Marco Soto-Arriaza

Interaction of gramicidin with DPPC/DODAB bilayer fragments

The interaction between the antimicrobial peptide gramicidin (Gr) and dipalmitoylphosphatidylcholine (DPPC)/dioctadecyldimethylammonium bromide (DODAB) 1:1 large unilamellar vesicles (LVs) or bilayer fragments (BFs) was evaluated by means of several techniques. The major methods were: 1) Gr intrinsic fluorescence and circular dichroism (CD) spectroscopy; 2) dynamic light scattering for sizing and zeta-potential analysis; 3) determination of the bilayer phase transition from extrinsic fluorescence of bilayer probes; 4) pictures of the dispersions for evaluation of coloidal stability over a range of time and NaCl concentration. For Gr in LVs, the Gr dimeric channel conformation is suggested from: 1) CD and intrinsic fluorescence spectra similar to those in trifluoroethanol (TFE); 2) KCl or glucose permeation through the LVs/Gr bilayer. For Gr in BFs, the intertwined dimeric, non-channel Gr conformation is evidenced by CD and intrinsic fluorescence spectra similar to those in ethanol. Both LVs and BFs shield Gr tryptophans against quenching by acrylamide but the Stern-Volmer quenching constant was slightly higher for Gr in BFs confirming that the peptide is more exposed to the water phase in BFs than in LVs. The DPPC/DODAB/Gr supramolecular assemblies may predict the behavior of other antimicrobial peptides in assemblies with lipids.

Differential Dynamic and Structural Behavior of Lipid-Cholesterol Domains in Model Membranes

Changes in the cholesterol (Chol) content of biological membranes are known to alter the physicochemical properties of the lipid lamella and consequently the function of membrane-associated enzymes. To characterize these changes, we used steady-state and time resolved fluorescence spectroscopy and two photon-excitation microscopy techniques. The membrane systems were chosen according to the techniques that were used: large unilamellar vesicles (LUVs) for cuvette and giant unilamellar vesicles (GUVs) for microscopy measurements; they were prepared from dipalmitoyl phosphatidylcholine (DPPC) and dioctadecyl phosphatidylcholine (DOPC) in mixtures that are well known to form lipid domains. Two fluorescent probes, which insert into different regions of the bilayer, were selected: 1,6-diphenyl-1,3,5-hexatriene (DPH) was located at the deep hydrophobic core of the acyl chain regions and 2-dimethylamino-6-lauroylnaphthalene (Laurdan) at the hydrophilic-hydrophobic membrane interface. Our spectroscopy results show that (i) the changes induced by cholesterol in the deep hydrophobic phospholipid acyl chain domain are different from the ones observed in the superficial region of the hydrophilic-hydrophobic interface, and these changes depend on the state of the lamella and (ii) the incorporation of cholesterol into the lamella induces an increase in the orientation dynamics in the deep region of the phospholipid acyl chains with a corresponding decrease in the orientation at the region close to the polar lipid headgroups. The microscopy data from DOPC/DPPC/Chol GUVs using Laurdan generalized polarization (Laurdan GP) suggest that a high cholesterol content in the bilayer weakens the stability of the water hydrogen bond network and hence the stability of the liquid-ordered phase (Lo).

Theoretical and experimental characterization of a novel pyridine benzimidazole: suitability for fluorescence staining in cells and antimicrobial properties

