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- POPC/Cholesterol lipid bilayers: A matter of polarityPublication . Cristo, Joana; M. Martins, JorgeWater is an important component of lipid membranes and contributes for the stabilization of the bilayer structure. It determines the polarity gradient in lipid bilayers, since the amount of water at the lipid/water interface is higher than in the acyl chains region.
- Sensing hydration and behavior of pyrene in POPC and POPC/cholesterol bilayers: A molecular dynamics studyPublication . Loura, Luís M. S.; Martins do Canto, Antonio M. T.; Martins, JorgeMolecular dynamics (MD) simulations of bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) with varying amounts of cholesterol (0, 5, 20, and 40 mol%) were carried out in the absence and presence of inserted pyrene molecules. Both fluorophore and bilayer parameters were computed, for characterization of probe location and dynamics, as well as its effects on the host bilayer. In agreement with previous studies in fluid disordered bilayers, pyrene prefers to be located in the hydrophobic acyl chain region of POPC bilayers, close to the glycerol group of lipid molecules and causes ordering of the lipid acyl chains. However, incorporation of pyrene in binary POPC/cholesterol bilayers decreases the acyl chain order parameter (especially near the end of the chains), opposing the ordering effect of cholesterol. These effects are modest and mainly felt locally. Significantly, as the bilayer is enriched with cholesterol, the relative position of pyrene and the POPC carbonyl and phosphocholine groups is invariant, and the local water density around the probe decreases. This work clarifies and supports the cautious use of pyrene Ham effect to effectively measure equivalent polarity in lipid bilayers. Within the time scale of the MD simulations, which is of the magnitude of the fluorescence lifetime of pyrene, the thermally averaged polarity of lipid bilayers is nearly out of influence of spurious uncertainty in the transverse location of pyrene in the bilayers. This renders the values of equivalent polarity measurements through the pyrene Ham effect more reliable and reproducible than previously expected. (C) 2012 Elsevier B.V. All rights reserved.
- On the concentration-dependent clustering of trehalose in aqueous media: a spectroscopic studyPublication . M. Martins, Jorge; Anjinho, M.Trehalose is a disaccharide of no reducing power, widely distributed in nature (bacteria, fungi, insects, invertebrates and plants), protecting cells and organisms against various stresses such as dryness, freezing and osmopressure [1].
- Bilayer polarity and its thermal dependency in the e(o) and e(d) phases of binary phosphatidylcholine/cholesterol mixturesPublication . Arrais, Dalila; Martins, JorgeDiverse variations in membrane properties are observed in binary phosphatidylcholine/cholesterol mixtures. These mixtures are nonideal, displaying single or phase coexistence, depending on chemical composition and other thermodynamic parameters. When compared with pure phospholipid bilayers, there are changes in water permeability, bilayer thickness and thermomechanical properties, molecular packing and conformational freedom of phospholipid acyl chains, in internal dipolar potential and in lipid lateral diffusion. Based on the phase diagrams for DMPC/cholesterol and DPPC/cholesterol, we compare the equivalent polarity of pure bilayers with specific compositions of these mixtures, by using the Py empirical scale of polarity. Besides the contrast between pure and mixed lipid bilayers, we find that liquid-ordered and liquid-disordered (e(d)) phases display significantly different polarities. Moreover, in the e(o) phase, the polarities of bilayers and their thermal dependences vary with the chemical composition, showing noteworthy differences for cholesterol proportions at 35, 40, and 45 mol%. At 20 degrees C, for DMPC/ cholesterol at 35 and 45 mol%, the equivalent dielectric constants are 21.8 and 23.8, respectively. Additionally, we illustrate potential implications of polarity in various membrane-based processes and reactions, proposing that for cholesterol containing bilayers, it may also go along with the occurrence of lateral heterogeneity in biological membranes. (c) 2007 Elsevier B.V. All rights reserved.
- Fluorescence sensing of microplastics on surfacesPublication . Costa, Camila Q. V.; Cruz, Joana; Martins, Jorge; Teodosio, Maria; Jockusch, Steffen; Ramamurthy, V.; Da Silva, José PauloMicroplastics, nanoplastics and related products have been recently found in marine ecosystems worldwide, calling for new analytical methods for rapid detection and risk assessment. Fluorescence is a sensitive technique that when used with polarity probes can potentially detect low-polarity plastic particles in environments. Here, we evaluated the fuorescence technique to sense polystyrene microparticles directly on salt, silica and sand surfaces, using Nile Red and pyrene as polarity probes. Results show that all probes displayed fuorescence on silica and sand, whereas strong fuorescence quenching was observed on NaCl. Polystyrene particles increase the fuorescence intensity due to probe migration into their nonpolar microenvironment. In the presence of polystyrene, the spectra of Nile Red are shifted to shorter wavelengths, while the ratio of vibronic bands I1/I3 of fuorescence of pyrene decreases to about 1. Pyrene showed similar sensitivity toward surface-modifed carboxyl polystyrene particles. On NaCl, the emission of pyrene increases linearly with polystyrene content for concentrations from 0.5 to 20 µg/g. The detection limit of polystyrene microparticles on natural sea salt using pyrene as probe is about 0.2 µg/g, while on sand, the sensitivity is about one order of magnitude lower. Overall, although being of relatively low selectivity, the fuorescence technique can be used to determine a maximum content of plastic particles of few micrometers size with little sample preparation. Fluorescence, when used in conjunction with pyrene probe, allows for detection and quantifcation of microplastic particles in the sub-ppm range.
- Analysis of the equilibrium distribution of Ligands in Heterogeneous Media – Approaches and pitfallsPublication . Moreno, Maria João; Loura, Luís M. S.; M. Martins, Jorge; Salvador, Armindo; Velazquez-Campoy, AdrianThe equilibrium distribution of small molecules (ligands) between binding agents in heterogeneous media is an important property that determines their activity. Heterogeneous systems containing proteins and lipid membranes are particularly relevant due to their prevalence in biological systems, and their importance to ligand distribution, which, in turn, is crucial to ligand’s availability and biological activity. In this work, we review several approaches and formalisms for the analysis of the equilibrium distribution of ligands in the presence of proteins, lipid membranes, or both. Special attention is given to common pitfalls in the analysis, with the establishment of the validity limits for the distinct approaches. Due to its widespread use, special attention is given to the characterization of ligand binding through the analysis of Stern–Volmer plots of protein fluorescence quenching. Systems of increasing complexity are considered, from proteins with single to multiple binding sites, from ligands interacting with proteins only to biomembranes containing lipid bilayers and membrane proteins. A new formalism is proposed, in which ligand binding is treated as a partition process, while considering the saturation of protein binding sites. This formalism is particularly useful for the characterization of interaction with membrane proteins.