Spectroscopic studies of molecular organization of antibiotic amphotericin B in monolayers and dipalmitoylphosphatidylcholine lipid multibilayers

Amphotericin B (AmB) is considered the gold-standard in the treatment of serious systemic mycoses despite its numerous adverse effects. Both the mechanism of antifungal action and the toxicity of this drug are dependent on its molecular organization. The effect of AmB on the organization of lipid membranes formed with dipalmitoylphosphatidylcholine (DPPC) was studied with application of the Langmuir-Blodgett technique and ATR-FTIR spectroscopy. The aim of this research was to analyze the physical interactions leading to the formation of aggregated forms of AmB molecules in one-component monolayers and lipid multibilayers. Analysis of FTIR spectra of two-component multibilayers suggests the possibility the mutual reorientation of the amino-sugar moiety (mycosamine) and macrolide ring. This effect may be significant in the explanation of the aggregation processes of AmB in biological systems.     

Phase behavior of lipid monolayers containing DPPC and cholesterol analogs

We investigate the miscibility phase behavior of lipid monolayers containing a wide variety of sterols. Six of the sterols satisfy a definition from an earlier study of "membrane-active sterols" in bilayers (cholesterol, epicholesterol, lathosterol, dihydrocholesterol, ergosterol, and desmosterol), and six do not (25-hydroxycholesterol, lanosterol, androstenolone, coprostanol, cholestane, and cholestenone). We find that monolayers containing dipalmitoyl phosphatidylcholine mixed with membrane-active sterols generally produce phase diagrams containing two distinct regions of immiscible liquid phases, whereas those with membrane-inactive sterols generally do not. This observation establishes a correlation between lipid monolayers and bilayers. It also demonstrates that the ability to form two regions of immiscibility in monolayers is not one of the biophysical attributes that explains cholesterol's predominance in animal cell membranes. Furthermore, we find unusual phase behavior for dipalmitoyl phosphatidylcholine monolayers containing 25-hydroxycholesterol, which produce both an upper and a lower miscibility transition. The lower transition correlates with a sharp change of slope in the pressure-area isotherm.     

Comparative molecular dynamics study of lipid membranes containing cholesterol and ergosterol

Sterol molecules are essential for maintaining the proper structure and function of eukaryotic cell membranes. The influence of cholesterol (the principal sterol of higher animals) on the lipid bilayer properties was extensively studied by both experimental and simulation methods. In contrast, the effect of ergosterol (the principal fungal sterol) on the membrane structure and dynamics is much less recognized. This work presents the results of comparative molecular dynamics simulation of the hydrated dimyristoylphosphatidylcholine bilayer containing approximately 25 mol % of cholesterol or ergosterol. A detailed analysis of the molecular properties (e.g., bilayer thickness, lipid order, diffusion, intermolecular interactions, etc.) of both sterol-induced liquid-ordered membrane phases is presented. Presence of sterols in the membrane significantly changes its property, especially fluidity and molecular packing. Moreover, in accordance with the experiments, our calculations show that, compared to cholesterol, ergosterol has higher ordering effect on the phospholipid acyl chains. This different influence on the properties of the lipid bilayer stems from differences in conformational freedom of sterol side chains. Additionally, obtained models of lipid membranes containing human and fungal sterols, constituting the result of our work, can be also utilized in other chemotherapeutic studies on interaction of selected ligands (e.g., antifungal compounds) with membranes.     

Recent progress in the study of the interactions of amphotericin B with cholesterol and ergosterol in lipid environments

In the past decade substantial progress has been made in understanding the organization and biological activity of amphotericin B (AmB) in the presence of sterols in lipid environments. This review concentrates mainly on interactions of AmB with lipids and sterols, AmB channel formation in membranes, AmB aggregation, AmB modifications important for understanding its biological activity, and AmB models explaining its mechanism of action. Most of the reviewed studies concern monolayers at the water–gas interface, monolayers deposited on a solid substrate by use of the Langmuir–Blodgett technique, micelles, vesicles, and multi-bilayers. Liposomal AmB formulations and drug delivery are intentionally omitted, because several reviews dedicated to this subject are already available.     

