Equations of state for POPX lipids at the air/water interface. A comprehensive study

Neutron Scattering Studies of the Effects of Formulating Amphotericin B with Cholesteryl Sulfate on the Drug's Interactions with Phospholipid and Phospholipid-Sterol Membranes

Langmuir surface pressure, small-angle neutron scattering (SANS), and neutron reflectivity (NR) studies have been performed to determine how formulation of the antifungal drug amphotericin B (AmB), with sodium cholesteryl sulfate (SCS)-as in Amphotec-affects its interactions with ergosterol-containing (model fungal cell) and cholesterol-containing (model mammalian cell) membranes. The effects of mixing AmB in 1:1 molar ratio with cholesteryl sulfate (yielding AmB-SCS micelles) are compared against those of free AmB, using monolayers and bilayers formed from palmitoyloleoylphosphatidylcholine (POPC) in the absence and presence of 30 mol % ergosterol or cholesterol, in all cases employing a 1:0.05 molar ratio of lipid:AmB. Analyses of the (bilayer) SANS and (monolayer) NR data indicate that the equilibrium changes in membrane structure induced in sterol-free and sterol-containing membranes are the same for free AmB and AmB-SCS. Stopped-flow SANS experiments, however, reveal that the structural changes to vesicle membranes occur far more rapidly following exposure to AmB-SCS vs free drug, with the kinetics of these changes varying with membrane composition. With POPC vesicles, the structural changes induced by AmB-SCS become apparent only after several minutes, and equilibrium is reached after ∼30 min. The corresponding onset of changes in POPC-ergosterol and POPC-cholesterol vesicles, however, occurs within ∼5 s, with equilibrium reached after 10 and 120 s, respectively. The rate of insertion of AmB into POPC-sterol membranes is thus increased through formulation as AmB-SCS. Moreover, the differences in monolayer surface pressure and SANS structure-change equilibration times suggest significant rearrangement of AmB within these membranes following insertion. The reduced times to equilibrium for the POPC-ergosterol vs POPC-cholesterol systems are consistent with the known differences in affinity of AmB for these two sterols, and the reduced time to equilibrium for AmB-SCS interaction with POPC-ergosterol membranes vs that for free AmB is consistent with the reduced host toxicity of Amphotec.

Interaction of amphotericin B with lipid monolayers

Langmuir isotherm, neutron reflectivity, and Brewster angle microscopy experiments have been performed to study the interaction of amphotericin B (AmB) with monolayers prepared from 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and mixtures of this lipid with cholesterol or ergosterol to mimic mammalian and fungal cell membranes, respectively. Isotherm data show that AmB causes a more pronounced change in surface pressure in the POPC/ergosterol system than in the POPC and POPC/cholesterol systems, and its interaction with the POPC/ergosterol monolayer is also more rapid than with the POPC and POPC/cholesterol monolayers. Brewster angle microscopy shows that, in interaction with POPC monolayers, AmB causes the formation of small domains which shrink and disappear within a few minutes. The drug also causes domain formation in the POPC/cholesterol and POPC/ergosterol monolayers; in the former case, these are formed more slowly than is seen with the POPC monolayers and are ultimately much smaller; in the latter case, they are formed rather more quickly and are more heterogeneous in size. Neutron reflectivity data show that the changes in monolayer structure following interaction with AmB are the same for all three systems studied: the data are consistent with the drug inserting into the monolayers with its macrocyclic ring intercalated among the lipid acyl chains and sterol ring systems, with its mycosamine moiety colocalizing with the sterol hydroxyl and POPC head groups. On the basis of these studies, it is concluded that AmB inserts in a similar manner into POPC, POPC/cholesterol, and POPC/ergosterol monolayers but does so with differing kinetics and with the formation of quite different in-plane structures. The more rapid time scale for interaction of the drug with the POPC/ergosterol monolayer, its more pronounced effect on monolayer surface pressure, and its more marked changes as regards domain formation are all consistent with the drug's selectivity for fungal vs mammalian cell membranes.

