Phase Transition Of Lipid Bilayer Research Articles

Probing Slipids Force Field for Phase Transitions in SOPC Lipid Bilayers with Various Cholesterol Concentrations

We explore the phase behavior of lipid bilayers containing SOPC (1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine) with various molar concentrations (0 mol%, 10 mol% and 30 mol%) of cholesterol. To this end, we performed extensive atomistic molecular dynamics simulations in conjunction with the Slipids force field with optimized parameters for the headgroups of phospholipids. We computed thermodynamic and structural quantities describing the ordering of the tails, the mobility of the heads and the arrangement of the lipids in the bilayers. We analyzed the behavior of the named quantities over the temperature range between 271 K and 283 K, where the experimentally determined melting temperature, Tm=279 K, lies, as well as at 400 K, which is used as a reference temperature. The obtained results are compared to available experimental data along with the outcome from molecular dynamics simulations of similar phospholipids containing different amounts of cholesterol. In the temperature interval of interest, we found evidence of the occurrence of a thermal-driven phase transition (melting) in both the pure system and the one with the lower concentration of cholesterol, while in the remaining system, the higher amount of cholesterol in the bilayer smears out the transitional behavior. Thus, we demonstrate the ability of the Slipids force field to predict the phase behavior of bilayers of SOPC and SOPC mixed with cholesterol.

Heating of nanoparticles and their environment by laser radiation and applications

This review considers the fundamental dynamic processes involved in the laser heating of metal nanoparticles and their subsequent cooling. Of particular interest are the absorption of laser energy by nanoparticles, the heating of a single nanoparticle or an ensemble thereof, and the dissipation of the energy of nanoparticles due to heat exchange with the environment. The goal is to consider the dependences and values of the temperatures of the nanoparticles and the environment, their time scales, and other parameters that describe these processes. Experimental results and analytical studies on the heating of single metal nanoparticles by laser pulses are discussed, including the laser thresholds for initiating subsequent photothermal processes, how temperature influences the optical properties, and the heating of gold nanoparticles by laser pulses. Experimental studies of the heating of an ensemble of nanoparticles and the results of an analytical study of the heating of an ensemble of nanoparticles and the environment by laser radiation are considered. Nanothermometry methods for nanoparticles under laser heating are considered, including changes in the refractive indices of metals and spectral thermometry of optical scattering of nanoparticles, Raman spectroscopy, the thermal distortion of the refractive index of an environment heated by a nanoparticle, and thermochemical phase transitions in lipid bilayers surrounding a heated nanoparticle. Understanding the sequence of events after radiation absorption and their time scales underlies many applications of nanoparticles. The application fields for the laser heating of nanoparticles are reviewed, including thermochemical reactions and selective nanophotothermolysis initiated in the environment by laser-heated nanoparticles, thermal radiation emission by nanoparticles and laser-induced incandescence, electron and ion emission of heated nanoparticles, and optothermal chemical catalysis. Applications of the laser heating of nanoparticles in laser nanomedicine are of particular interest. Significant emphasis is given to the proposed analytical approaches to modeling and calculating the heating processes under the action of a laser pulse on metal nanoparticles, taking into account the temperature dependences of the parameters. The proposed models can be used to estimate the parameters of lasers and nanoparticles in the various application fields for the laser heating of nanoparticles.

Studies of some bio-inspired liquid crystals

ABSTRACTThe main phase transition of lipid bilayers corresponds to a defined change from an ordered (gel) to a disordered (fluid liquid crystal) state of hydrocarbon chains, occurring in general at a defined temperature Tm. However, aqueous dispersions of pure anionic phospholipids, and particularly DMPG, may exhibit a ‘melting regime’, from Tmon until Tmoff, over ~10°C. The melting process depends on pH, ionic strength and several details of sample preparation. This paper makes a review on seven papers of the author with different collaborations, with the original proposal of pores in the DMPG melting transition. The focus is on structural results obtained from X-ray scattering, varying concentration and temperature, but integrating with results from other experimental techniques. Initially the effect of salt addition was separated from the effect of DMPG concentration. At DMPG concentrations higher than 70 mM a lamellar phase starts and a detailed study of the temperature variation of 150 mM DMPG allowed arrival to detailed pore model, focused in the surface fractions of pores in the bilayers. The model admits large and small pores in the melting regime and agrees with the new Lamellar Phase with pores (Lp) starting at T3. The large biological relevance is stressed.

