Molecular organisation of amphotericin B at the air–water interface in the presence of sterols: a monolayer study

Abstract

Using the monolayer technique to study the surface behaviour of systems consisting of amphotericin B (AmB) and various sterols, the components were found to interact with each other. The interactions observed are accounted for by postulating that, at low surface pressures, AmB and different sterols form mixed films where the former lies parallel and the latter normal to the air-water interface in such a way that the polar groups in both components establish hydrogen bonds that lead to the formation of an AmB-sterol 'complex' of 2:1 stoichiometry at the interface. At high surface pressures, AmB molecules rearrange themselves normal to the interface; this gives rise to the Van der Waals interactions between non-polar chains of both components that vary with the nature and composition of the system. The occurrence of these hydrophobic interactions prevents the desorption of AmB into the subphase, which is consistent with the positive excess areas of mixing obtained under these surface pressure conditions. Among the four sterols studied, ergosterol exhibits the strongest interaction with AmB and beta-sitosterol the weakest. Cholesterol and stigmasterol show intermediate behaviour.

Desorption of Amphotericin B from Mixed Monolayers with Cholesterol at the Air/Water Interface

“…In contrast, for mixtures with X AmB = 0.5 and 0.7, the overall desorption process is controlled by two steps (dissolution and diffusion), reaching eventually a steady state. This accounts for the existence of an interaction between AmB and cholesterol and indicates the presence of a stable complex, the formation of which has already been predicted from the excess free energy of mixing values (4,7). Comparing the mixtures of X AmB = 0.5 and 0.7 at the same surface pressure (15 mN/m) it is to be pointed out that although the rate of dissolution for both cases is practically the same, the values for θ * are different (90 min for X AmB = 0.7, and 75 min for X AmB = 0.5), indicating that for the mixture containing a 0.7 molar fraction of AmB, the steady state is attained after a longer period of time than for the 1 : 1 mixture.…”

“…The characteristics of mixed AmB/cholesterol monolayers have already been reported; however, the experimental conditions applied therein were different than those in the present work. Nevertheless, the overall behavior is similar and for a detailed analysis of the mixed Langmuir isotherms the readers are referred to our previous papers (4,6). The relaxation experiments carried out for the AmB monolayer alone (9) confirm that amphotericin molecules undergo desorption from the surface.…”

“…Interestingly, when AmB was mixed with a sterol, the stability of the monolayer was increased, implying complex formation with 2 : 1 stoichiometry (4-7), the structure of which is presented in Ref. (4). The deviations from ideality were manifested by negative excess areas and excess free energies of mixing at low surface pressures and positive at high pressures above the transition.…”

“…It is believed that AmB acts through linking to membrane sterols (2) to make complexes responsible for the formation of internal channels (3) by which crucial elements escape from the cell, causing its damage and eventually death. To get further insight into AmB's mechanism of action, we have been researching the surface behavior of AmB alone as well as its interaction with sterols in monolayers at the air/water interface (4)(5)(6). The pressure/area (π/A) isotherm of AmB exhibits a characteristic transition plateau which separates two regions of liquid-expanded and liquid-condensed states (7,8).…”

Study at the Air/Water Interface of a Hepatitis A N-Acetylated and C-Amidated Synthetic Peptide (AcVP3(110–121)–NH2)

“…Deviations were quantified calculating G EX M and H (11, 13) following Eqs. [2], [4], and [5]. The interaction parameter (α) was also calculated using Eq.…”