The Conventional Langmuir Trough Technique as a Convenient Means to Determine the Solubility of Sparingly Soluble Surface Active Molecules: Case Study Cholesterol

The feasibility of a method based on mass preservation [G. Schwarz, J. Zhang, Chem. Phys. Lipids, 110 (2001) 35-45] to determine the solubility of Cholesterol in water from monomolecular films on air/water interface was investigated. Using a mass balance equation, it was found that Cholesterol undergoes an exponential desorption at very low surface pressures followed by an almost linear desorption into the subphase at higher surface pressures until monolayer collapse. Processing of the surface pressure measurements as a function of trough area in accord with the theory, enabled the accurate determination of the molecular dimensions of Cholesterol as a function of surface pressure. Slight modification of the theory enabled accurate quantification of the surface pressure-independent apparent solubility of Cholesterol and the amount of Cholesterol desorbed into the subphase as a function of surface pressure, in the nanomolar range.

Interactions of amphotericin B derivative of low toxicity with biological membrane components—the Langmuir monolayer approach

Amphotericin B (AmB)--a polyene macrolide antibiotic--exhibits strong antifungal activity, however, is known to be very toxic to mammalian cells. In order to decrease AmB toxicity, a number of its derivatives have been synthesized. Basing on in vitro and in vivo research, it was evidenced that one of AmB derivatives, namely N-methyl-N-D-fructopyranosylamphotericin B methyl ester (in short MF-AME) retained most of the antifungal activity of the parent antibiotic, however, exhibited dramatically lower animal toxicity. Therefore, MF-AME seems to be a very promising modification product of AmB. However, further development of this derivative as potential new antifungal drug requires the elucidation of its molecular mechanism of reduced toxicity, which was the aim of the present investigations. Our studies were based on examining the binding energies by determining the strength of interaction between MF-AME and membrane sterols (ergosterol-fungi sterol, and cholesterol-mammalian sterol) and DPPC (model membrane phospholipid) using the Langmuir monolayer technique, which serves as a model of cellular membrane. Our results revealed that at low concentration the affinity of MF-AME to ergosterol is considerably stronger as compared to cholesterol, which correlates with the improved selective toxicity of this drug. It is of importance that the presence of phospholipids is essential since--due to very strong interactions between MF-AME and DPPC--the antibiotic used in higher concentration is "immobilized" by DPPC molecules, which reduces the concentration of free antibiotic, thus enabling it to selectively interact with both sterols.

Examination of bilayer lipid membranes for ‘pin-hole’ character

BLM prepared on electrode substrates by supporting or tethering were tested for 'pin-hole' character, comparing data from cyclic voltammetry (CV), surface plasmon resonance (SPR) and rotating disc electrodes (RDE). 1-hexadecylamine tethered BLMs on SAM modified gold electrodes were compared with BLMs assembled on modified polyHEMA or sol-gel layers. BLM formation followed by SPR showed that the initial phase of the assembly was complete in 5-20 minutes and produced layers of thickness >5 nm, compared with the expected final BLM thickness of approximately 3 nm. The CVs of the K(3)[Fe(CN)(6)] couple were significantly suppressed irrespective of the method of BLM assembly, without major differences emerging for the different methods. However, data from the RDE distinguished the 'pin-hole' character of the different preparations. The data were consistent with incomplete initial (<1 h, SPR estimated BLM thickness >5 nm) vesicle fusion leaving 'pin-holes' of approximately 2 microm (HDA-11-mercaptoundecanoic acid (MUA) tethered BLM) to approximately 3 microm (tetraethylorthosilicate sol-gel supported BLM) followed by a slow maturation (>15 h; impedance spectroscopy estimated thickness approximately 3 nm) and lateral spreading and fusion, resulting in loss of 'pin-hole' character (<1 microm). The BLM could be used in conjunction with potentiometric measurement to observe the incorporation of nystatin into the BLM and the rate of incorporation adjusted according to original permeability of the BLM. The 'pin-hole-free' BLM construction with lowest permeability (TEOS supported, 4 x 10(-10) cm s(-1) compared with HDA-MUA, 3 x 10(-9) cm s(-1)) gave a potentiometric signal independent of bulk ion-concentration across 5 decades change in concentration. Formed on an ion-selective electrode, nystatin incorporation could be followed as a change in potential, over >2 h, whereas the TEOS supported BLM with permeability 1 x 10(-9) cm s(-1) shows nystatin incorporation within 1 h. In this instance, addition of ConA reduced the potential to the same value as prior to nystatin incorporation, consistent with nystatin channel closure.

