Autonomous Learning of Features for Control: Experiments with Embodied and Situated Agents

As discussed in previous studies, the efficacy of evolutionary or reinforcement learning algorithms for continuous control optimization can be enhanced by including a neural module dedicated to feature extraction trained through self-supervised methods. In this paper we report additional experiments supporting this hypothesis and we demonstrate how the advantage provided by feature extraction is not limited to problems that benefit from dimensionality reduction or that involve agents operating on the basis of allocentric perception. We introduce a method that permits to continue the training of the feature-extraction module during the training of the policy network and that increases the efficacy of feature extraction. Finally, we compare alternative feature-extracting methods and we show that sequence-to-sequence learning yields better results than the methods considered in previous studies

Synthesis and characterization of trans-di-(4-pyridyl)porphyrin dimers

Preparation and characterization of a small library of symmetric trans-di(4-pyridyl) porphyrin dimers, obtained by either Glaser\u2013Hay or Sonogashira coupling reactions from appropriately prepared trans-di-4-pyridylporphyrin precursors, is presented. The porphyrin dimers are differentiated by a phenyl-alkynyl bridge of increasing length at one meso-position, while for all the derivatives the two remaining opposite meso-positions are tailored with a phenyl moiety bearing a short polyether chain. Coordination of the four pyridyl groups with appropriate metal fragments may be exploited to construct tubular hollow structures, with varied internal sizes, depending on the choice of the porphyrin dimer component

Spontaneous insertion of carbon nanotube bundles inside biomembranes: a hybrid particle-field coarse-grained molecular dynamics study

The processes of CNTs bundle formation and insertion/rearrangement inside lipid bilayers, as models of cellular membranes, is described and analyzed in details using simulations on the microsecond scale. Molecular Dynamics simulations employing hybrid particle-field models (MD–SCF) show that during the insertion process lipid molecules coat bundles surfaces. The distortions of bilayers are more pronounced for systems undergoing to insertion of bundles made of longer CNTs. In particular, when the insertion occurs in perpendicular orientation, adsorption of lipids on CNTs surfaces promotes a transient poration. This result suggests mechanism of membrane disruption operated by bundles causing the formation of solvent-rich pockets

Hybrid Particle-Field Coarse-Grained Models for Biological Phospholipids

In the framework of a recently developed scheme for a hybrid particle-field simulation technique where self-consistent field theory (SCF) and molecular dynamics (MD) are combined [J. Chem. Phys. 2009, 130, 214106], specific coarse-grained models for phospholipids and water have been developed. We optimized the model parameters, which are necessary in evaluating the interactions between the particles and the density fields, so that the coarse-grained model can reproduce the structural properties of the reference particle-particle simulations. The development of these specific coarse-grained models suitable for hybrid particle-field simulations opens the way toward simulations of large-scale systems employing models with chemical specificity, especially for biological systems. © 2011 American Chemical Society

Understanding the Interaction of Block Copolymers with DMPC Lipid Bilayer using Coarse-Grained Molecular Dynamics Simulations

In this paper, we present a computational model of the adsorption and percolation mechanism of poloxamers (poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) triblock copolymers) across a 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid bilayer. A coarse-grained model was used to cope with the long time scale of the percolation process. The simulations have provided details of the interaction mechanism of Pluronics with lipid bilayer. In particular, the results have shown that polymer chains containing a PPO block with a length comparable to the DMPC bilayer thickness, such as P85, tends to percolate across the lipid bilayer. On the contrary, Pluronics with a shorter PPO chain, such as L64 and F38, insert partially into the membrane with the PPO block part while the PEO blocks remain in water on one side of the lipid bilayer. The percolation of the polymers into the lipid tail groups reduces the membrane thickness and increases the area per lipid. These effects are more evident for P85 than L64 or F38. Our findings are qualitatively in good agreement with published small-angle X-ray scattering experiments that have evidenced a thinning effect of Pluronics on the lipid bilayer as well as the role of the length of the PPO block on the permeation process of the polymer through the lipid bilayer. Our theoretical results complement the experimental data with a detailed structural and dynamic model of poloxamers at the interface and inside the lipid bilayer