504 research outputs found
Equilibrium spherically curved 2D Lennard-Jones systems
To learn about basic aspects of nano-scale spherical molecular shells during
their formation, spherically curved two-dimensional N-particle Lennard-Jones
systems are simulated, studying curvature evolution paths at zero-temperature.
For many N-values (N<800) equilibrium configurations are traced as a function
of the curvature radius R. Sharp jumps for tiny changes in R between
trajectories with major differences in topological structure correspond to
avalanche-like transitions. For a typical case, N=25, equilibrium
configurations fall on smooth trajectories in state space which can be traced
in the E-R plane. The trajectories show-up with local energy minima, from which
growth in N at steady curvature can develop.Comment: 10 pages, 2 figures, to be published in Journal of Chemical Physic
Dynamics of Air-Fluidized Granular System Measured by the Modulated Gradient Spin-echo
The power spectrum of displacement fluctuation of beads in the air-fluidized
granular system is measured by a novel NMR technique of modulated gradient
spin-echo. The results of measurement together with the related spectrum of the
velocity fluctuation autocorrelation function fit well to an empiric formula
based on to the model of bead caging between nearest neighbours; the cage
breaks up after a few collisions \cite{Menon1}. The fit yields the
characteristic collision time, the size of bead caging and the diffusion-like
constant for different degrees of system fluidization. The resulting mean
squared displacement increases proportionally to the second power of time in
the short-time ballistic regime and increases linearly with time in the
long-time diffusion regime as already confirmed by other experiments and
simulations.Comment: 4 figures. Submited to Physical Review Letters, April 200
Blood particulate analogue fluids: A review
Microfluidics has proven to be an extraordinary working platform to mimic and study blood flow phenomena and the dynamics of components of the human microcirculatory system. However, the use of real blood increases the complexity to perform these kinds of in vitro blood experiments due to diverse problems such as coagulation, sample storage, and handling problems. For this reason, interest in the development of fluids with rheological properties similar to those of real blood has grown over the last years. The inclusion of microparticles in blood analogue fluids is essential to reproduce multiphase effects taking place in a microcirculatory system, such as the cell-free layer (CFL) and Fähraeus–Lindqvist effect. In this review, we summarize the progress made in the last twenty years. Size, shape, mechanical properties, and even biological functionalities of microparticles produced/used to mimic red blood cells (RBCs) are critically exposed and analyzed. The methods developed to fabricate these RBC templates are also shown. The dynamic flow/rheology of blood particulate analogue fluids proposed in the literature (with different particle concentrations, in most of the cases, relatively low) is shown and discussed in-depth. Although there have been many advances, the development of a reliable blood particulate analogue fluid, with around 45% by volume of microparticles, continues to be a big challengeThis research was funded by the Spanish Ministry of Science and Education Grant No.
PID2019-108278RB-C32 / AEI / 10.13039/501100011033, and Junta de Extremadura (Spain) Grant
Nos. GR18175 and IB18005 (partially financed by FEDER funds). The authors also acknowledge
the Fundação para a Ciência e a Tecnologia (FCT) for partially financing the research under the
strategic grants UIDB/04077/2020, UIDB/00532/2020, and the project NORTE-01-0145-FEDER030171 (PTDC/EME-SIS/30171/2017) funded by COMPETE2020, NORTE 2020, PORTUGAL 2020,
Lisb@2020, and FEDE
Critical swelling of particle-encapsulating vesicles
We consider a ubiquitous scenario where a fluctuating, semipermeable vesicle
is embedded in solution while enclosing a fixed number of solute particles. The
swelling with increasing number of particles or decreasing concentration of the
outer solution exhibits a continuous phase transition from a fluctuating state
to the maximum-volume configuration, whereupon appreciable pressure difference
and surface tension build up. This criticality is unique to
particle-encapsulating vesicles, whose volume and inner pressure both
fluctuate. It implies a universal swelling behavior of such vesicles as they
approach their limiting volume and osmotic lysis.Comment: 4 pages, 1 figur
Fluctuation spectrum of fluid membranes coupled to an elastic meshwork: jump of the effective surface tension at the mesh size
We identify a class of composite membranes: fluid bilayers coupled to an
elastic meshwork, that are such that the meshwork's energy is a function
\textit{not} of the real microscopic membrane area ,
but of a \textit{smoothed} membrane's area , which corresponds to the
area of the membrane coarse-grained at the mesh size . We show that the
meshwork modifies the membrane tension both below and above the scale
, inducing a tension-jump . The
predictions of our model account for the fluctuation spectrum of red blood
cells membranes coupled to their cytoskeleton. Our results indicate that the
cytoskeleton might be under extensional stress, which would provide a means to
regulate available membrane area. We also predict an observable tension jump
for membranes decorated with polymer "brushes"
A Sucrose Solution Application to the Study of Model Biological Membranes
The small-angle X-ray and neutron scattering, time resolved X-ray small-angle
and wide-angle diffraction coupled with differential scanning calorimetry have
been applied to the investigation of unilamellar and multilamellar
dimyristoylphosphatidylcholine (DMPC) vesicles in sucrose buffers with sucrose
concentrations from 0 to 60%. Sucrose buffer decreased vesicle size and
polydispersity and increased an X-ray contrast between phospholipid membrane
and bulk solvent sufficiently. No influence of sucrose on the membrane
thickness or mutual packing of hydrocarbon chains has been detected. The region
of sucrose concentrations 30%-40% created the best experimental conditions for