Breaking sensitivity limits of QCM-D: An integrated nano-honeycomb sensor for characterization molecular interactions with lipid bilayers.

In the last few years Quartz Crystal Microbalance with Dissipation (QCM-D) emerged as an indispensable label-free method for monitoring formation of lipid bilayers on surfaces and assessing quality of the bilayers formed. QCM-D provides time-resolved data on mass and viscoelastic properties of molecular layers at the sensor surface and therefore is suitable for detecting vesicle adhesion, rupture, and spreading to form planar surface-supported bilayers. However, the small surface areas of QCM sensors impose severe limitation on mass sensitivity of the method. This is, perhaps, the main reason why QCM-D is rarely employed for studying lipid bilayer phase transitions and membrane binding events. To overcome these limitations, we have integrated the QCM sensor with an array of rigid honeycomb-packed anodic aluminum oxide (AAO) nanopores of ca. 25-80 nm in diameter and 2-10 μm in length by anodizing aluminum film deposited on the electrode surface. The resulting nanostructured QCM sensor increased surface area and improved sensitivity of the method by >100-fold. AAO-QCM electrode also provided a nanoscale template for self-assembly of lipid bilayers into nanotubular structures without any chemical attachment. We then employed such nanopore-confined bilayers to observe the main and more difficult to characterize pre-transition of membranes prepared from either pure 1,2-dimyristoyl-sn-glycerol-3-phosphocholine (DMPC) or 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) lipids. Observed changes in QCM frequency reflected variations in lipid density upon phase transitions and occurred at temperatures in agreement with literature data. The signal-to-noise ratio of the phase transition peaks increased by 300-fold when compared to conventional QCM-D sensors. To further explore potential applications of AAO-QCM-D, ethanol-induced biphasic effects of phospholipid bilayers were studied. High sensitivity of AAO-QCM electrodes makes the method highly attractive for developing biosensing applications based on detecting changes in lipid packing at the nanoscale.

Preparation and Characterization of Trastuzumab Fab-Conjugated Liposomes (Immunoliposomes).

Immunoliposomes are made by conjugating antibodies or antibody fragments on liposome surfaces. Antibody fragments Fab', single-chain Fv fragments (scFv), or new constructs such as nanobodies are commonly used instead of whole IgGs for reduced risk immunogenicity. Here we described the preparation and characterization of immunoliposome-containing trastuzumab Fabs on the surface. The targeting ligand Fab-PEG-DSPE was synthesized by site-specific coupling between the C-terminal cysteine of the Fab and the maleimide group at the distal end of a DSPE-PEG. It was incorporated into preformed liposomes at 60°C above the lipid bilayer phase transition temperature. The binding avidity of the immunoliposomes containing different Fab valencies was characterized using biolayer interferometry.

Improvement of Thermal Stability of Amphipathic Peptide-Phospholipid Nanodiscs via Lateral Association of α-Helices by Disulfide Cross-Linking.

Amphipathic α-helical peptides have been reported to form discoidal particles or nanodiscs with phospholipids, in which a lipid bilayer patch is encircled by peptides. Peptide-based nanodiscs have broad applicability because of their ease of preparation, size flexibility, and structural plasticity. We previously revealed that the nanodiscs formed by apolipoprotein-A-I-derived peptide 18A showed temperature-dependent structural destabilization above the gel-to-liquid-crystalline phase transition temperature of the lipid bilayer. It has been suggested that this destabilization is due to the migration of peptides bound to the edge of the discs to the bilayer surface. In this study, we designed a peptide that could stabilize nanodisc structures against the phase transition of lipid bilayers by disulfide cross-linking of peptides. An 18A-dimer cross-linked by a proline residue, 37pA (Ac-18A-P-18A-CONH2), also showed thermal destabilization of nanodiscs like 18A. However, cross-linking the sides of the two α-helices of the cysteine-substituted analogue 37pA-C2 with disulfide bonds led to the formation of nanodiscs that were more stable to temperature changes. This stabilizing effect was mainly due to the formation of a cyclic 37pA-C2 monomer by intramolecular disulfide cross-linking. These results suggest that the lateral association of two α-helices, which is the basis of the double-belt structure, is an important factor for the implementation of stable nanodiscs. The results of this study will help in development of more stable nanoparticles with membrane proteins in the future.

