Heat transfer from nanoparticles: a corresponding state analysis

In this contribution, we study situations in which nanoparticles in a fluid are strongly heated, generating high heat fluxes. This situation is relevant to experiments in which a fluid is locally heated using selective absorption of radiation by solid particles. We first study this situation for different types of molecular interactions, using models for gold particles suspended in octane and in water. As already reported in experiments, very high heat fluxes and temperature elevations (leading eventually to particle destruction) can be observed in such situations. We show that a very simple modeling based on Lennard-Jones interactions captures the essential features of such experiments, and that the results for various liquids can be mapped onto the Lennard-Jones case, provided a physically justified (corresponding state) choice of parameters is made. Physically, the possibility of sustaining very high heat fluxes is related to the strong curvature of the interface that inhibits the formation of an insulating vapor film

Conformational Mechanics of Polymer Adsorption Transitions at Attractive Substrates

Conformational phases of a semiflexible off-lattice homopolymer model near an attractive substrate are investigated by means of multicanonical computer simulations. In our polymer-substrate model, nonbonded pairs of monomers as well as monomers and the substrate interact via attractive van der Waals forces. To characterize conformational phases of this hybrid system, we analyze thermal fluctuations of energetic and structural quantities, as well as adequate docking parameters. Introducing a solvent parameter related to the strength of the surface attraction, we construct and discuss the solubility-temperature phase diagram. Apart from the main phases of adsorbed and desorbed conformations, we identify several other phase transitions such as the freezing transition between energy-dominated crystalline low-temperature structures and globular entropy-dominated conformations.Comment: 13 pages, 15 figure

Investigating the specificity of peptide adsorption on gold using molecular dynamics simulations

We report all-atom molecular dynamics simulations following adsorption of gold-binding and non-gold-binding peptides on gold surfaces modeled with dispersive interactions. We examine the dependence of adsorption on both identity of the amino acids and mobility of the peptides. Within the limitations of the approach, results indicate that when the peptides are solvated, adsorption requires both configurational changes and local flexibility of individual amino acids. This is achieved when peptides consist mostly of random coils or when their secondary structural motifs (helices, sheets) are short and connected by flexible hinges. In the absence of solvent, only affinity for the surface is required: mobility is not important. In combination, these results suggest the barrier to adsorption presented by displacement of water molecules requires conformational sampling enabled through mobility.Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – SFRH/BPD/20555/2004/0GV

Placental lactogens induce serotonin biosynthesis in a subset of mouse beta cells during pregnancy

AIMS/HYPOTHESIS: Upregulation of the functional beta cell mass is required to match the physiological demands of mother and fetus during pregnancy. This increase is dependent on placental lactogens (PLs) and prolactin receptors, but the mechanisms underlying these events are only partially understood. We studied the mRNA expression profile of mouse islets during pregnancy to gain a better insight into these changes. METHODS: RNA expression was measured ex vivo via microarrays and quantitative RT-PCR. In vivo observations were extended by in vitro models in which ovine PL was added to cultured mouse islets and MIN6 cells. RESULTS: mRNA encoding both isoforms of the rate-limiting enzyme of serotonin biosynthesis, tryptophan hydroxylase (TPH), i.e. Tph1 and Tph2, were strongly induced (fold change 25- to 200-fold) during pregnancy. This induction was mimicked by exposing islets or MIN6 cells to ovine PLs for 24 h and was dependent on janus kinase 2 and signal transducer and activator of transcription 5. Parallel to Tph1 mRNA and protein induction, islet serotonin content increased to a peak level that was 200-fold higher than basal. Interestingly, only a subpopulation of the beta cells was serotonin-positive in vitro and in vivo. The stored serotonin pool in pregnant islets and PL-treated MIN6 cells was rapidly released (turnover once every 2 h). CONCLUSIONS/INTERPRETATION: A very strong lactogen-dependent upregulation of serotonin biosynthesis occurs in a subpopulation of mouse islet beta cells during pregnancy. Since the newly formed serotonin is rapidly released, this lactogen-induced beta cell function may serve local or endocrine tasks, the nature of which remains to be identified

Exploring Functional β-Cell Heterogeneity In Vivo Using PSA-NCAM as a Specific Marker

BACKGROUND:The mass of pancreatic beta-cells varies according to increases in insulin demand. It is hypothesized that functionally heterogeneous beta-cell subpopulations take part in this process. Here we characterized two functionally distinct groups of beta-cells and investigated their physiological relevance in increased insulin demand conditions in rats. METHODS:Two rat beta-cell populations were sorted by FACS according to their PSA-NCAM surface expression, i.e. beta(high) and beta(low)-cells. Insulin release, Ca(2+) movements, ATP and cAMP contents in response to various secretagogues were analyzed. Gene expression profiles and exocytosis machinery were also investigated. In a second part, beta(high) and beta(low)-cell distribution and functionality were investigated in animal models with decreased or increased beta-cell function: the Zucker Diabetic Fatty rat and the 48 h glucose-infused rat. RESULTS:We show that beta-cells are heterogeneous for PSA-NCAM in rat pancreas. Unlike beta(low)-cells, beta(high)-cells express functional beta-cell markers and are highly responsive to various insulin secretagogues. Whereas beta(low)-cells represent the main population in diabetic pancreas, an increase in beta(high)-cells is associated with gain of function that follows sustained glucose overload. CONCLUSION:Our data show that a functional heterogeneity of beta-cells, assessed by PSA-NCAM surface expression, exists in vivo. These findings pinpoint new target populations involved in endocrine pancreas plasticity and in beta-cell defects in type 2 diabetes