Molecular organization of antibiotic amphotericin B in dipalmitoylphosphatidylcholine monolayers induced by K(+) and Na(+) ions: the Langmuir technique study

The effect of potassium (K(+)) and sodium (Na(+)) ions on the self-association of antibiotic amphotericin B (AmB) in the lipid membrane was reported. Mixed Langmuir monolayers of AmB and dipalmitoylphosphatidylcholine (DPPC) were investigated by recording surface pressure-area isotherms spread on aqueous buffers containing physiological concentration of K(+) and Na(+) ions. The analyses of the π-A isotherms and compressional modulus curves indicate the interactions in the AmB-DPPC system. The strength of the AmB-DPPC interactions and the stability of the mixed monolayers were examined on the basis of the excess free energy of mixing values. The obtained results proved a high affinity of AmB towards lipids induced by the presence of K(+) than Na(+) ions. The most stable monolayers in the presence of K(+) and Na(+) ions were formed by AmB and DPPC with the 1:1 and 2:1 stoichiometry. The understanding of the AmB aggregation processes at the molecular level should contribute to elucidate the mechanisms of action and toxicity of this widely used drug. The presented results are potentially valuable in respect to develop more efficient and less toxic AmB formulations.     

Interactions of amphotericin B derivatives with lipid membranes--a molecular dynamics study

Amphotericin B (AmB) is a well-known polyene macrolide antibiotic used to treat systemic fungal infections. AmB targets more efficiently fungal than animal membranes. However, there are only minor differences in the mode of action of AmB against both types of membranes, which is a source of AmB toxicity. In this work, we analyzed interactions of two low toxic derivatives of AmB (SAmE and PAmE), synthesized in our laboratory, with lipid membranes. Molecular dynamics simulations of the lipid bilayers containing ergosterol (fungal cells) or cholesterol (animal cells) and the studied antibiotic molecules were performed to compare the structural and dynamic properties of AmB derivatives and the parent drug inside the membrane. A number of differences was found for AmB and its derivatives' behavior in cholesterol- and ergosterol-containing membranes. We found that PAmE and SAmE can penetrate deeper into the hydrophobic region of the membrane compared to AmB. Modification of the amino and carboxyl group of AmB also resulted in the conformational transition within the antibiotic's polar head. Wobbling dynamics differentiation, depending on the sterol present, was discovered for the AmB derivatives. These differences may be interpreted as molecular factors responsible for the improved selectivity observed macroscopically for the studied AmB derivatives.     

Thermodynamic and spectroscopic features of the behavior of amphotericin B in aqueous medium

The interaction between amphotericin B molecules in aqueous medium solution was studied using absorption and circular dichroism approaches. The results showed that at concentrations below 1 microM of amphotericin B, an equilibrium between the monomer and aggregate occurred with a constant of approximately 0.6x10(6) M(-1). The aggregate formation constant was dependent on the experimental conditions of the medium: its value increased at acidic pH values, while alkaline medium induced the equilibrium displacement to the monomer formation. Either neutral salts or chaotropic agents such as urea prevented the formation of the aggregate. The presence of net electrical charge on the amine and carboxyl groups plays a role in the thermodynamic stability of the aggregate. A hydrophobic effect was also found between the monomer form and the water molecules of neighbours. In the aggregate formation water molecules were released contributing to an increase in the entropic change.     

