compounds by ITC, thereby eliminating the need for specific reporter groups.     

Length dependence of the coil <--> beta-sheet transition in a membrane environment

The most abundant structural element in protein aggregates is the beta-sheet. Designed peptides that fold into a beta-sheet structure upon binding to lipid membranes are useful models to elucidate the thermodynamic characteristics of the random coil <-->beta-structure transition. Here, we examine the effect of strand length on the random coil <--> beta-sheet transition of the (KIGAKI)n peptide with the total chain length varying between 7 and 30 amino acids. The beta-sheet content of the peptides in the presence and absence of membranes was measured with circular dichroism spectroscopy. The peptides were titrated with small unilamellar lipid vesicles, and the thermodynamic binding parameters were determined with isothermal titration calorimetry (ITC). Membrane binding includes at least two processes, namely (i) the transfer of the peptide from the aqueous phase to the lipid surface and (ii) the conformational change from a random coil conformation to a beta-sheet structure. CD spectroscopy and ITC analysis demonstrate that beta-sheet formation depends cooperatively on the peptide chain length with a distinct increase in beta-structure for n > 10-12. Binding to the lipid membrane is an entropy-driven process as the binding enthalpy is always endothermic. The contribution of the beta-sheet folding reaction to the overall process was determined with analogues of the KIGAKI repeat where two adjacent amino acids were replaced by their D-enantiomers. The folding reaction for peptides with n >or= 12 is characterized by a negative free folding energy of DeltaG(degree)beta approximately equal -0.15 kcal/mol per amino acid residue. The folding step proper is exothermic with DeltaH(degree)(beta) approximately equal -0.2 to -0.6 kcal/mol per residue and counteracted by a negative entropy term TDeltaS(degree)(beta) = -0.1 to -0.5 kcal/mol per residue, depending on the chain length (18 相似文献   

Structural stability against disintegration by anionic lipids rationalizes the efficiency of cationic liposome/DNA complexes

Reported here is the correlation between the transfection efficiency of cationic liposome/DNA complexes (lipoplexes) and the structural evolution that they undergo when interacting with anionic membrane lipids. Multicomponent lipoplexes, incorporating from three to six lipid species simultaneously, presented a much higher transfection efficiency than binary lipoplexes, which are more commonly used for gene-delivery purposes. The discovery that a high transfection efficiency can be achieved by employing multicomponent complexes at a lower-than-ever-before membrane charge density of lipoplexes was of primary significance. Synchrotron small-angle X-ray diffraction (SAXD) experiments showed that anionic liposomes made of dioleoylphosphatidylglycerol (DOPG) disintegrated the lamellar phase of lipoplexes. DNA unbinding was measured by electrophoresis on agarose gels. Most importantly, structural changes induced by anionic lipids strictly depended on the lipid composition of lipoplexes. We found evidence of the existence of three different regimes of stability related to the interaction between complexes and anionic membranes. Both unstable (with low membrane charge density, sigmaM) and highly stable lipoplexes (with high sigmaM) exhibited low transfection efficiency whereas highly efficient multicomponent lipoplexes exhibited an "optimal stability". This intermediate regime reflects a compromise between two opposing constraints: protection of DNA in the cytosol and endosomal escape. Here we advance the concept that structural stability, upon interaction with cellular anionic lipids, is a key factor governing the transfection efficiency of lipoplexes. Possible molecular mechanisms underlying experimental observations are also discussed.     

Kinetics and thermodynamics of chlorpromazine interaction with lipid bilayers: effect of charge and cholesterol

Passive transport across cell membranes is the major route for the permeation of xenobiotics through tight endothelia such as the blood–brain barrier. The rate of passive permeation through lipid bilayers for a given drug is therefore a critical step in the prediction of its pharmacodynamics. We describe a detailed study on the kinetics and thermodynamics for the interaction of chlorpromazine (CPZ), an antipsychotic drug used in the treatment of schizophrenia, with neutral and negatively charged lipid bilayers. Isothermal titration calorimetry was used to study the partition and translocation of CPZ in lipid membranes composed of pure POPC, POPC:POPS (9:1), and POPC:Chol:POPS (6:3:1). The membrane charge due to the presence of POPS as well as the additional charge resulting from the introduction of CPZ in the membrane were taken into account, allowing the calculation of the intrinsic partition coefficients (K(P)) and the enthalpy change (ΔH) associated with the process. The enthalpy change upon partition to all lipid bilayers studied is negative, but a significant entropy contribution was also observed for partition to the neutral membrane. Because of the positive charge of CPZ, the presence of negatively charged lipids in the bilayer increases both the observed amount of CPZ that partitions to the membrane (KP(obs)) and the magnitude of ΔH. However, when the electrostatic effects are discounted, the intrinsic partition coefficient was smaller, indicating that the hydrophobic contribution was less significant for the negatively charged membrane. The presence of cholesterol strongly decreases the affinity of CPZ for the bilayer in terms of both the amount of CPZ that associates with the membrane and the interaction enthalpy. A quantitative characterization of the rate of CPZ translocation through membranes composed of pure POPC and POPC:POPS (9:1) was also performed using an innovative methodology developed in this work based on the kinetics of the heat evolved due to the interaction of CPZ with the membranes.     