The molecular design of drug-ionic liquids for transdermal drug delivery: Mechanistic study of counterions structure on complex formation and skin permeation

Though ionic liquids (ILs) as novel enhancers had garnered wide attention, detailed studies elucidating molecular design of drug-ILs were missing and mechanisms of their formation and skin permeation were still lacking. Herein, we systematically investigated effects of counterions structures on formation and skin permeation of drug-ILs. Firstly, effects of counterions on formation of drug-ILs were dependent on polarizability, molecular weight (M.W.) and polar surface area of counterions. It was caused by strong charge assisted hydrogen bond and van der Waals interactions revealed through FT-IR, X-ray photoelectron spectroscopy and molecular docking, which undermined ionic interactions and reduced total interaction strength, thereby produced lower lattice energy. Then, skin permeability of drug-ILs had a good parabola relationship with M.W., polarizability and log P of counterions. The underlying mechanism was the increased drug miscibility with stratum corneum, which caused conformational disorder and phase transition of lipid bilayers characterized by ATR-FTIR, DSC and confocal laser scanning microscopy. Finally, the drug-ILs proved to be non-irritating using in vivo skin erythema analysis. In conclusion, the quantitative structure-activity relationship models based on counterions structure to predict formation and skin permeation of drug-ILs were developed, which provided basic theory for design of drug-ILs with high permeation-enhancing efficiency.

Biomimetic light-activatable graphene-based nanoarchitecture for synergistic chemophotothermal therapy

• Lipid coated biocompatible rGO-mesoporous-silica nanocomposites with was prepared with enhanced hydrophilicity and stability. • The high loading capacity of the nanodrug carriers was proved. • The in vivo antitumor evaluations confirmed the efficacy of nanodrug carriers. The development of biomimetic technology offers excellent opportunities for the construction of therapeutic platforms with enhanced biocompatibility and antitumor performance. In this study, lipid-bilayer-coated reduced graphene oxide (rGO) modified with mesoporous-silica nanosheets was constructed via facile lipid self-assembly for effective synergistic chemophotothermal therapy. These nanodrug carriers were suitable for a high loading rate (53.3%) of doxorubicin (DOX). In vitro , after cellular internalization of these nanosheets, upon near infrared light (NIR) irradiation, the rGO in the nanoarchitecture generated heat not only for hyperthermia treatment but also for triggering lipid bilayer phase transition and on-demand controlled DOX release for chemotherapy. As expected, systematic in vivo antitumor evaluations confirmed the efficient solid tumor removal without recurrence. Our results revealed the excellent integration of the mesoporous-silica-coated rGO and lipid bilayer, which enabled the construction of a platform with the merits of both materials within a single unit. Thus, our results also indicate the great potential for the applications of graphene-based nanomaterials in biomedicine.

A machine learning study of the two states model for lipid bilayer phase transitions.

We have adapted a set of classification algorithms, also known as machine learning, to the identification of fluid and gel domains close to the main transition of dipalmitoyl-phosphatidylcholine (DPPC) bilayers. Using atomistic molecular dynamics conformations in the low and high temperature phases as learning sets, the algorithm was trained to categorise individual lipid configurations as fluid or gel, in relation with the usual two-states phenomenological description of the lipid melting transition. We demonstrate that our machine can learn and sort lipids according to their most likely state without prior assumption regarding the nature of the order parameter of the transition. Results from our machine learning study provide strong support in favour of a two-states model approach of membrane fluidity.

A REVIEW ON PHYSICO-CHEMICAL PARAMETERS OF LIPOSOMAL DOXORUBICIN

The development of a generic liposomal doxorubicin product requires the study of critical physicochemical properties of the formulation. Food and Drug Administration (FDA) draft guideline has suggested few parameters to be tested for in vitro bioequivalence study which include liposomal composition, state of encapsulated drug, internal environment, liposomal morphology and number of lamellae, lipid bilayer phase transition, liposomal size distribution, grafted Polyethylene Glycol (PEG) at liposomal surface, electric surface potential or charge and in vitro leakage under multiple conditions. Characteristic features of components of liposomal doxorubicin formulation and detail of parameters to be studied have been discussed. This review compile specific, current and historical research outcomes on in vitro analysis of liposomal doxorubicin and highlights the important features that have a critical impact on properties of liposomal doxorubicin formulation. It will provide a better insight to the generic manufacturers and will help them to identify the critical quality attributes during the formulation development phase.