Effects of cholesterol on the structure and collapse of DPPC monolayers

Cholesterol induces faster collapse by compressed films of pulmonary surfactant. Because collapse prevents films from reaching the high surface pressures achieved in the alveolus, most therapeutic surfactants remove or omit cholesterol. The studies here determined the structural changes by which cholesterol causes faster collapse by films of dipalmitoyl phosphatidylcholine, used as a simple model for the functional alveolar film. Measurements of isobaric collapse, with surface pressure held constant at 52 mN/m, showed that cholesterol had little effect until the mol fraction of cholesterol, X_chol, exceeded 0.20. Structural measurements of grazing incidence X-ray diffraction at ambient laboratory temperatures and a surface pressure of 44 mN/m, just below the onset of collapse, showed that the major structural change in an ordered phase occurred at lower X_chol. A centered rectangular unit cell with tilted chains converted to an untilted hexagonal structure over the range of X_chol = 0.0–0.1. For X_chol = 0.1–0.4, the ordered structure was nearly invariant; the hexagonal unit cell persisted, and the spacing of the chains was essentially unchanged. That invariance strongly suggests that above X_chol = 0.1, cholesterol partitions into a disordered phase, which coexists with the ordered domains. The phase rule requires that for a binary film with coexisting phases, the stoichiometries of the ordered and disordered regions must remain constant. Added cholesterol must increase the area of the disordered phase at the expense of the ordered regions. X-ray scattering from dipalmitoyl phosphatidylcholine/cholesterol fit with that prediction. The data also show a progressive decrease in the size of crystalline domains. Our results suggest that cholesterol promotes adsorption not by altering the unit cell of the ordered phase but by decreasing both its total area and the size of individual crystallites.     

Binding of antibiotic amphotericin B to lipid membranes: a 1H NMR study

The (1)H NMR technique was applied to study binding of AmB, an antifungal drug, to lipid membranes formed with egg yolk phosphatidylcholine. The analysis of (1)H NMR spectra of liposomes, containing also cholesterol and ergosterol (at 40 mol%), shows that AmB binds preferentially to the polar headgroups. Such a binding restricts molecular motion of the choline fragment in the hydrophilic region at the surface of liposomes but increases the segmental motional freedom in the hydrophobic core. The same effects are also observed in the sterol-containing membranes, except that the effect on the hydrophobic core was exclusively observed in the membranes containing ergosterol.     

Interactions of the drug amphotericin B with phospholipid membranes containing or not ergosterol: new insight into the role of ergosterol

Amphotericin B (AmB) is an amphipathic polyene antibiotic which permeabilizes ergosterol-containing membranes, supposedly by formation of pores. In water, AmB forms chiral aggregates, modelled as stacks of planar dimers in which the joined polyene chains in each dimer turn round, from one dimer to the following in these stacks, by forming a helical array. Studies of the binding of AmB with L-dipalmitoylphosphatidylcholine (L-DPPC) and L-dilauroylphosphatidylcholine (L-DLPC) bilayers disclose the main following results. (1) An inversion of the helicity of the L-DPPC-bound AmB aggregates, when the L-DPPC bilayers are in the gel phase, is inferred from the evolution of the circular dichroism spectra of AmB+L-DPPC mixtures. (2) An AmB-induced gel-to-subgel transformation of L-DPPC bilayers, in the previous mixtures, is revealed by a differential scanning calorimetry study. (3) The role played by ergosterol in the location of phospholipid-bound AmB aggregates with respect to a phospholipid bilayer is directly demonstrated from atomic force microscopy observations of mica-supported AmB+L-DLPC mixtures, in the presence or absence of ergosterol. While in the absence of ergosterol AmB aggregates remained at the surface of the bilayer, in the presence of ergosterol they appeared embedded within this bilayer and became hollow-centered. As such an embedding in the hydrophobic core of a bilayer requires a rearrangement of the aggregates with respect to their architecture in water, this rearrangement is held responsible for the hollowing of aggregates. The hollow-centered sublayer-embedded AmB aggregates are thought to be the precursors of the formation of AmB pores.     

Interactions of amphotericin B derivatives with lipid membranes—A molecular dynamics study

Amphotericin B (AmB) is a well-known polyene macrolide antibiotic used to treat systemic fungal infections. AmB targets more efficiently fungal than animal membranes. However, there are only minor differences in the mode of action of AmB against both types of membranes, which is a source of AmB toxicity. In this work, we analyzed interactions of two low toxic derivatives of AmB (SAmE and PAmE), synthesized in our laboratory, with lipid membranes. Molecular dynamics simulations of the lipid bilayers containing ergosterol (fungal cells) or cholesterol (animal cells) and the studied antibiotic molecules were performed to compare the structural and dynamic properties of AmB derivatives and the parent drug inside the membrane. A number of differences was found for AmB and its derivatives' behavior in cholesterol- and ergosterol-containing membranes. We found that PAmE and SAmE can penetrate deeper into the hydrophobic region of the membrane compared to AmB. Modification of the amino and carboxyl group of AmB also resulted in the conformational transition within the antibiotic's polar head. Wobbling dynamics differentiation, depending on the sterol present, was discovered for the AmB derivatives. These differences may be interpreted as molecular factors responsible for the improved selectivity observed macroscopically for the studied AmB derivatives.     