Physicochemical mechanism for the enhanced ability of lipid membrane penetration of polyarginine

Arginine-rich, cell-penetrating peptides (e.g., Tat-peptide, penetratin, and polyarginine) are used to carry therapeutic molecules such as oligonucleotides, DNA, peptides, and proteins across cell membranes. Two types of processes are being considered to cross the cell membranes: one is an endocytic pathway, and another is an energy-independent, nonendocytic pathway. However, the latter is still not known in detail. Here, we studied the effects of the chain length of polyarginine on its interaction with an anionic phospholipid large unilamellar vesicle (LUV) or a giant vesicle using poly-l-arginine composed of 69 (PLA69), 293 (PLA293), or 554 (PLA554) arginine residues, together with octaarginine (R8). ζ-potential measurements confirmed that polyarginine binds to LUV via electrostatic interactions. Circular dichroism analysis demonstrated that the transition from the random coil to the α-helix structure upon binding to LUV occurred for PLA293 and PLA554, whereas no structural change was observed for PLA69 and R8. Fluorescence studies using membrane probes revealed that the binding of polyarginine to LUV affects the hydration and packing of the membrane interface region, in which the degree of membrane insertion is greater for the longer polyarginine. Isothermal titration calorimetry measurements demonstrated that although the binding affinity (i.e., the Gibbs free energy of binding) per arginine residue is similar among all polyarginines the contribution of enthalpy to the energetics of binding of polyarginine increases with increasing polymer chain length. In addition, confocal laser scanning microscopy showed that all polyarginines penetrate across giant vesicle membranes, and the order of the amount of membrane penetration is R8 ≈ PLA69 < PLA293 ≈ PLA554. These results suggest that the formation of α-helical structure upon lipid binding drives the insertion of polyarginine into the membrane interior, which appears to enhance the membrane penetration of polyarginine.     

Responses of Transmembrane Peptide and Lipid Chains to Hydrophobic Mismatch

Hydrophobic mismatch between the hydrophobic length of membrane proteins and hydrophobic thickness of membranes is a crucial factor in controlling protein function and assembly. We combined fluorescence with circular dichroism(CD) and attenuated total reflection infrared(ATR-IR) spectroscopic methods to investigate the behaviors of the peptide and lipids under hydrophobic mismatch using a model peptide from the fourth transmembrane domain of natural resistance-associated macrophage protein 1(Nramp1), the phosphatidylcholines(PCs) and phosphatidylglycerols(PGs) with different lengths of acyl chains(14:0, 16:0 and 18:0). In all PG lipid membranes, the peptide forms stable a-helix structure, and the helix axis is parallel to lipid chains. The helical span and orientation hardly change in varying thickness of PG membranes, while the lipid chains can deform to accommodate to the hydrophobic surface of embedded peptide. By comparison, the helical structures of the model peptide in PC lipid membranes are less stable. Upon incorporation with PC lipid membranes, the peptide can deform itself to accommodate to the hydrophobic thickness of lipid membranes in response to hydrophobic mismatch. In addition, hydrophobic mismatch can increase the aggregation propensity of the peptide in both PC and PG lipid membranes and the peptide in PC membranes has more aggregation tendency than that in PG membranes.     

Effect of chlorogenic acid on the phase transition in phospholipid and phospholipid/cholesterol membranes

Chlorogenic acid (CGA) is present in many plants, especially in green coffee, dry plums, and bilberries. It is an important bioactive polyphenol. Studies showed that CGA has an antioxidative, bacteriostatic, anticancer, antiviral, and anti-inflammatory activity. Despite great interest in this compound, its interaction with the lipid model membrane has not yet been investigated. To better understand the relationship between the biological activity of CGA and its interaction with biological membranes, the thermotropic behavior of model lipid membranes was investigated. The effect of CGA on the model lipid membrane, specifically on the lipid bilayer phase transitions, was examined by the combined methods: differential scanning calorimetry and fluorescence spectroscopy. In particular, the degree of packing order of the hydrophilic phase of the lipid bilayer was determined using the fluorimetric method with Laurdan and Prodan probes, while the fluorescence anisotropy of the hydrophobic phase with the DPH and TMA-DPH probes. The results of the study show that CGA incorporates mainly into the hydrophilic part of membrane, changing the packing order of the polar heads of lipids. No significant changes were recorded in membrane fluidity of the hydrophobic membrane region, for the fluorescence anisotropy practically did not change. One can thus infer that CGA does not penetrate deep into the hydrophobic area of the membrane.