Experimental and Molecular Dynamics Simulation Study of the Effects of Lignin Dimers on the Gel-to-Fluid Phase Transition in DPPC Bilayers.

High resolution differential scanning calorimetry (DSC) and molecular dynamics (MD) simulations were used to investigate the effect of three lignin dimers on the gel to fluid phase transition in DPPC lipid bilayers. The goal of this research is to begin to understand the partitioning of model lignin dimers into lipid bilayers and its effects on the gel to fluid transition temperature (Tm). The long-term objective is to establish structure-function relationships for well-defined lignin derivatives at biologically relevant surfaces. This work uses a newly synthesized guiacylglycerol guaiacol ester with a hydroxypropenyl (HOC3H4-) group resembling natural lignin (GG dimer), compared with a truncated GG dimer without the HOC3H4- and benzyl-modified GG dimers. The DSC results show that the dimer most like natural lignin (with a hydroxypropenyl tail) has log K = 2.72 ± 0.05, and MD simulations show that it associates with the headgroups of the lipid but does not penetrate strongly into the interior of the bilayer. Therefore, this dimer has little effect on the Tm value. In contrast, the truncated dimer, which has been used as a representative GG dimer in prior studies, partitions into the bilayer, as seen in MD simulations, and shifts Tm because of its increased lipophilicity (DSC log K = 3.45 ± 0.20). Similarly, modification of the natural GG dimer by benzylation of the phenol makes it lipophilic (DSC log K = 3.38 ± 0.28), causing it to partition into the bilayer, as seen in MD simulations and shift Tm. In MD, we capture the transition from gel to fluid phase by defining and analyzing a normalized deuterium order parameter averaged over all carbon atoms located in the middle of the lipid tails. In this way, the phase transition can be clearly observed and, importantly, MD results show the same trend of transition temperature shifts as the DSC results. Furthermore, we compare partition coefficients estimated from free energy profiles calculated in MD to those obtained from experiment and they are in qualitative agreement. The success at predicting the structural effects of lignin dimers on lipid bilayers suggests that MD simulations can be used in the future to screen the interactions of lignin oligomers and their derivatives with lipid bilayers.

Investigation of phase transitions of saturated phosphocholine lipid bilayers via molecular dynamics simulations

Lipid bilayers play an important role in biological systems as they protect cells against unwanted chemicals and provide a barrier for material inside a cell from leaking out. In this paper, nearly 30 μs of molecular dynamics (MD) simulations were performed to investigate phase transitions of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dipalmitoyl-sn-glycero-phosphocholine (DPPC) lipid bilayers from the liquid crystalline (Lα) to the ripple (Pβ) and to the gel phase (Lβ). Our MD simulations accurately predict the main transition temperature for the single-component bilayers. A key focus of this work is to quantify the structure of the Pβ phase for DMPC and compare with measures from x-ray experiments. The Pβ major arm has similar structure to that of the Lβ, while the thinner minor arm has interdigitated chains and the transition region between these two regions has large chain splay and disorder. At lower temperatures, our MD simulations predict the formation of the Lβ phase with tilted fatty acid chains. The Pβ and Lβ phases are studied for mixtures of DMPC and DPPC and compare favorably with experiment. Overall, our MD simulations provide evidence for the relevancy of the CHARMM36 lipid force field for structures and add to our understanding of the less-defined Pβ phase.

Interpretation of solution scattering data from lipid nanodiscs.

