Runguang Sun

The behavior of the adsorption of cytochrome C on lipid monolayers: A study by the Langmuir-Blodgett technique and theoretical analysis.

Cytochrome c (Cyt c) is an essential component of the inner mitochondrial respiratory chain because of its function of transferring electrons. The feature is closely related to the interaction between Cyt c and membrane lipids. We used Langmuir-Blodgett monolayer technique combined with AFM to study the interaction of Cyt c with lipid monolayers at air-buffer interface. In our work, by comparing the mixed Cyt c-anionic (DPPS) and Cyt c-zwitterionic (DPPC/DPPE) monolayers, the adsorption capacity of Cyt c on lipid monolayers is DPPS>DPPE>DPPC, which is attributed to their different headgroup structures. π-A isothermal data show that Cyt c (v=2.5 μL) molecules are at maximum adsorption quantity on lipid monolayer. Moreover, Cyt c molecules would form aggregations and drag some lipids with them into subphase if the protein exceeds the maximum adsorption quantity. π-T curve indicates that it takes more time for Cyt c molecular conformation to rearrange on DPPE monolayer than on DPPC. The compressibility study reveals that the adsorption or intermolecular aggregation of Cyt c molecules on lipid monolayer will change the membrane fluidization. In order to quantitatively estimate Cyt c molecular adsorption properties on lipid monolayers, we fit the experimental isotherm with a simple surface state equation. A theoretical model is also introduced to analyze the liquid expanded (LE) to liquid condensed (LC) phase transition of DPPC monolayer. The results of theoretical analysis are in good agreement with the experiment.

Interfacial interactions and nanostructure changes in DPPG/HD monolayer at the air/water interface

Lung surfactant (LS) plays a crucial role in regulating surface tension during normal respiration cycles by decreasing the work associated with lung expansion and therefore decreases the metabolic energy consumed. Monolayer surfactant films composed of a mixture of phospholipids and spreading additives are of optional utility for applications in lung surfactant-based therapies. A simple, minimal model of such a lung surfactant system, composed of 1,2-dipalmitoyl-sn-glycero-3-[phosphor-rac-(1-gylcerol)] (DPPG) and hexadecanol (HD), was prepared, and the surface pressure-area (π-A) isotherms and nanostructure characteristics of the binary mixture were investigated at the air/water interface using a combination of Langmuir-Blodgett (LB) and atomic force microscopy (AFM) techniques. Based on the regular solution theory, the miscibility and stability of the two components in the monolayer were analyzed in terms of compression modulus (Cs-1), excess Gibbs free energy (Δexcπ), activity coefficients (γ), and interaction parameter (ξ). The results of this paper provide valuable insight into basic thermodynamics and nanostructure of mixed DPPG/HD monolayers; it is helpful to understand the thermodynamic behavior of HD as spreading additive in LS monolayer with a view toward characterizing potential improvements to LS performance brought about by addition of HD to lung phospholipids.

A Langmuir and AFM study on interfacial behavior of binary monolayer of hexadecanol/DPPE at the air-water interface

Hexadecanol is chemically stable and can be used as an effective addition in synthetic clinical lung surfactant preparations to improve their spreading properties. In this work, a detailed thermodynamic and structural characterization of a simple model system, which based on a hexadecanol-phospholipid mixture is reported. Langmuir monolayers of binary mixtures of hexadecanol/1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) have been studied with thermodynamic parameters and monolayer structure. The extent of the thermodynamic parameters has been evaluated by the direct related parameters, such as mean molecular area, percent of condensation, surface excess Gibbs free energies, isothermal compressibility, interaction energy parameter, activity coefficient and two-dimensional phase diagram. Monolayer film structure has been characterized by atomic force microscopy (AFM) technique. Combining thermodynamic and AFM data indicate that there exist repulsive and attractive interactions between the two film forming molecules and the binary films behave as non-ideal mixtures, which can be portrayed by the mole fraction of hexadecanol. At low mole fraction of hexadecanol, the monolayer is phase-separated and the interactions between hexadecanol and DPPE is more repulsive; while the content of hexadecanol up to 0.6, the monolayer becomes miscible and stable, the interaction between different molecules is more attractive. The addition of hexadecanol in the DPPE monolayer clearly affects the lateral organization of membranes and improves its surface tension kinetics. The results discussed in this context will be expected to be potential contribution for exogenous lung surfactant researches.

