Monte Carlo Simulations of Vesicles and Fluid Membranes Transformations

The appearance of compartmentalization is recognized as a key step in biogenesis. The study of the dynamical behaviour of amphiphilic close membranes at equilibrium or under some external stress (osmotic pressure or dehydration process) can be useful in order to better elucidate the role of vesicles in the origin of life and to get insight into the molecular and membrane properties that bring to a spontaneous vesicle division. A Monte Carlo approach to simulate the evolution of close membranes under an external stress will be presented. This approach is mainly based on the accepted surface energy model introduced by Helfrich (1973) and Seifert (1997a). Some preliminary results will be also illustrated and possible developments and limits of this method discusse

Novel Dicyano-Phenylenevinylene Fluorophores for Low-Doped Layers: A Highly Emissive Material for Red OLEDs

Two efficient deep red (DR)-emitting organic dicyano-phenylenevinylene derivatives with terminal withdrawing or donor groups were synthesized. The spectroscopic properties of the neat solids and the low-doped layers in polystyrene or polyvinylcarbazole host matrixes were analyzed, and the luminescence performance was explained using density functional theory (DFT) analysis. A noteworthy 89% fluorescence quantum yield was observed for the brightest red-emissive polyvinylcarbazole (PVK) blend. This result pushed us to successfully produce an emissive red organic light-emitting device (OLED) as a preliminary feasibility test

The significance of lipid composition for membrane activity: new concepts and ways of assessing function

In the last decade or so, it has been realised that membranes do not just have a lipid-bilayer structure in which proteins are embedded or with which they associate. Structures are dynamic and contain areas of heterogeneity which are vital for their formation. In this review, we discuss some of the ways in which these dynamic and heterogeneous structures have implications during stress and in relation to certain human diseases. A particular stress is that of temperature which may instigate adaptation in poikilotherms or appropriate defensive responses during fever in mammals. Recent data emphasise the role of membranes in sensing temperature changes and in controlling a regulatory loop with chaperone proteins. This loop seems to need the existence of specific membrane microdomains and also includes association of chaperone (heat stress) proteins with the membrane. The role of microdomains is then discussed further in relation to various human pathologies such as cardiovascular disease, cancer and neurodegenerative diseases. The concept of modifying membrane lipids (lipid therapy) as a means for treating such pathologies is then introduced. Examples are given when such methods have been shown to have benefit. In order to study membrane microheterogeneity in detail and to elucidate possible molecular mechanisms that account for alteration in membrane function, new methods are needed. In the second part of the review, we discuss ultra-sensitive and ultra-resolution imaging techniques. These include atomic force microscopy, single particle tracking, single particle tracing and various modern fluorescence methods. Finally, we deal with computing simulation of membrane systems. Such methods include coarse-grain techniques and Monte Carlo which offer further advances into molecular dynamics. As computational methods advance they will have more application by revealing the very subtle interactions that take place between the lipid and protein components of membranes – and which are so essential to their function

An Amphiphilic Pyridinoyl-hydrazone Probe for Colorimetric and Fluorescence pH Sensing

A new pH sensor based on a substituted aroylhydrazide with a flexible side chain and a terminal trimethyl ammonium group (PHA+) was designed and synthesized. The terminal quaternary ammonium guarantees excellent solubility in water. At the same time, the probe is very soluble in hydrophobic envirornments. The pyridinoyl-hydrazone moiety acts as the pH-sensitive fluorophore/chromophore probe. Extensive physicochemical characterization has been performed on the bromide salt PHABr. DFT calculations, based on single-crystal X-ray data, permitted to rationalize the optical behavior. Molecular dynamics simulations permitted to clarify the mode of interaction with lipid membrane. The ability of the probe to change color and fluorescence in response to different pH and media of different polarity has been investigated. PHABr shows a remarkable pH-dependent behavior in both absorption and fluorescence spectra with high sensitivity and strong on-off switch effect at neutral pH, perceptible even to the naked eye

A Water Soluble 2-Phenyl-5-(pyridin-3-yl)-1,3,4-oxadiazole Based Probe: Antimicrobial Activity and Colorimetric/Fluorescence pH Response