Benzimidazoles presenting intramolecular hydrogen bonding interactions have been normally used to better understand the role of H-bonding in biological processes. Here, we present an experimental and theoretical study of a new compound [2,4-di-tert-butyl-6-(3H-imidazo[4,5-c]pyridine-2-yl)phenol]; (B2), a benzimidazole derivate, exhibiting an intramolecular hydrogen bond. B2 was synthesized and characterized by its 1H, HHCOSY, FT-IR and mass spectra (EI-MS 323 M+). The electronic and optical properties of B2 were studied with theoretical calculations using density functional theory (DFT) and time-dependent DFT (TDDFT). B2 showed luminescent emission at room temperature in different solvents, with a large Stokes shift (e.g.; λex = 335 nm; λem = 510 nm in acetonitrile). Also, the quantum yield (φ = 0.21) and theoretical band emission are reported. We found that B2 exhibited a fluorescence emission at around 500 nm in ethanol and in acetonitrile that could be quenched by aqueous solutions of Hg(NO3)2 in the range of micro molar concentrations. Cyclic voltammetry in acetonitrile showed a strong anodic response due to a quasireversible process, with reduction and oxidation waves at −1.28 and −0.47 V vs. SCE. Regarding the biological properties, we assessed the antimicrobial activity of B2 in Salmonella enterica (bacteria), Cryptococcus spp. (yeast), Candida albicans (yeast), Candida tropicalis (yeast) and Botrytis cinerea (mold). To this end, we determined the minimal inhibitory concentration (MIC) (for bacteria and yeasts), the growth inhibition halos (for yeasts), and the inhibition of mycelial growth (for the mold). We observed that B2 exerted an antifungal effect against Cryptococcus spp. and Botrytis cinerea. In addition, due to its fluorescence properties, B2 has proven to be a suitable marker to observe bacteria (Salmonella enterica and an Escherichia coli derivative), yeasts (Candida albicans), and even human cells (SKOV-3 and HEK-293) by confocal microscopy.

Effect of cholesterol content on the structural and dynamic membrane properties of DMPC/DSPC large unilamellar bilayers.

In this study, we report the effect of cholesterol content on the dynamic and structural properties of a dimyristoyl-phosphatidylcholine and distearoyl-phosphatidylcholine mixture in large unilamellar vesicles. The range of cholesterol concentrations studied varied around approximately 33.3mol%, where it has been postulated that an abrupt change in bilayer organization occurs. Steady-state fluorescence measurements demonstrated a typical behavior; at low temperatures in the main phase transition, the cholesterol concentration did not affect the gel phase, but at 37.5°C (phase coexistence) and in the liquid crystalline phase, the presence of cholesterol produced an increase in the fluorescence anisotropy of DPH and the generalized polarization of Laurdan. The greater effect was observed in the liquid crystalline phase, in which the bilayer became a mixture of fluid-like and liquid-ordered phases. The results obtained at approximately 33.3mol% of Cholesterol demonstrated that the Generalized Polarization of Laurdan, the DPH lifetime, the limiting anisotropy and the rotational correlation time, as well as the fluorescence quenching of DPH by TEMPO, are at maxima, while the fluorescence intensity of dehydroergosterol and the lipid solubility in TritonX-100 are at minima. These results correlate well with the hypothesis of domain segregation in the DMPC/DSPC/Cholesterol LUV system. In this context, we postulate that at 33.3mol% of Cho, the proportion of ordered domains reaches a maximum.

The Self-Assembly of a Cyclic Lipopeptides Mixture Secreted by a B. megaterium Strain and Its Implications on Activity against a Sensitive Bacillus Species

Cyclic lipopeptides are produced by a soil Bacillus megaterium strain and several other Bacillus species. In this work, they are detected both in the Bacillus intact cells and the cells culture medium by MALDI-TOF mass spectrometry. The cyclic lipopeptides self-assemble in water media producing negatively charged and large aggregates (300–800 nm of mean hydrodynamic radius) as evaluated by dynamic light scattering and zeta-potential analysis. The aggregate size depends on pH and ionic strength. However, it is not affected by changes in the osmolarity of the outer medium suggesting the absence of an internal aqueous compartment despite the occurrence of low molecular weight phospholipids in their composition as determined from inorganic phosphorus analysis. The activity against a sensitive Bacillus cereus strain was evaluated from inhibition halos and B. cereus lysis. Essential features determining the antibiotic activity on susceptible Bacillus cereus cells are the preserved cyclic moiety conferring cyclic lipopeptides resistance to proteases and the medium pH. The aggregates are inactive per se at the pH of the culture medium which is around 6 or below. The knock out of the sensitive cells only takes place when the aggregates are disassembled due to a high negative charge at pH above 6.