Publications

Abstract Ionic liquids (ILs) have been widely used for energy storage and conversion devices due to their negligible vapor pressure, high thermal stability, and outstanding interfacial properties. Notably, the interfacial nanostructure and the wettability of thin ionic liquid films on solid surfaces are of utmost relevance in nanosurface science and technology. Herein, a reproducible physical vapor deposition methodology was used to fabricate thin films of four alkylimidazolium bis(trifluoromethylsulfonyl)imide ILs. The effect of the cation alkyl chain length on the wettability of ILs was explored on different surfaces: gold (Au); silver (Ag); indium-tin oxide (ITO). High-resolution scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to evaluate the morphology of the produced micro- and nanodroplets and films. SEM and AFM results revealed an island growth for all the ILs deposited on ITO and Ag surfaces, with a lower minimum free area to promote nucleation (MFAN) in Ag and higher wettability for ILs having larger non-polar domains. The low wettability of ITO by the studied ILs was highlighted. For long-chain ILs, nucleation and growth mechanisms were strongly conditioned by coalescence processes. The results also supported the higher affinity of the ILs to the Au surface. The increase in the length of the cation alkyl chain was found to promote a better film adhesion inducing a 2D growth and higher wetting ability.

Abstract Hypothesis: The injection of air into the sample cell of an isothermal titration calorimeter containing a liquid provides a rich-in-information signal, with a periodic contribution arising from the creation, growing and release of bubbles. The identification and analysis of such contributions allow the accurate determination of the surface tension of the target liquid. Experiments: Air is introduced at a constant rate into the sample cell of the calorimeter containing either a pure liquid or a solution. The resulting calorimetric signal is analyzed by a new algorithm, which is implemented into a computational code. Findings: The thermal power generated by our experiments is often noisy, thus hiding the periodic signal arising from the bubbles' formation and release. The new algorithm was tested with a range of different types of calorimetric raw data, some of them apparently being just noise. In all cases, the contribution of the bubbles to the signal was isolated and the corresponding period was successfully determined in an automated way. It is also shown that two reference measurements suffice to calibrate the instrument at a given temperature, regardless the injection rate, allowing the direct determination of surface tension values for the liquid contained in the sample cell. (c) 2021 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Abstract Fluorescence spectroscopy is used to characterize the partition of three second-generation D,L-alpha-cyclic peptides to two lipid model membranes. The peptides have proven antimicrobial activity, particularly against Gram positive bacteria, and the model membranes are formed of either with 1,2-dimyristoyl-sn-glycero-3-phospho- (1'- rac-glycerol) (DMPG) or its mixture with 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE), at a molar ratio of (1:1). The peptide's intrinsic fluorescence was used in the Steady State and/or Time Resolved Fluorescence Spectroscopy experiments, showing that the peptides bind to the membranes, and the extent of their partition is thereof quantified. The peptide-induced membrane leakage was followed using an encapsulated fluorescent dye. Overall, the partition is mainly driven by electrostatics, but also involves hydrophobic interactions. The introduction of a hydrocarbon tail in one of the residues of the parent peptide, CPR, adjacent to the tryptophan (Trp) residue, significantly improves the partition of the modified peptides, CPRT10 and CPRT14, to both membrane systems. Further, we show that the length of the tail is the main distinguishing factor for the extension of the partition process. The parent peptide induces very limited leakage, at odds with the peptides with tail, that promote fast leakage, increasing in most cases with peptide concentration, and being almost complete for the highest peptide concentration and negatively charged membranes. Overall, the results help the unravelling of the antimicrobial action of these peptides and are well in line with their proven high antimicrobial activity.

