Linking surface tension to water polarization with a new hypothesis: The Ling-Damodaran Isotherm.

Studying aqueous solutions of complex (bio)polymers is essential from both theoretical and practical perspectives. To understand the principles that govern the properties of these solutions is pivotal for the study of biological processes, considering that the most distinguished components of the cells are polymers (proteins, nucleic acids). These macromolecular aqueous systems, known as colloids, has raise the interest of scientists in recent years. It is known that several physicochemical properties deviate from ideal behaviour in this kind of solutions and that the physical state of water is different compared to its pure state. Particularly, the surface tension of such mixtures often shows a peculiar profile at semi-dilute and concentrated conditions. Here, we joined the colloidal concept of water polarization (proposed in the Association-Induction Hypothesis) with Damodaran's formalism for surface tension to theoretically derive a compelling mathematical model that explains the behaviour of polymer solutions. We measured the surface tension and osmolarity of different polyethylene oxide solutions and we used the ACDAN fluorescence probe to assess the water dipolar relaxation (polarization) in these mixtures. As a proof of concept, we also studied the influence of these polymer solutions on lipid interfaces. Our isotherm model explains the experimental observations with a unifying view that correlates with other measured properties, such as osmolarity and water dipolar relaxation. This provides a link between interfacial and bulk physicochemical properties of polymer solutions, also giving a new framework for studying the interaction of colloidal systems with lipid membranes interfaces.

HIV-1 Gag specificity for PIP2 is regulated by macromolecular electric properties of both protein and membrane local environments.

HIV-1 assembly occurs at the plasma membrane, with the Gag polyprotein playing a crucial role. Gag association with the membrane is directed by the matrix domain (MA), which is myristoylated and has a highly basic region that interacts with anionic lipids. Several pieces of evidence suggest that the presence of phosphatidylinositol-(4,5)-bisphosphate (PIP2) highly influences this binding. Furthermore, MA also interacts with nucleic acids, which is proposed to be important for the specificity of GAG for PIP2-containing membranes. It is hypothesized that RNA has a chaperone function by interacting with the MA domain, preventing Gag from associating with unspecific lipid interfaces. Here, we study the interaction of MA with monolayer and bilayer membrane systems, focusing on the specificity for PIP2 and on the possible effects of a Gag N-terminal peptide on impairing the binding for either RNA or membrane. We found that RNA decreases the kinetics of the protein association with lipid monolayers but has no effect on the selectivity for PIP2. Interestingly, for bilayer systems, this selectivity increases in presence of both the peptide and RNA, even for highly negatively charged compositions, where MA alone does not discriminate between membranes with or without PIP2. Therefore, we propose that the specificity of MA for PIP2-containing membranes might be related to the electrostatic properties of both membrane and protein local environments, rather than a simple difference in molecular affinities. This scenario provides a new understanding of the regulation mechanism, with a macromolecular view, rather than considering molecular interactions within a ligand-receptor model.

Introducing the multi-dimensional spectral phasors: a tool for the analysis of fluorescence excitation-emission matrices.

The use of phasors to analyze fluorescence data was first introduced for time-resolved studies for a simpler mathematical analysis of the fluorescence-decay curves. Recently, this approach was extended to steady-state experiments with the introduction of the spectral phasors (SP), derived from the Fourier transform of the fluorescence emission spectrum. In this work, we revise key mathematical aspects that lead to an interpretation of SP as the characteristic function of a probability distribution. This formalism allows us to introduce a new tool, called multi-dimensional spectral phasor (MdSP) that seize, not only the information from the emission spectrum, but from the full excitation-emission matrix (EEM). In addition, we developed a homemade open-source Java software to facilitate the MdSP data processing. Due to this mathematical conceptualization, we settled a mechanism for the use of MdSP as a tool to tackle spectral signal unmixing problems in a more accurate way than SP. As a proof of principle, with the use of MdSP we approach two important biophysical questions: protein conformational changes and protein-ligand interactions. Specifically, we experimentally measure the EEM changes upon denaturation of human serum albumin (HSA) or during its association with the fluorescence dye 1,8-anilinonaphtalene sulphate (ANS) detected via tryptophan-ANS Förster Resonance Energy Transfer (FRET). In this sense, MdSP allows us to obtain information of the system in a simpler and finer way than the traditional SP. Specifically, understanding a protein's EEM as a molecular fingerprint opens new doors for the use of MdSP as a tool to analyze and comprehend protein conformational changes and interactions.

Differential activity of lytic α-helical peptides on lactobacilli and lactobacilli-derived liposomes.

Eukaryotic antimicrobial peptides (AMPs) interact with plasma membrane of bacteria, fungi and eukaryotic parasites. Noteworthy, Lactobacillus delbrueckii subsp. lactis (CIDCA 133) and L. delbrueckii subsp. bulgaricus (CIDCA 331) show different susceptibility to human beta-defensins (ß-sheet peptides). In the present work we extended the study to α-helical peptides from anuran amphibian (Aurein 1.2, Citropin 1.1 and Maculatin 1.1). We studied the effect on whole bacteria and liposomes formulated with bacterial lipids through growth kinetics, flow cytometry, leakage of liposome content and studies of peptide insertion in lipid monolayers. Growth of strain CIDCA 331 was dramatically inhibited in the presence of all three peptides and minimal inhibitory concentrations were lower than those for strain CIDCA 133. Flow cytometry revealed that AMPs lead to the permeabilization of bacteria. In addition, CIDCA 331-derived liposomes showed high susceptibility, leading to content leakage and structural disruption. Accordingly, peptide insertion in lipid monolayers demonstrated spontaneous interaction of AMPs with CIDCA 331 lipids. In contrast, lipids monolayers from strain CIDCA 133 were less susceptible. Summarizing we demonstrate that the high resistance of the probiotic strain CIDCA 133 to AMPs extends to α helix peptides Aurein, Citropin and Maculatin. This behavior could be ascribed in part to differences in membrane composition. These findings, along with the previously demonstrated resistance to ß defensins from human origin, suggest that strain CIDCA 133 is well adapted to host innate immune effectors from both mammals and amphibians thus indicating conserved mechanisms of interaction with key components of the innate immune system.