Calorimetric and Spectroscopic Studies of the Effects of the Cell Penetrating Peptide Pep‑1 and the Antimicrobial Peptide Combi‑2 on Vesicles Mimicking <i>Escherichia coli</i> Membrane

The objective of this study is to measure and compare the effects of the cell penetrating peptide (CPP) Pep-1 and the antimicrobial peptide (AMP) combi-2 on vesicles of membranes mimicking <i>Escherichia coli</i> (<i>E. coli</i>). To characterize the effects of Pep-1 and combi-2 on <i>E. coli</i> membrane vesicles, a combination of five biophysical techniques was employed: fluorescence, infrared, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) techniques. Upon addition of <i>E. coli</i> membranes, tryptophan fluorescence intensity of Pep-1 showed a sudden blue-shift and decreased in a nonconcentration-dependent manner while the intensity of combi-2 decreased in a concentration-dependent manner, most significantly for a very low peptide-to-lipid ratio of 1:40. Complexes of Pep-1 and combi-2 with <i>E. coli</i> membrane mimicking vesicles having shown a significant blue-shift in fluorescence intensity were then prepared and studied in freeze-dried states. IR results indicate that Pep-1 and combi-2 adopt a major 3₁₀-helix structure in the presence of <i>E. coli</i> membrane mimicking vesicles at low peptide concentration. Pep-1 and combi-2 have a similar effect on <i>E. coli</i> membrane mimicking vesicles at low concentration even though combi-2 is in the interfacial region of the bilayer while Pep-1 is located between the interfacial region and the hydrophobic region. Combi-2 at low concentration acts as a CPP. TGA and DSC results reveal that combi-2 has a stabilizing effect on <i>E. coli</i> at any concentration while Pep-1 stabilizes the <i>E. coli</i> membrane only at high concentration. Both peptides show a preferential interaction with one of the anionic lipids leading to clustering in <i>E. coli</i> membrane. SEM images reveal that Pep-1 and combi-2 form superstructures including fibrils in the presence of <i>E. coli</i> membrane mimicking vesicles. Calorimetric and spectroscopic techniques may be used in a complementary way with imaging techniques to gain more insights into peptide–lipid interactions

Direct Visualization of the Hydration Layer on Alumina Nanoparticles with the Fluid Cell STEM in situ

Rheological behavior of aqueous suspensions containing nanometer-sized powders is of relevance to many branches of industry. Unusually high viscosities observed for suspensions of nanoparticles compared to those of micron size powders cannot be explained by current viscosity models. Formation of so-called hydration layer on alumina nanoparticles in water was hypothesized, but never observed experimentally. We report here on the direct visualization of aqueous suspensions of alumina with the fluid cell in situ. We observe the hydration layer formed over the particle aggregates and show that such hydrated aggregates constitute new particle assemblies and affect the flow behavior of the suspensions. We discuss how these hydrated nanoclusters alter the effective solid content and the viscosity of nanostructured suspensions. Our findings elucidate the source of high viscosity observed for nanoparticle suspensions and are of direct relevance to many industrial sectors including materials, food, cosmetics, pharmaceutical among others employing colloidal slurries with nanometer-scale particles