We have recorded the swarming-like collective migration of a large number of keratocytes (tissue ... more We have recorded the swarming-like collective migration of a large number of keratocytes (tissue cells obtained from the scales of goldfish) using long-term videomicroscopy. By increasing the overall density of the migrating cells, we have been able to demonstrate experimentally a kinetic phase transition from a disordered into an ordered state. Near the critical density a complex picture emerges with interacting clusters of cells moving in groups. Motivated by these experiments we have constructed a flocking model that exhibits a continuous transition to the ordered phase, while assuming only short-range interactions and no explicit information about the knowledge of the directions of motion of neighbors. Placing cells in microfabricated arenas we found spectacular whirling behavior which we could also reproduce in simulations.
Leaf venation is a pervasive example of a complex biological network, endowing leaves with a tran... more Leaf venation is a pervasive example of a complex biological network, endowing leaves with a transport system and mechanical resilience. Transport networks optimized for efficiency have been shown to be trees, i.e. loopless. However, dicotyledon leaf venation has a large number of closed loops, which are functional and able to transport fluid in the event of damage to any vein, including the primary veins. Inspired by leaf venation, we study two possible reasons for the existence of a high density of loops in transport networks: resilience to damage and fluctuations in load. In the first case, we seek the optimal transport network in the presence of random damage by averaging over damage to each link. In the second case, we seek the network that optimizes transport when the load is sparsely distributed: at any given time most sinks are closed. We find that both criteria lead to the presence of loops in the optimum state.
Physica A: Statistical Mechanics and its Applications, 2004
We present a thermodynamically consistent mesoscopic model of protein adsorption at liquid-solid ... more We present a thermodynamically consistent mesoscopic model of protein adsorption at liquid-solid interfaces. First describing the equilibrium state under varying protein concentration of the solution and binding conditions, we predict a non-trivial (nonmonotonic) dependence of the experimentally observable properties of the adsorbed layer (such as the surface density and surface coverage) on these parameters. We subsequently proceed to develop a dynamical model consistent with the equilibrium description, which qualitatively reproduces known experimental phenomena and offers a promising way of studying the exchange of the adsorbed proteins by the proteins of the solution. r (G.J. Szo¨ll + osi).
The use of one equivalent of benzylamine increased the enantioselectivity in the heterogeneous ca... more The use of one equivalent of benzylamine increased the enantioselectivity in the heterogeneous catalytic hydrogenation of α,β-unsaturated carboxylic acids over cinchonidine-modified 5% Pd/Al 2 O 3 catalyst. The beneficial effect of the amine additive was dependent on substrate structure. The highest effect, a more than sixfold increase in the enantiomeric excess, was obtained in the hydrogenation of itaconic acid. This is the first report on the enantioselective hydrogenation of a prochiral α,β-unsaturated dicarboxylic acid over a supported heterogeneous metal catalyst. 2005 Elsevier Inc. All rights reserved.
A study on the origin of rate enhancement (RE) in the enantioselective heterogeneous catalytic hy... more A study on the origin of rate enhancement (RE) in the enantioselective heterogeneous catalytic hydrogenation of methyl benzoylformate (MBF), ketopantolactone (KPL) and pyruvic aldehyde dimethyl acetal (PA) under the Orito reaction conditions over Pt catalyst modified with parent cinchona alkaloids, as compared to the unmodified catalyst is presented. The hydrogenations were carried out in continuousflow fixed-bed reactor system over 20-100 mg Pt/Al 2 O 3 catalyst in 1 mL min −1 flow of toluene/acetic acid 9/1 solvent mixture under 40-80 bar H 2 pressure, at 283 or 293 K using 0.044-2 mM modifier concentration and 45 mM substrate concentration. Our results obtained using racemic hydrogenations followed by three changes of the chiral modifier (on the same catalyst) supported the so-called "ligand acceleration" phenomenon in the enantioselective hydrogenation of activated ketones such as MBF, KPL and PA. In our opinion, RE produced by the first modifier added after racemic hydrogenation can also be explained by the purifying effect of the cinchona. REs observed following further exchanges of modifiers are indicative of the intrinsic character of the phenomenon. This research suggested that the origin of enantiodifferentiation and rate enhancement is the same, namely, both may be traced back-probably in different ways-to the role of the intermediate complexes of the hydrogenation, to its formation and transformation, which in turn depends on numerous factors.