The structural information contained in solution scattering data from empty lipid nanodiscs is examined in the context of a multi-component geometric model. X-ray scattering data were collected on nanodiscs of different compositions at scattering vector magnitudes up to 2.0 Å-1. Through the calculation of the partial form factor for each of the nanodisc components before the isotropic average, structural parameters in the model were correlated to the features observed in the X-ray scattering data and to the corresponding distance distribution function. It is shown that, in general, the features at ∼0.3-0.6 Å-1 in the scattering data correlate to the bilayer structure. The data also support the argument that the elliptical shape of nanodiscs found in model fitting is physical, rather than an artefact due to the nanodisc size distribution. The lipid chain packing peak at ∼1.5 Å-1 is visible in the data and reflects the lipid bilayer phase transition. The shape change in the distance distribution function across the phase transition suggests that the nanodiscs are more circular in the fluid phase. The implication of these findings for model fitting of empty and protein-loaded nanodiscs is discussed.

X-ray scattering reveals molecular tilt is an order parameter for the main phase transition in a model biomembrane.

Synchrotron diffuse x-ray scattering data reveal a dramatic softening of the molecular tilt modulus K_{θ} of the model biomembrane composed of DMPC lipids as the temperature is lowered towards the main phase transition temperature at T_{M}=24^{∘}C. Spontaneous tilt occurs below T_{M}, suggesting that tilt is a symmetry breaking order parameter. Consistent with this hypothesis, it is also found that a different lipid POPS has no spontaneous tilt below its T_{M} at 14^{∘}C and correspondingly its tilt modulus did not soften as T_{M} was approached from above. As previously known, the bending modulus K_{C} of DMPC also softens close to T_{M}, but unlike the tilt modulus, K_{C} has a maximum 3^{∘} above T_{M}, which also marks the limit of the well-known anomalous swelling regime. Tilt adds a different perspective to our previous understanding of the main phase transition in lipid bilayers.

Enhanced Sampling of Phase Transitions in Coarse-Grained Lipid Bilayers.

Freezing and melting of dipalmitoylphosphatidylcholine (DPPC) bilayers are simulated in both the explicit (Wet) and implicit solvent (Dry) coarse-grained MARTINI force fields with enhanced sampling, via the isobaric, molecular dynamics version of the generalized replica exchange method (gREM). Phase transitions are described with the entropic viewpoint, based upon the statistical temperature as a function of enthalpy, TS(H) = 1/(dS(H)/dH), where S is the configurational entropy. Bilayer thickness, area per lipid, and the second-rank order parameter (P2) are calculated vs temperature in the transition range. In a 32-lipid Wet MARTINI system, transitions in the lipid and water subsystems are strongly coupled, giving rise to considerable structure in TS(H) and the need to specify the state of the water when reporting a lipid transition temperature. For gel lipid + liquid water → fluid lipid + liquid water, we find 292.4 K. The small system is influenced by finite-size effects, but it is argued that the entropic approach is well suited to revealing them, which will be particularly relevant for studies of finite nanosystems where there is no thermodynamic limit. In a 390-lipid Dry MARTINI system, two-dimensional analogues of the topographies of coexisting states ("subphases") seen in pure fluids are found. They are not seen in the 32-lipid Wet or Dry system, but the Dry lipids show a new type of state with gel in one leaflet and tilted gel in the other. Dry bilayer transition temperatures are 333.3 K (390 lipids) and 338 K (32 lipids), indicating that the 32-lipid system is not too small for a qualitative study of the transition. Physical arguments are given for Dry lipid system size dependence and for the difference between Wet and Dry systems.

Calorimetry-Derived Composition Vectors to Resolve Component Raman Spectra in Phospholipid Phase Transitions.

Multidimensional least squares analysis is a well-established technique for resolving component vibrational spectra from mixed samples or systems. Component resolution of temperature-dependent vibrational spectra is challenging, however, due to the lack of a suitable model for the variation in sample composition with temperature. In this work, analysis of temperature-dependent Raman spectra of lipid membranes is accomplished by using "concentration" vectors independently derived from enthalpy changes determined by differential scanning calorimetry. Specifically, the lipid-bilayer phase transitions of DMPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) are investigated through Raman spectra acquired from individual, optically trapped vesicles in suspension as a function of temperature. Heat capacity profiles of the same vesicle suspension are measured using differential scanning calorimetry and numerically integrated to generate enthalpy change curves of each phase transition, which are in turn used to construct composition vectors. Multidimensional least squares analysis optimized for a fit to these composition vectors allows resolution of the component spectra corresponding to gel, ripple, and liquid-crystalline phases of the DMPC. The quality of fit of the calorimetry-derived results is confirmed by unstructured residual differences between the data and the model, and a composition variation predicted by the resolved spectra that matches the calorimetry results. This approach to analysis of temperature-dependent spectral data could be readily applied in other areas of materials characterization, where one is seeking to learn about structural changes that occur through temperature-dependent phase transitions.