A long-lived amphiphilic fluorescent probe studied in POPC air-water monolayer and solution bilayer systems

A novel amphiphilic fluorescent probe (Fluorazophore-L) with a strongly dipolar, nonionic azoalkane as headgroup and a palmitoyl tail has been synthesized and characterized. Pure Fluorazophore-L was found to be sufficiently amphiphilic to form stable air-water monolayers. An analysis of the surface pressure versus area suggests an area per molecule of about 34+/-2 A(2) at 29 mN m(-1). The partitioning into a lipid membrane model was quantified at the air-water interface by spreading 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) monolayers. Measurements with different molar fractions of Fluorazophore-L revealed a small but significant reduction of the mean area in the mixed monolayer. The excess free energy of mixing (-0.5+/-0.1 kT) indicated a weakly attractive interaction slightly above thermal energy, suggesting a good miscibility of the fluorescent probe within the lipid monolayer without major structural modifications. Spectroscopic measurements confirmed the incorporation of Fluorazophore-L into POPC vesicles. The fluorescence lifetime was very long (125+/-5 ns under air) with monoexponential fluorescence decays.

Chain length dependence of lipid partitioning between the air/water interface and its subphase: thermodynamic and structural implications

We have investigated phosphatidylcholines with the same two saturated hydrocarbon chains of 12, 10 and 8 C-atoms. Langmuir trough data could be evaluated towards even small lipid subphase desorption when applying a novel approach that had recently been developed in our laboratory. The C12 lipid turned out to form a nearly insoluble monolayer with slight desorption only beyond 15 mN/m for an area/volume ratio around 1 cm(-1). Above 22 mN/m micellation in the subphase apparently terminates further accumulation in the interface forcing additionally added lipid to enter the bulk volume. A comparatively substantial increase of solubilization was observed for the C10 monolayer. When turning to the C8 lipid partitioning proved to take place in nearly equal parts. In that case, strong multimeric aggregation is indicated to occur in both the interfacial and the bulk volume domains. All the results are quantitatively discussed in the light of basic thermodynamic and structural considerations.

A universal thermodynamic approach to analyze biomolecular binding experiments

Binding processes of any kind can be characterized as an association of a given ligand with some binding factor. This includes macromolecules as well as supramolecular aggregates such as micelles or membranes. The underlying molecular binding mechanism may be more or less complicated due to various intermediate steps (involving for instance conformational changes, aggregation, cooperativity, etc.). A sensible discussion of possible binding models naturally calls for a model-independent access to basic thermodynamic properties. The present contribution will demonstrate how this can quite generally be accomplished by a pertinent processing of properly selected experimental data. The method requires a series of titration measurements comprising the use of variable amounts of both the ligand and the binding factor. It leads to a linear mass conservation plot (i.e. amount of the ligand vs. a matching amount of the binding factor) whose slope and ordinate intercept are equal to the binding ratio (i.e. bound ligand per binding factor) and the free ligand concentration, respectively. This establishes the specific binding isotherm. The approach also reveals latent structurally determined features of the applied physical measuring signal. A number of examples including specific binding, unspecific adsorption and insertion in two-dimensional molecular films will illustrate the methodology.