X-ray small-angle experiments with phospholipid vesicles.Comment: PDF: 10 pages, 6 figures. MS Word sours
Fluorescence studies on new potential antitumoral benzothienopyran-1-ones in solution and in liposomes
Fluorescence properties of four new potential
antitumoral compounds, 3-arylbenzothieno[2,3-c]pyran-1-ones, were studied in solution and in lipid membranes of dipalmitoyl phosphatidylcholine (DPPC), egg yolk phosphatidylcholine (Egg-PC) and dioctadecyldimethylammonium bromide (DODAB). The 3-(4-methoxyphenyl)benzothieno[2,3-c]pyran-1-one (1c) exhibits the higher fluorescence quantum yields in all solvents studied. All compounds present a solvent sensitive emission, with significant red shifts in polar solvents for the methoxylated compounds. The results point to an ICT character of the excited state, more pronounced for compound 1c. Fluorescence (steady-state) anisotropy measurements of the compounds incorporated in liposomes of DPPC, DODAB and Egg-PC indicate that all compounds have two different locations, one due to a deep penetration in the lipid membrane and another corresponding to a more hydrated environment. In general, the methoxylated compounds prefer hydrated environments inside the liposomes. The 3-(4-
fluorophenyl)benzothieno[2,3-c]pyran-1-one (1a) clearly prefers a hydrated environment, with some molecules located at the outer part of the liposome interface. On the contrary, the preferential location of 3-(2-fluorophenyl)benzothieno[2,3-c]pyran-1-one (1b) is in the region of lipid hydrophobic tails. Compounds with a planar geometry (1a and 1c) have higher mobility in the lipid membranes when phase transition occurs.Portugal and FEDER (Fundo Europeu de Desenvolvimento Regional), for financial support through Centro de Física (CFUM) and Centro de Química (CQ-UM) of University of Minho and through the Project PTDC/QUI/81238/2006. M.S.D. Carvalho and R.C. Calhelha acknowledge FCT for their PhD grants SFRH/BD/47052/2008 and SFRH/BD/29274/2006, respectively.Fundação para a Ciência e a Tecnologia (FCT
ALA and ALA hexyl ester in free and liposomal formulations for the photosensitisation of tumour organ cultures
In spite of the wide range of tumours successfully treated with 5-aminolevulinic acid mediated photodynamic therapy, the fact that 5-aminolevulinic acid has low lipid solubility, limits its clinical application. More lipophilic 5-aminolevulinic acid prodrugs and the use of liposomal carriers are two approaches aimed at improving 5-aminolevulinic acid transmembrane access. In this study we used both 5-aminolevulinic acid and its hexyl ester in their free and encapsulated formulations to compare their corresponding endogenous synthesis of porphyrins. Employing murine tumour cultures, we found that neither the use of hexyl ester nor the entrappment of either 5-aminolevulinic acid or hexyl ester into liposomes increase the rate of tumour porphyrin synthesis. By light and electronic microscopy it was demonstrated that exposure of tumour explants to either free or liposomal 5-aminolevulinic acid and subsequent illumination induces the same type of subcellullar damage. Mitochondria, endoplasmic reticulum and plasma membrane are the structures mostly injured in the early steps of photodynamic treatment. In a later stage, cytoplasmic and nuclear disintegration are observed. By electronic microscopy the involvement of the endocytic pathway in the incorporation of liposomal 5-aminolevulinic acid into the cells was shown
Characterization of monoolein-based lipoplexes using fluorescence spectroscopy
Lipoplexes are commonly used as delivery systems in vitro and in vivo, the role of a neutral lipid as helper being of extreme importance in these systems. Cationic liposomes composed of dioctadecyldimethylammonium bromide (DODAB) with monoolein (MO) as a helper, at different molar ratios (1:2; 1:1 and 1:0.5) were prepared, and subsequently titrated to DNA. The structural and physicochemical properties of the lipid/DNA complexes were assessed by Ethidium Bromide (EtBr) exclusion, 90º Static Light Scattering (90º SLS) assays and Fluorescence Resonance Energy Transfer (FRET). In EtBr exclusion assays, the steady-state fluorescence spectra of EtBr were decomposed into the sum of two lognormal emissions, emanating from two different environments – H2O and DNA, and the effect of charge ratio (+/-) was observed. 90º SLS assays gave an important contribution, detecting size variations in systems with different MO fractions on the lipoplexes. In FRET assays, 2-(3-(diphenylhexatrienyl)propanoyl)-1-hexadecanoyl-sn-glycero-3-phosphocholine (DPH-HPC) was used as donor and EtBr as acceptor. The DNA component previously calculated by EtBr exclusion, was used to determine the energy transfer efficiency, as an indirect measurement of the lipoplexes structural and physicochemical properties. Our results demonstrate that the inclusion of monoolein in the cationic liposomes formulation significantly modifies the rate of DNA complexation, being DODAB:MO (1:1) the system with higher DNA condensation efficiency.Fundação para a Ciência e a Tecnologia (FCT
DODAB and DODAC bilayer-like aggregates in the micromolar surfactant concentration domain
In the millimolar concentration domain (typically 1 mM), dioctadecyldimethylammonium bromide and chloride (DODAX, X representing Br- or Cl- counterions) molecules assemble in water as large unilamellar vesicles. Differential scanning calorimetry (DSC) is a suitable technique to obtain the melting temperature (Tm) characteristic of surfactant bilayers, while fluorescence spectroscopy detects formation of surfactant aggregates, like bilayers. These two techniques were combined to investigate the assemble of DODAX molecules at micromolar concentrations, from 10 to 100 micromolar. At 1 mM surfactant, Tm ~ 45 ºC and 49 oC, respectively for DODAB and DODAC. DSC and fluorescence of Nile Red were used to show the formation of DODAX aggregates, at the surfactant concentration as low as 10 micromolar, whose Tm decreases monotonically with increasing DODAX concentration to attain the value for the ordinary vesicles. The data indicate that these aggregates are organized as bilayer-like structures.Fundação para a Ciência e a Tecnologia (FCT
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