Photothermally Triggered Lipid Bilayer Phase Transition and Drug Release from Gold Nanorod and Indocyanine Green Encapsulated Liposomes.

In light-activated liposomal drug delivery systems (DDSs), the light sensitivity can be obtained by a photothermal agent that converts light energy into heat. Excess heat increases the drug permeability of the lipid bilayer, and drug is released as a result. In this work, two near-IR responsive photothermal agents in a model drug delivery system are studied: either gold nanorods (GNRs) encapsulated inside the liposomes or indocyanine green (ICG) embedded into the lipid bilayer. The liposome system is exposed to light, and the heating effect is studied with fluorescent thermometers: laurdan and CdSe quantum dots (QDs). Both photothermal agents are shown to convert light into heat in an extent to cause a phase transition in the surrounding lipid bilayer. This phase transition is also proven with laurdan generalized polarization (GP). In addition to the heating results, we show that the model drug (calcein) is released from the liposomal cavity with both photothermal agents when the light power is sufficient to cause a phase transition in the lipid bilayer.

Flexible String Model Analytical Description of Main Phase Transition in Lipid Bilayers

Flexible string model [1,2,3] expanded with van der Waals attraction [4]: VdW= -g/a∧{5/2}, between hydrophobic chains, occupying each an average area a (in dimensionless units) in bilayer plane, is shown to provide analytical description of the main phase transition in lipid bilayers. Single lipid is modeled as a flexible string with a given bending rigidity and a finite incompressible area. Entropic repulsion between fluctuating lipid tails is represented by a parabolic potential, its stiffness parameter, b, is found self-consistenly: ∂F/∂b = ν·(√a-1)², where ν≪1 is small parameter of the theory. The free energy with added van der Waals interaction between adjacent hydrocarbon chains: F = log(b) + ∑log(1 + b/c·4ₙ⁴) + VdW. Here b is dimensionless entropic repulsion parameter, and cₙ coefficients are characteristics of energies of lipid's elementary modes. The theory, being microscopic, allows to study analytically an evolution of membrane properties in the vicinity of the main transition such as temperature dependence of the lateral lipid self-diffusion coefficient. It is also possible to study thermodynamic fluctuations of the lateral pressure near phase transition temperature in the random-phase approximation. [1] S.Mukhin, S.Baoukina. 2005. Analytical derivation of thermodynamic characteristics of lipid bilayer from a flexible string model. Phys. Rev. E. 71: 061918. [2] S. Mukhin, B. Kheyfets. 2010. Analytical Approach to Thermodynamics of Bolalipid Membranes. Phys. Rev. E. 82: 051901. [3] S. Mukhin, B. Kheyfets. 2014. Pore formation phase diagrams for lipid membranes. JETP Lett. 99: 358-362. [4] L. Salem. 1962. Attractive Forces between Long Saturated Chains at Short Distances. J. Chem. Phys. 37: 2100-2113.

Integrating DIY and Traditional Biochemical Laboratory Experiences into an Undergraduate Biophysics Course

Many undergraduate physics majors will go directly to industry after graduation. Incorporating laboratory research into senior level courses facilitates the teaching of interdisciplinary skills such as designing experiments, careful measurement, and troubleshooting experimental difficulties. In addition to these skills hacking and making are also valuable for physics majors seeking employment directly after graduation. Two experiments were utilized to engage students in building these skills. In the first, traditional biochemical studies of phase behavior in lipid membranes was done with fluorescent probes. Fluorescence spectroscopy assays were used to measure main chain phase transitions in lipid bilayers and transbilayer flip-flop rates for phospholipid/cholesterol liposomes. Results from both assay experiments were compiled across teams of students. Each student was also responsible for producing a technical written lab report focusing on the experimental methods and results. In a second experience students attempted to build a Brewster Angle microscope using 3D printed parts and a simple cell phone camera. This project introduced students to CAD software and 3D printing.

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.