Abstract A novel formulation of cationic liposomes was studied by mixing dipalmitoylphosphatidylcholine (DPPC) with tetradecyltrimethylammonium bromide gemini surfactants with different alkane spacer groups lengths attached to their ammonium head-groups. The physicochemical characterization of the cationic liposomes was obtained by combining experimental results from differential scanning microcalorimetry (DSC) with molecular dynamic simulations, in order to understand their structural configuration. An adapted Ising model was used to interpret the results in terms of cooperativity of the phase transitions. The gemini surfactants partition into the lipid bilayer of DPPC liposomes, and the induced changes in colloidal stability and phase transition were analyzed in detail.The DPPC liposomes became positively charged upon gemini surfactant partition, showing increased colloidal stability. Our results show signif-icant differences in structural configuration between gemini surfactants with short and long spacer lengths. While gemini with shorter spacers allocate within the lipid bilayer with both headgroups in the same layer, geminis with longer spacers unexpectedly intercalate in the lipid membrane in a partic-ular zig-zag configuration, with each headgroup located at a different side of the bilayer, altering the cou-pling degree parameters of the membrane's phase transition.The extraordinary increase of colloidal stability of DPPC liposomes with gemini surfactants at very low molar ratio and the possibility to tune the physicochemical properties of the membrane by control de spacer length of the geminis opens new possibilities for cationic liposomal formulations with potential applications in vaccines, drug/gene delivery or biosensing.(c) 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

Abstract Isothermal titration calorimetry (ITC) is currently widely used in many applied areas of research, spanning protein-ligand binding, metal-ligand interactions, DNA/DNA or protein/DNA interactions, partition to membranes, and polymer surfactant interactions, to mention just a few. This is due to the availability of commercial instruments, and thus the production and spread of an accepted and widely followed SOP is felt by most users, in an effort to produce results that are scientifically correct and comparable. Therefore, within the efforts of Working Group 4 of the ARBRE-MOBIEU COST Action (CA15126), this ITC SOP was generated, alongside SOPs for several other biophysical techniques. Here, we discuss the factors that are fundamental for good experimental design and that need to be carefully considered, as well as machine calibration, in particular chemical calibration, linked to another outcome of Working Group 4 on ITC benchmarking, to be also published in this Special Issue.

Abstract Hypothesis: Amphiphilic molecules spontaneously adsorb to fluid polar-nonpolar interfaces. The time scale of such adsorption depends on the molecular size and structure of the solute. This process should be accompanied by a power heat exchange that could be detected by commercial isothermal calorimeters. Experiments: Air is injected in the bulk of different aqueous solutions contained in the sample cell of an isothermal titration calorimeter. The formation of the resulting bubbles leads to a liquid/air interface to which the solute molecules spontaneously adsorb. Continuous injection experiments to produce multiple bubbles as well as experiments with static bubbles stand from the capillary tip, aiming to observe slow adsorption processes, were performed. Findings: The power associated with the formation, growth and release of air bubbles in different liquids was measured. Different independent contributions that can be associated to the pressure change in the gas phase, the evaporation-condensation of the solvent, the increase of interfacial area, the change in the heat capacity of the sample cell content, and the release of the bubble were observed. The periodic pattern produced by the continuous injection of air at a constant rate is used to determine the surface tension of different liquids, including solutions of different molecules and (bio)macromolecules.

Abstract Equilibrium binding constants (Kb) between chemical compounds and target proteins or between interacting proteins provide a quantitative understanding of biological interaction mechanisms. Reported uncertainties of measured experimental parameters are critical for decision-making in many scientific areas, e.g., in lead compound discovery processes and in comparing computational predictions with experimental results. Uncertainties in measured Kb values are commonly represented by a symmetric normal distribution, often quoted in terms of the experimental value plus–minus the standard deviation. However, in general, the distributions of measured Kb (and equivalent Kd) values and the corresponding free energy change ΔGb are all asymmetric to varying degree. Here, using a simulation approach, we illustrate the effect of asymmetric Kb distributions within the realm of isothermal titration calorimetry (ITC) experiments. Further we illustrate the known, but perhaps not widely appreciated, fact that when distributions of any of Kb, Kd and ΔGb are transformed into each other, their degree of asymmetry is changed. Consequently, we recommend that a more accurate way of expressing the uncertainties of Kb, Kd, and ΔGb values is to consistently report 95% confidence intervals, in line with other authors’ suggestions. The ways to obtain such error ranges are discussed in detail and exemplified for a binding reaction obtained by ITC. © 2021, European Biophysical Societies' Association.