Interfacial behavior of cholesterol, ergosterol, and lanosterol in mixtures with DPPC and DMPC

Binary mixtures of cholesterol, ergosterol, and lanosterol with phosphatidylcholines differing in the length of the saturated acyl chains, viz 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-myristoyl-sn-glycero-3-phosphocholine (DMPC), were analyzed using a Langmuir balance for recording force-area (pi-A) and surface potential-area (psi-A) isotherms. A progressive disappearance of the liquid expanded-liquid condensed transition was observed in mixed monolayers with DPPC after the increase in the content of all three sterols. For fluid DMPC matrix, no modulation of the monolayer phase behavior due to the sterols was evident with the exception of lanosterol, for which a pronounced discontinuity between mole fractions of X = 0.3 and X = 0.75 was discernible in the compression isotherms. Condensing and expanding effects in force-area (pi-A) isotherms due to varying X(sterols) and differences in the monolayer physical state were assessed from the values for the interfacial compression moduli. Surface potential measurements support the notion that cholesterol and ergosterol, but not lanosterol, reduce the penetration of water into the lipid monolayers. Examination of the excess free energy of mixing revealed an enhanced stability of binary monolayers containing cholesterol compared to those with ergosterol or lanosterol; the differences are emphasized in the range of surface pressure values found in natural membranes.     

Interactions of the drug amphotericin B with phospholipid membranes containing or not ergosterol: new insight into the role of ergosterol.

Amphotericin B (AmB) is an amphipathic polyene antibiotic which permeabilizes ergosterol-containing membranes, supposedly by formation of pores. In water, AmB forms chiral aggregates, modelled as stacks of planar dimers in which the joined polyene chains in each dimer turn round, from one dimer to the following in these stacks, by forming a helical array. Studies of the binding of AmB with L-dipalmitoylphosphatidylcholine (L-DPPC) and L-dilauroylphosphatidylcholine (L-DLPC) bilayers disclose the main following results. (1) An inversion of the helicity of the L-DPPC-bound AmB aggregates, when the L-DPPC bilayers are in the gel phase, is inferred from the evolution of the circular dichroism spectra of AmB+L-DPPC mixtures. (2) An AmB-induced gel-to-subgel transformation of L-DPPC bilayers, in the previous mixtures, is revealed by a differential scanning calorimetry study. (3) The role played by ergosterol in the location of phospholipid-bound AmB aggregates with respect to a phospholipid bilayer is directly demonstrated from atomic force microscopy observations of mica-supported AmB+L-DLPC mixtures, in the presence or absence of ergosterol. While in the absence of ergosterol AmB aggregates remained at the surface of the bilayer, in the presence of ergosterol they appeared embedded within this bilayer and became hollow-centered. As such an embedding in the hydrophobic core of a bilayer requires a rearrangement of the aggregates with respect to their architecture in water, this rearrangement is held responsible for the hollowing of aggregates. The hollow-centered sublayer-embedded AmB aggregates are thought to be the precursors of the formation of AmB pores.     

Interaction of amphotericin B with monolayers of egg lecithin and cholesterol: polorized absorption spectra