The growing demand of responsive tools for biological and biomedical applications pushestowards new low-cost probes easy to synthesize and versatile. Current optical probes are theranostictools simultaneously responsive to biological parameters/analyte and therapeutically operating.Among the optical methods for pH monitoring, simple small organic molecules including multifunc-tional probes for simultaneous biological activity being highly desired by scientists and technicians.Here, we present a novel pH-responsive probe with a three-ring heteroaromatic pattern and a flexiblecationic chain. The novel molecule shows real-time naked-eye colorimetric and fluorescence responsein the slightly acidic pH range besides its excellent solubility both in the organic phase and in water.In addition, the small probe shows significant antibacterial activity, particularly againstEscherichia coli.Single-crystal X-ray study and density functional theory (DFT) calculations rationalize the moleculespectroscopic response. Finally, molecular dynamics (MD) elucidate the interactions between theprobe and a model cell membrane

Transmembrane Peptides as Sensors of the Membrane Physical State

Cell membranes are commonly considered fundamental structures having multiple roles such as confinement, storage of lipids, sustain and control of membrane proteins. In spite of their importance, many aspects remain unclear. The number of lipid types is orders of magnitude larger than the number of amino acids, and this compositional complexity is not clearly embedded in any membrane model. A diffused hypothesis is that the large lipid palette permits to recruit and organize specific proteins controlling the formation of specialized lipid domains and the lateral pressure profile of the bilayer. Unfortunately, a satisfactory knowledge of lipid abundance remains utopian because of the technical difficulties in isolating definite membrane regions. More importantly, a theoretical framework where to fit the lipidomic data is still missing. In this work, we wish to utilize the amino acid sequence and frequency of the membrane proteins as bioinformatics sensors of cell bilayers. The use of an alignment-free method to find a correlation between the sequences of transmembrane portion of membrane proteins with the membrane physical state (MPS) suggested a new approach for the discovery of antimicrobial peptides

The VMC survey - XI : Radial Stellar Population Gradients in the Galactic Globular Cluster 47 Tucanae

Copyright American Astronomical SocietyWe present a deep near-infrared color-magnitude diagram of the Galactic globular cluster 47 Tucanae, obtained with the Visible and Infrared Survey Telescope for Astronomy (VISTA) as part of the VISTA near-infrared Y, J, Ks survey of the Magellanic System (VMC). The cluster stars comprising both the subgiant and red giant branches exhibit apparent, continuous variations in color-magnitude space as a function of radius. Subgiant branch stars at larger radii are systematically brighter than their counterparts closer to the cluster core; similarly, red-giant-branch stars in the cluster's periphery are bluer than their more centrally located cousins. The observations can very well be described by adopting an age spread of ~0.5 Gyr as well as radial gradients in both the cluster's helium abundance (Y) and metallicity (Z), which change gradually from (Y = 0.28, Z = 0.005) in the cluster core to (Y = 0.25, Z = 0.003) in its periphery. We conclude that the cluster's inner regions host a significant fraction of second-generation stars, which decreases with increasing radius; the stellar population in the 47 Tuc periphery is well approximated by a simple stellar population.Peer reviewe

First XMM-Newton Observations of the Globular Cluster M22

We have examined preliminary data of the globular cluster, M22, from the EPIC MOS detectors on board XMM-Newton. We have detected 27 X-ray sources within the centre of the field of view, 24 of which are new detections. Three sources were found within the core of the cluster. From spectral analysis of the X-ray sources, it is possible that the object at the centre of the core is a quiescent X-ray transient and those lying further out are maybe cataclysmic variables.Comment: 6 pages, 3 figures, accepted to be published in Astronomy and Astrophysic

The evolution of the binary population in globular clusters: a full analytical computation

I present a simplified analytical model that simulates the evolution of the binary population in a dynamically evolving globular cluster. A number of simulations have been run spanning a wide range in initial cluster and environmental conditions by taking into account the main mechanisms of formation and destruction of binary systems. Following this approach, I investigate the evolution of the fraction, the radial distribution, the distribution of mass ratios and periods of the binary population. According to these simulations, the fraction of surviving binaries appears to be dominated by the processes of binary ionization and evaporation. In particular, the frequency of binary systems changes by a factor 1-5 depending on the initial conditions and on the assumed initial distribution of periods. The comparison with the existing estimates of binary fractions in Galactic globular clusters suggests that significant variations in the initial binary content could exist among the analysed globular cluster. This model has been also used to explain the observed discrepancy found between the most recent N-body and Monte Carlo simulations in the literature.Comment: 18 pages, 12 figures, accepted for publication by MNRA