ABSTRACT The first example of enantioselective heterogeneous catalytic hydrogenation of an α-fluo... more ABSTRACT The first example of enantioselective heterogeneous catalytic hydrogenation of an α-fluoro ketone is reported. The hydrogenation of racemic 2-fluorocyclohexanone over cinchonidine- or methoxycinchonidine-modified Pt/Al2O3 resulted in diastereoselective and enantioselective formation of the cis-(1R,2S)-2-fluorocyclohexanol. Due to the preferential hydrogenation of the S enantiomer, the kinetic resolution of the substrate was possible; the unreacted R enantiomer was accumulated in the reaction mixture. Under our experimental conditions, high diastereoselectivities (up to 85%) and good enantioselectivities (up to 59%) were obtained, demonstrating that activation of a ketone by a single α fluorine atom is efficient for obtaining enantiodiscrimination in this heterogeneous catalytic system.
The sonochemical enantioselective hydrogenation of various prochiral carbonyl compounds to the co... more The sonochemical enantioselective hydrogenation of various prochiral carbonyl compounds to the corresponding (R)-hydroxy derivatives over different platinum catalysts using cinchonidine as chiral modifier is described. The carbonyl derivatives studied were trifluoromethyl ketones and ␣and -ketoesters. The sonochemical pretreatment of the catalyst-modifier system was found to be mostly advantageous in increasing the optical yield. Moreover, the ultrasonically promoted reactions provided the best enantiomeric excesses (ee%) ever obtained in this heterogeneous catalytic system (methyl pyruvate 95 ee%, ethyl 4-phenyl-2-oxobutyrate 95 ee%, and ethyl benzoylformate 92 ee%). In a few cases, however, the insonation did not result in enhancement in enantioselectivities. On the basis of the comparison with conventional reactions the limitations of the ultrasonic irradiation in asymmetric hydrogenations are also drawn.
Conventional kinesin is a motor protein, which is able to walk along a microtubule processively. ... more Conventional kinesin is a motor protein, which is able to walk along a microtubule processively. The exact mechanism of the stepping motion and force generation of kinesin is still far from clear. In this paper we argue that neck linker docking is a crucial element of this mechanism, without which the experimentally observed dwell times of the steps could not be explained under a wide range of loading forces. We also show that the experimental data impose very strict constraints on the lengths of both the neck linker and its docking section, which are compatible with the known structure of kinesin.
Leaf venation is a pervasive example of a complex biological transport network that is necessary ... more Leaf venation is a pervasive example of a complex biological transport network that is necessary for the survival of land plants. Transport networks optimized for efficiency have been shown to be trees, i.e. loopless. However, dicotyledon leaf venation has a large number of functional closed loops. Inspired by leaf venation, we study two possible reasons for the existence of a
We have recorded the swarming-like collective migration of a large number of keratocytes (tissue ... more We have recorded the swarming-like collective migration of a large number of keratocytes (tissue cells obtained from the scales of goldfish) using long-term videomicroscopy. By increasing the overall density of the migrating cells, we have been able to demonstrate experimentally a kinetic phase transition from a disordered into an ordered state. Near the critical density a complex picture emerges with interacting clusters of cells moving in groups. Motivated by these experiments we have constructed a flocking model that exhibits a continuous transition to the ordered phase, while assuming only short-range interactions and no explicit information about the knowledge of the directions of motion of neighbors. Placing cells in microfabricated arenas we found spectacular whirling behavior which we could also reproduce in simulations.
Leaf venation is a pervasive example of a complex biological network, endowing leaves with a tran... more Leaf venation is a pervasive example of a complex biological network, endowing leaves with a transport system and mechanical resilience. Transport networks optimized for efficiency have been shown to be trees, i.e. loopless. However, dicotyledon leaf venation has a large number of closed loops, which are functional and able to transport fluid in the event of damage to any vein, including the primary veins. Inspired by leaf venation, we study two possible reasons for the existence of a high density of loops in transport networks: resilience to damage and fluctuations in load. In the first case, we seek the optimal transport network in the presence of random damage by averaging over damage to each link. In the second case, we seek the network that optimizes transport when the load is sparsely distributed: at any given time most sinks are closed. We find that both criteria lead to the presence of loops in the optimum state.
Physica A: Statistical Mechanics and its Applications, 2004
We present a thermodynamically consistent mesoscopic model of protein adsorption at liquid-solid ... more We present a thermodynamically consistent mesoscopic model of protein adsorption at liquid-solid interfaces. First describing the equilibrium state under varying protein concentration of the solution and binding conditions, we predict a non-trivial (nonmonotonic) dependence of the experimentally observable properties of the adsorbed layer (such as the surface density and surface coverage) on these parameters. We subsequently proceed to develop a dynamical model consistent with the equilibrium description, which qualitatively reproduces known experimental phenomena and offers a promising way of studying the exchange of the adsorbed proteins by the proteins of the solution. r (G.J. Szo¨ll + osi).
