Biological Principles in Self-Organization of Young Brain - Viewed from Kohonen Model

Variants of the Kohonen model are proposed to study biological principles of self-organization in a model of young brain. We suggest a function to measure aquired knowledge and use it to auto-adapt the topology of neuronal connectivity, yielding substantial organizational improvement relative to the standard model. In the early phase of organization with most intense learning, we observe that neural connectivity is of Small World type, which is very efficient to organize neurons in response to stimuli. In analogy to human brain where pruning of neural connectivity (and neuron cell death) occurs in early life, this feature is present also in our model, which is found to stabilize neuronal response to stimuli

Ensayo aleatorizado del cierre de orejuela izquierda vs varfarina para la prevención de accidentes cerebrovasculares tromboembólicos en pacientes con fibrilación auricular no relacionada con valvulopatía. Estudio PREVAIL

The successful application of poly­(<i>N</i>-vinylcaprolactam)-based microgels requires a profound understanding of their synthesis. For this purpose, a validated process model for the microgels synthesis by precipitation copolymerization with the cross-linker <i>N</i>,<i>N</i>′-methylenebis­(acrylamide) is formulated. Unknown reaction rate constants, reaction enthalpies, and partition coefficients are obtained by quantum mechanical calculations. The remaining parameter values are estimated from reaction calorimetry and Raman spectroscopy measurements of experiments with different monomer/cross-linker compositions. Because of high cross-propagation reaction rate constants, simulations predict a fast incorporation of the cross-linker. This agrees with reaction calorimetry measurements. Furthermore, the gel phase is predicted as the major reaction locus. The model is utilized for a prediction of the internal particle structure regarding its cross-link distribution. The highly cross-linked core reported in the literature corresponds to the predictions of the model

Orbital and Skeletal Structure of a Single Molecule on a Metal Surface Unveiled by Scanning Tunneling Microscopy

Atomic-scale spatial characteristics of a phthalocyanine orbital and skeleton are obtained on a metal surface with a scanning tunneling microscope and a CO-functionalized tip. Intriguingly, the high spatial resolution of the intramolecular electronic patterns is achieved without resonant tunneling into the orbital and despite the hybridization of the molecule with the reactive Cu substrate. The resolution can be fine-tuned by the tip–molecule distance, which controls the p-wave and s-wave contribution of the molecular probe to the imaging process. The detailed structure is deployed to minutely track the translation of the molecule in a reversible interconversion of rotational variants and to quantify relaxations of the adsorption geometry. Entering into the Pauli repulsion imaging mode, the intramolecular contrast loses its orbital character and reflects the molecular skeleton instead. The assignment of pyrrolic-hydrogen sites becomes possible, which in the orbital patterns remains elusive

Orbital and Skeletal Structure of a Single Molecule on a Metal Surface Unveiled by Scanning Tunneling Microscopy

Atomic-scale spatial characteristics of a phthalocyanine orbital and skeleton are obtained on a metal surface with a scanning tunneling microscope and a CO-functionalized tip. Intriguingly, the high spatial resolution of the intramolecular electronic patterns is achieved without resonant tunneling into the orbital and despite the hybridization of the molecule with the reactive Cu substrate. The resolution can be fine-tuned by the tip–molecule distance, which controls the p-wave and s-wave contribution of the molecular probe to the imaging process. The detailed structure is deployed to minutely track the translation of the molecule in a reversible interconversion of rotational variants and to quantify relaxations of the adsorption geometry. Entering into the Pauli repulsion imaging mode, the intramolecular contrast loses its orbital character and reflects the molecular skeleton instead. The assignment of pyrrolic-hydrogen sites becomes possible, which in the orbital patterns remains elusive

Molecular Nanocrystals on Ultrathin NaCl Films on Au(111)

International audienc

Branching Defects in Dendritic Molecules: Coupling Efficiency and Congestion Effects

An analytical model supplemented by Monte Carlo simulations specifies the statistics of branching defects in dendritic molecules as a function of the generation <i>g</i> as well as the maximal <i>g</i> for which defect-free synthesis is possible, <i>g</i>_max. The defects arise because of (i) imperfect coupling efficiency characterized by a constant fraction <i>P</i> ≤ 1 of successful add-on reactions in the absence of excluded volume effects and (ii) packing constraints associated with steric congestion at high <i>g</i> when the maximal density is approached. The model specifies <i>n</i>_<i>g</i>, the number of junctions, and the number of defects for both <i>g</i> ≤ <i>g</i>_max and <i>g</i> > <i>g</i>_max, as well as <i>g</i>_max and its dependence on <i>P</i>. The branching polydispersity is characterized by the average number of junction–junction bonds, <i>X</i>_<i>g</i>^eff. For <i>g</i> < <i>g</i>_max and efficient synthesis <i>X</i>_<i>g</i>^eff is weakly reduced with respect to <i>X</i>, its value in defect-free molecules, and <i>n</i>_g ∼ (<i>X</i>^eff – 1)^<i>g</i> increases exponentially. In the congested regime, at <i>g</i> > <i>g</i>_max, branching is strongly reduced, and <i>X</i>_<i>g</i>^eff slowly approaches 2 as <i>X</i>_<i>g</i>^eff – 2 ∼ 1/<i>g</i> while <i>n</i>_<i>g</i> eventually exhibits power law growth: <i>n</i>_<i>g</i> ∼ <i>g</i>³ for dendrimers and <i>n</i>_<i>g</i> ∼ <i>g</i>² for dendronized polymers. The branching defects can be interrogated by different forms of end-group analysis utilizing the theory framework proposed

Pushing Synthesis toward the Maximum Generation Range of Dendritic Macromolecules

The maximum generation <i>g</i>_max of a dendritic molecule denotes the value of the generation number <i>g</i>, above which such a compound cannot be synthesized without defects anymore due to steric constraints. For dendronized polymers (DPs), such a densely packed regime is entered far earlier (<i>g</i>_max ≈ 6) than it is for comparable dendrimers (<i>g</i>_max ≥ 10) because dendritic side chains are confined to a cylindrical rather than a spherical volume. We here report a long sought-after improvement to a key step in the divergent synthesis of high-<i>g</i> DPs which enabled obtaining the polymers of <i>g</i> = 6, 7, and 8. These DPs are of unprecedented dendritic perfection, and the representatives with <i>g</i> > 6 are to our knowledge the first molecules for which <i>g</i>_max has been surpassed. We suggest a straightforward parameter α which allows to assess whether any dendritic molecule is above <i>g</i>_max, given sufficiently efficient chemistry and the possibility of accurately determining the number of defects. Finally, we correlate gel permeation chromatography results and atomic force microscopic images with defect rates

Spectroscopic Line Shapes of Vibrational Quanta in the Presence of Molecular Resonances

Line shapes of molecular vibrational quanta in inelastic electron tunneling spectroscopy may indicate the strength of electron-vibration coupling, the hybridization of the molecule with its environment, and the degree of vibrational damping by electron–hole pair excitation. Bare as well as C₆₀-terminated Pb tips of a scanning tunneling microscope and clean as well as C₆₀-covered Pb­(111) surfaces were used in low-temperature experiments. Depending on the overlap of orbital and vibrational spectral ranges different spectroscopic line shapes of molecular vibrational quanta were observed. The energy range covered by the molecular resonance was altered by modifying the adsorption configuration of the molecule terminating the tip apex. Concomitantly, the line shapes of different vibrational modes were affected. The reported observations represent an experimental proof to theoretical predictions on the contribution from resonant processes to inelastic electron tunneling