Polarized absorption spectra have been obtained of the antibiotic polyene, amphotericin B, interacting with monolayers of egg lecithin, cholesterol and equimolar egg lecithin-cholesterol at low and high surface pressures. An expression is derived which enables the determination from the polarization data of the orientations of the transition moments of the polyene absorption bands at 4077 and 3645 Å. For some of the systems the 3645-Å band is replaced by a previously unreported band appearing between 3610 and 3632 Å. The orientation of the 4077-Å transition moment (parallel to the long molecular axis) is found to vary from an angle of 64 ° with the surface for the low-pressure monolayers of cholesterol to 21 ° for the high-pressure films of egg lecithin-cholesterol. For the band between 3610 Å and 3645 Å, the angle varies from 90 ° for cholesterol to 18 ° for the high-pressure mixed-lipid film. It is found that a large increase in surface pressure of the cholesterol and egg lecithin-cholesterol monolayers causes a decrease in the angle of the 4077-Å moment for both films and that of the higher energy moment for the mixed film. Increasing the content of cholesterol in these monolayers rotates the orientation of the transition moments for both bands toward the surface normal, the change being greatest for the low-pressure films. The effectiveness of amphotericin B in lowering the surface tension of these lipid monolayers is related to its binding, orientation and extent of penetration. For low-pressure cholesterol films where the surface interaction with the polyene is greatest, the binding and penetration are large and the polyene molecule is oriented with its long dimension nearly perpendicular to the surface.     

Spectrophotometric analysis of organisation of dipalmitoylphosphatidylcholine bilayers containing the polyene antibiotic amphotericin B

Amphotericin B (AmB) is a polyene antibiotic widely used in the treatment of deep-seated fungal infections. The mode of action of AmB is directly related to the effect of the drug on the lipid phase of biomembranes. In the present work the effect of AmB on the properties of lipid bilayers formed with dipalmitoylphosphatidylcholine (DPPC) and the effect of the lipid phase on the molecular organisation of AmB were studied with application of spectrophotometry in the UV-Vis region. The absorption spectra of AmB in lipid membranes display a complex structure with hypsochromically and bathochromically shifted bands indicative of formation of molecular aggregates of the drug. Formation of molecular aggregates was analysed at different concentrations of the drug in the lipid phase in the range 0.05--5 mol% and at different temperatures in the range 5--55 degrees C. The aggregation level of AmB in the ordered phase of DPPC displayed a minimum corresponding to a concentration of 1 mol% with respect to the lipid. An increase in the aggregation level was observed in the temperature region corresponding to the main phase transition. The structure of molecular aggregates of AmB is analysed on the basis of spectroscopic effects in terms of the exciton splitting model. Analysis of the position of the absorption maximum of AmB in the lipid phase of DPPC in terms of the theory of solvatochromc effects makes it possible to ascribe the refractive indices n=1.40 and n=1.49 to the hydrophobic core of the membrane in the L(alpha) and the P(beta)' phase respectively. Analysis of the aggregation of AmB in the lipid phase in relation to the physical state of the membrane reveals that the temperature range of the main phase transition of a lipid cluster in the immediate vicinity of AmB depends on its concentration. The termination of the phase transition temperature, as read from the AmB aggregation, varies between 42 degrees C at 1 mol% AmB in DPPC and 49 degrees C at 5 mol% AmB in DPPC. The exciton splitting theory applied to the analysis of the spectroscopic data makes it possible to calculate the diameter of the AmB pore as 2.8 A in the gel phase and 3.6 A in the fluid phase of the DPPC membrane, on the assumption that the pore is formed by nine AmB molecules.     

Competitive binding of cholesterol and ergosterol to the polyene antibiotic nystatin. A fluorescence study

Competition studies between cholesterol and ergosterol were carried out to gain insight into the binding interactions between nystatin and these sterols. Lipid vesicles were prepared with mixtures of palmitoyloleoylphosphocholine/ergosterol/cholesterol, and both sterol molar ratio and total content were varied. The inhibitory effect of cholesterol toward the ergosterol ability to induce the formation of long-lived fluorescent antibiotic species was used to detect nystatin-cholesterol interactions. It was found that the key factor controlling nystatin photophysical properties in the ternary lipid mixtures was their ergosterol/cholesterol molar ratio and not their overall sterol content. Moreover, permeabilization studies showed that nystatin was able to form pores in all the mixed vesicles, but the initial rate of pore formation was also dependent on the ergosterol/cholesterol molar ratio. Our data show that ergosterol is displaced by competing cholesterol, indirectly confirming cholesterol's ability to coassemble with nystatin. The distinct spectroscopic properties emphasize the different molecular architecture adopted by nystatin-cholesterol and -ergosterol complexes, and therefore are relevant to understanding the interaction of the antibiotic with membranes.     