The use of one equivalent of benzylamine increased the enantioselectivity in the heterogeneous ca... more The use of one equivalent of benzylamine increased the enantioselectivity in the heterogeneous catalytic hydrogenation of α,β-unsaturated carboxylic acids over cinchonidine-modified 5% Pd/Al 2 O 3 catalyst. The beneficial effect of the amine additive was dependent on substrate structure. The highest effect, a more than sixfold increase in the enantiomeric excess, was obtained in the hydrogenation of itaconic acid. This is the first report on the enantioselective hydrogenation of a prochiral α,β-unsaturated dicarboxylic acid over a supported heterogeneous metal catalyst. 2005 Elsevier Inc. All rights reserved.
A study on the origin of rate enhancement (RE) in the enantioselective heterogeneous catalytic hy... more A study on the origin of rate enhancement (RE) in the enantioselective heterogeneous catalytic hydrogenation of methyl benzoylformate (MBF), ketopantolactone (KPL) and pyruvic aldehyde dimethyl acetal (PA) under the Orito reaction conditions over Pt catalyst modified with parent cinchona alkaloids, as compared to the unmodified catalyst is presented. The hydrogenations were carried out in continuousflow fixed-bed reactor system over 20-100 mg Pt/Al 2 O 3 catalyst in 1 mL min −1 flow of toluene/acetic acid 9/1 solvent mixture under 40-80 bar H 2 pressure, at 283 or 293 K using 0.044-2 mM modifier concentration and 45 mM substrate concentration. Our results obtained using racemic hydrogenations followed by three changes of the chiral modifier (on the same catalyst) supported the so-called "ligand acceleration" phenomenon in the enantioselective hydrogenation of activated ketones such as MBF, KPL and PA. In our opinion, RE produced by the first modifier added after racemic hydrogenation can also be explained by the purifying effect of the cinchona. REs observed following further exchanges of modifiers are indicative of the intrinsic character of the phenomenon. This research suggested that the origin of enantiodifferentiation and rate enhancement is the same, namely, both may be traced back-probably in different ways-to the role of the intermediate complexes of the hydrogenation, to its formation and transformation, which in turn depends on numerous factors.
ABSTRACT The first example of enantioselective heterogeneous catalytic hydrogenation of an α-fluo... more ABSTRACT The first example of enantioselective heterogeneous catalytic hydrogenation of an α-fluoro ketone is reported. The hydrogenation of racemic 2-fluorocyclohexanone over cinchonidine- or methoxycinchonidine-modified Pt/Al2O3 resulted in diastereoselective and enantioselective formation of the cis-(1R,2S)-2-fluorocyclohexanol. Due to the preferential hydrogenation of the S enantiomer, the kinetic resolution of the substrate was possible; the unreacted R enantiomer was accumulated in the reaction mixture. Under our experimental conditions, high diastereoselectivities (up to 85%) and good enantioselectivities (up to 59%) were obtained, demonstrating that activation of a ketone by a single α fluorine atom is efficient for obtaining enantiodiscrimination in this heterogeneous catalytic system.
The sonochemical enantioselective hydrogenation of various prochiral carbonyl compounds to the co... more The sonochemical enantioselective hydrogenation of various prochiral carbonyl compounds to the corresponding (R)-hydroxy derivatives over different platinum catalysts using cinchonidine as chiral modifier is described. The carbonyl derivatives studied were trifluoromethyl ketones and ␣and -ketoesters. The sonochemical pretreatment of the catalyst-modifier system was found to be mostly advantageous in increasing the optical yield. Moreover, the ultrasonically promoted reactions provided the best enantiomeric excesses (ee%) ever obtained in this heterogeneous catalytic system (methyl pyruvate 95 ee%, ethyl 4-phenyl-2-oxobutyrate 95 ee%, and ethyl benzoylformate 92 ee%). In a few cases, however, the insonation did not result in enhancement in enantioselectivities. On the basis of the comparison with conventional reactions the limitations of the ultrasonic irradiation in asymmetric hydrogenations are also drawn.
Conventional kinesin is a motor protein, which is able to walk along a microtubule processively. ... more Conventional kinesin is a motor protein, which is able to walk along a microtubule processively. The exact mechanism of the stepping motion and force generation of kinesin is still far from clear. In this paper we argue that neck linker docking is a crucial element of this mechanism, without which the experimentally observed dwell times of the steps could not be explained under a wide range of loading forces. We also show that the experimental data impose very strict constraints on the lengths of both the neck linker and its docking section, which are compatible with the known structure of kinesin.
Leaf venation is a pervasive example of a complex biological transport network that is necessary ... more Leaf venation is a pervasive example of a complex biological transport network that is necessary for the survival of land plants. Transport networks optimized for efficiency have been shown to be trees, i.e. loopless. However, dicotyledon leaf venation has a large number of functional closed loops. Inspired by leaf venation, we study two possible reasons for the existence of a
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