Behavior of lysozyme adsorbed onto biological liquid crystal lipid monolayer at the air/water interface*

The interaction between proteins and lipids is one of the basic problems of modern biochemistry and biophysics. The purpose of this study is to compare the penetration degree of lysozyme into 1,2-diapalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphoethano-lamine (DPPE) by analyzing the data of surface pressure–area (π–A) isotherms and surface pressure–time (π–T) curves. Lysozyme can penetrate into both DPPC and DPPE monolayers because of the increase of surface pressure at an initial pressure of 15 mN/m. However, the changes of DPPE are larger than DPPC, indicating stronger interaction of lysozyme with DPPE than DPPC. The reason may be due to the different head groups and phase state of DPPC and DPPE monolayers at the surface pressure of 15 mN/m. Atomic force microscopy reveals that lysozyme was absorbed by DPPC and DPPE monolayers, which leads to self-aggregation and self-assembly, forming irregular multimers and conical multimeric. Through analysis, we think that the process of polymer formation is similar to the aggregation mechanism of amyloid fibers.

Analysis of the induction of the myelin basic protein binding to the plasma membrane phospholipid monolayer

Myelin basic protein (MBP) is an essential structure involved in the generation of central nervous system (CNS) myelin. Myelin shape has been described as liquid crystal structure of biological membrane. The interactions of MBP with monolayers of different lipid compositions are responsible for the multi-lamellar structure and stability of myelin. In this paper, we have designed MBP-incorporated model lipid monolayers and studied the phase behavior of MBP adsorbed on the plasma membrane at the air/water interface by thermodynamic method and atomic force microscopy (AFM). By analyzing the pressure–area (π–A) and pressure–time (π–T) isotherms, univariate linear regression equation was obtained. In addition, the elastic modulus, surface pressure increase, maximal insertion pressure, and synergy factor of monolayers were detected. These parameters can be used to modulate the monolayers binding of protein, and the results show that MBP has the strongest affinity for 1,2-dipalmitoyl-sn-glycero-3- phosphoserine (DPPS) monolayer, followed by DPPC/DPPS mixed and 1,2-dipalmitoyl-sn-glycero-3-phospho-choline (DPPC) monolayers via electrostatic and hydrophobic interactions. AFM images of DPPS and DPPC/DPPS mixed monolayers in the presence of MBP (5 nM) show a phase separation texture at the surface pressure of 20 mN/m and the incorporation of MBP put into the DPPC monolayers has exerted a significant effect on the domain structure. MBP is not an integral membrane protein but, due to its positive charge, interacts with the lipid head groups and stabilizes the membranes. The interaction between MBP and phospholipid membrane to determine the nervous system of the disease has a good biophysical significance and medical value.

Influence of K+, Na+ or Ca2+ ions on the interaction between AmB and saturated phospholipids by Langmuir technique

Interaction between AmB and cell membrane is influenced by different metal cations. In the presence of K+, Na+ or Ca2+ ions, the surface pressure-area isotherms and the elastic modulus of an amphotericin-dipalmitoyl-phosphatidylcholine(AmB-DPPC) mixed monolayer were discussed. And the excess free energy and entropies of mixing were calculated according to the surface pressure-area isotherms. The phase transition of the mixed monolayer needed a higher concentration of AmB in the sequence Na+>pure buffer>K+>Ca2+. When the molar fraction of AmB(xAmB) was 0.5, the molecular interaction changed from attraction to repulsion and the mixed monolayer turned to ordered state from disorder state under the induction of K+ or Ca2+ ions at all surface pressure in our experiment. At high surface pressure, the disorder of monolayer enhanced in the presence of Na+ ions at xAmB>0.1. At different molar ratios of AmB, the influences of these metal cations were discrepant. These cations may influence AmB molecules to form pores on the monolayer. It is helpful to understand the reduction of AmB’s toxicity as theoretical reference.

The mechanism of the interaction between curcumin and bovine serum albumin using fluorescence spectrum

The interaction between curcumin (CUR) and bovine serum albumin (BSA) in physiological buffer (pH 7.4) was investigated by fluorescence and UV-vis absorption spectroscopy at 298, 306 and 313 K. The results revealed that CUR could strongly quench the intrinsic fluorescence of BSA through a static quenching procedure. The binding constant K and number of binding sites n of CUR with BSA were measured by fluorescence quenching method. The thermodynamic parameters, enthalpy change (ΔH) and entropy change (ΔS), were calculated to be–64.11 kJ mol–1 < 0 and–95.53 J mol–1 K–1 < 0, which respectively indicated that the interaction of CUR with BSA was driven mainly by the van der Waals force or hydrogen bond formation. The UV and AFM results found that the CUR and BSA could interact to form complex structures.