Organization of antibiotic amphotericin B in model lipid membranes. A mini review

Amphotericin B (AmB) is a polyene antibiotic frequently applied in the treatment of fungal infections. According to the general understanding, the mode of action of AmB is directly related to the molecular organization of the drug in the lipid environment, in particular to the formation of pore-like molecular aggregates. Electronic absorption and fluorescence techniques were applied to investigate formation of molecular aggregates of AmB in the lipid environment of liposomes and monomolecular layers formed at the argon-water interface. It appears that AmB dimers, stabilized by van der Waals interactions, are present in the membrane environment along with the aggregates formed by a greater number of molecules. Linear dichroism measurements reveal that AmB is distributed between two fractions of molecules, differently oriented with respect to the bilayer. Molecules in one fraction remain parallel to the plane of the membrane and molecules in the other one are perpendicular. Scanning Force Microscopy imaging of the surface topography of the monolayers formed with AmB in the presence of lipids reveals formation of pore-like structures characterized by the external diameter close to 17 A and the internal diameter close to 6 A. All the findings are discussed in terms of importance of the molecular organization of AmB in the pharmacological action, as well as of the toxic side effects of the drug.     

Oligonucleotide delivery by a cationic derivative of the polyene antibiotic amphotericin B. II: study of the interactions of the oligonucleotide/cationic vector complexes with lipid monolayers and lipid unilamellar vesicles

We report a study of the behavior of oligodeoxyribonucleotide (ODN)/amphotericin B3-(N'-dimethylamino)propylamide (AMA) complexes, in the presence of lipid monolayers and large unilamellar vesicles. This study follows the recent discovery of the capacity of AMA, as a new cationic vector, to enhance ODN cellular uptake and efficacy. It aims at investigating the internalization mode of a nucleic acid by AMA. A first study at the air-water interface of AMA and AMA/ODN by surface pressure measurement shows that only free AMA would adsorb at the air-water interface. Second, in the presence of zwitterionic phospholipid- and sterol-containing mixture, ODN-AMA interactions in solution would be higher than lipid-AMA interactions at the interface. In monolayer or with large unilamellar vesicles, AMA monomers adsorb mainly at the phospholipid interface. These results favor a crossing mechanism through AMA channel formation, despite the size of ODN.     

A calorimetric and spectroscopic comparison of the effects of ergosterol and cholesterol on the thermotropic phase behavior and organization of dipalmitoylphosphatidylcholine bilayer membranes

We performed comparative DSC and FTIR spectroscopic measurements of the effects of cholesterol (Chol) and ergosterol (Erg) on the thermotropic phase behavior and organization of DPPC bilayers. Ergosterol is the major sterol in the biological membranes of yeasts, fungi and many protozoa. It differs from Chol in having two additional double bonds, one in the steroid nucleus at C7-8 and another in the alkyl chain at C22-23. Erg also has an additional methyl group in the alkyl chain at C24. Our DSC studies indicate that the incorporation of Erg is more effective than Chol is in reducing the enthalpy of the pretransition. At lower concentrations Erg is also more effective than Chol in reducing the enthalpies of both the sharp and broad components of main phase transition. However, at sterol concentrations from 30 to 50 mol%, Erg is generally less effective at reducing the enthalpy of the broad components and does not completely abolish the cooperative hydrocarbon chain-melting phase transition at 50 mol%, as does Chol. Nevertheless, in this higher ergosterol concentration range, there is no evidence of the formation of ergosterol crystallites. Our FTIR spectroscopic studies demonstrate that Erg incorporation produces a similar ordering of liquid-crystalline DPPC bilayers as does Chol, but an increased degree of hydrogen bonding of the fatty acyl carbonyl groups in the glycerol backbone region of the DPPC bilayer. These and other results indicate that Erg is less miscible in DPPC bilayers at higher concentrations than is Chol. Finally, we provide a tentative molecular explanation for the comparative experimental and computation results obtained for Erg and Chol in phospholipid bilayers, emphasizing the dynamic conformational differences between these two sterols.