Chain conformation: A key parameter driving clustering or dispersion in polyelectrolyte - Colloid systems.

The work presents the characterization of Pluronic F127 micellar solutions in presence of hyaluronic acid in semi-dilute regime. The effects of the nature and salt concentration are investigated by differential scanning calorimetry and small angle neutron scattering. Hyaluronic acid reduces the critical micellar temperature to the same extend as an increase of the ionic strength. Within the investigated HA concentration range, the size and shape of the micelles are not modified by the addition of HA but their dispersion state depends on the salt concentration. By increasing the ionic strength we observe the formation of small micellar clusters which organize into a face-centered cubic liquid crystalline phase at high salt concentration. This behavior is reinforced by increasing the HA concentration or molecular weight. The nature of the salt plays also a role and divalent cations such as Ca2+ promote the clustering of micelles and their crystallization. The origin of the aggregative behavior is the change of the HA chain conformation -from stretched to coil- by addition of salt which in turn induces an excluded volume around the micelles and exerts a depletion interaction.

The fluctuating ribosome: thermal molecular dynamics characterized by neutron scattering.

Conformational changes associated with ribosome function have been identified by X-ray crystallography and cryo-electron microscopy. These methods, however, inform poorly on timescales. Neutron scattering is well adapted for direct measurements of thermal molecular dynamics, the 'lubricant' for the conformational fluctuations required for biological activity. The method was applied to compare water dynamics and conformational fluctuations in the 30 S and 50 S ribosomal subunits from Haloarcula marismortui, under high salt, stable conditions. Similar free and hydration water diffusion parameters are found for both subunits. With respect to the 50 S subunit, the 30 S is characterized by a softer force constant and larger mean square displacements (MSD), which would facilitate conformational adjustments required for messenger and transfer RNA binding. It has been shown previously that systems from mesophiles and extremophiles are adapted to have similar MSD under their respective physiological conditions. This suggests that the results presented are not specific to halophiles in high salt but a general property of ribosome dynamics under corresponding, active conditions. The current study opens new perspectives for neutron scattering characterization of component functional molecular dynamics within the ribosome.

Dynamics of nanoscale polarization fluctuations in a uniaxial relaxor.

We have studied neutron diffuse scattering in a Sr(0.61)Ba(0.39)Nb(2)O(6) single crystal by neutron backscattering at sub-µeV energy resolution. We can identify two response components with transverse polarization: an elastic (resolution limited) central peak, which monotonically increases with decreasing temperature, and a quasielastic central peak, having a maximum intensity around the ferroelectric phase transition close to 350 K. In contrast to previous neutron experiments on this and other relaxor materials, we were able to observe a temperature dependence of the characteristic frequency of these fluctuations, obeying the same Vogel-Fulcher law as the dynamic part of the dielectric permittivity of this material. In this way our findings provide a first direct link between the Vogel-Fulcher-type frequency dependence of dielectric permittivity and dynamic nanoscale lattice modulations with a transverse correlation length of about 5-10 unit cells.

Evidence of dynamical constraints imposed by water organization around a bio-hydrophobic interface.

Molecular dynamics simulations and elastic neutron scattering experiments have been used to highlight how the structural organization of hydration water is able in some cases to locally constrain atomic movements at biologic interfaces. Using fully hydrated small peptides as models of protein interfaces, we show that the length of the side chains and the hydrophilic backbone have specific signatures. The dynamics of the side chain, which is part of biomolecules, have not only a crucial role in the whole flexibility as compared to the backbone, but also modify the values of transition temperatures. The analysis of the activation energies of methyl group dynamics suggests that the interaction between hydrophobic side chain and surrounding water plays an important role in the whole flexibility as well. We suggest that the progressive water cluster organization, around hydrophobic interfaces increases the activation energy and that a plateau regime is reached only when an extended hydrogen-bond network is established. The cluster size corresponds to a single layer of water molecules.

From chain collapse to new structures: spectroscopic properties of poly(3-thiophene acetic acid) upon binding by alkyl trimethylammonium bromide surfactants.

The binding of cationic surfactants with varying alkyl chain length to a regiorandom conjugated polyanion, poly(3-thiophene acetic acid) (PTAA), is studied in an aqueous buffer by using absorption and emission spectroscopies, photon correlation spectroscopy, isothermal titration calorimetry, and cryogenic transmission electron microscopy. We study the mixed solutions as a function of composition ratio R of surfactant molecules to monomer units molar concentrations, at low polymer concentration and in a very wide composition range (10(-6) < R < 10(2)) below the critical micellar concentration. Upon surfactant binding, the molecularly dispersed chains first collapse progressively and then form new structures as the mixed aggregates get enriched in surfactant. The collapse leads to a strong decrease of the conjugation length and to a blue shift of the absorption spectra by 30 to 50 nm. The new structures are responsible for a new intense emission band at about 600 nm, red-shifted by nearly 130 nm from the initial emission maximum of the polymer (~472 nm). As the surfactant tail becomes shorter, the blue shift of the absorption spectra and the intensity raise of the new emission are delayed to larger composition ratios while their variations become smoother functions of the surfactant concentration. These particular spectroscopic properties of PTAA seem related to its unique combination of a strongly hydrophobic backbone, a large ratio of contour length to persistence length, and an overall good aqueous solubility. Our results show that such features are well suited to design a colorimetric biosensor at small composition ratio, and a fluorescent biomarker at large composition ratio.

Alanine metabolism in isolated human kidney tubules--Use of a mathematical model.

To gain insight into the fate of alanine nitrogen and carbon taken up by the human kidney under certain conditions, isolated human kidney cortex tubules were incubated in Krebs-Henseleit medium with L-alanine as substrate. The tubules metabolized alanine at high rates and in a dose-dependent manner. Most of the alanine nitrogen removed was recovered as ammonia and to a lesser extent as glutamate. Glucose, lactate and glutamate were also found to be significant products of alanine carbon metabolism. A simple mathematical model allowing one to calculate flux of alanine carbon through the various metabolic steps involved is proposed and applied to data obtained in experiments in which 5 mM [U-14C]-,[1-14C]-, [2-14C]- and [3-14C]alanine were used as substrates in parallel. About 40% of the alanine carbon removed was recovered as CO2 and oxidation of C1 of alanine accounted for most of the CO2 released from alanine. Calculations reveal that the ATP produced exceeded 3.2-fold the ATP consumed in relation to alanine metabolism. It is concluded that, in human kidney, alanine may serve as an energy supplier and as a precursor of glucose and ammonia.

A simple model for alanine metabolism in isolated rat hepatocytes.

A simple model describing reactions of alanine metabolism in isolated hepatocytes from fasted rats is proposed and applied to radioactive data obtained in experiments in which L-[1-14C]-, L-[2-14C]-, L-[3-14C]-, and L-[U-14C]alanine as well as L-alanine plus NaH14CO3 were used as substrates in parallel. Measurements of the rates of incorporation of the label into glucose and CO2 and of accumulation of [1-14C]pyruvate, [1-14C]lactate, [1-14C]alanine and [1-14C]glutamate plus [1-14C]glutamine from the different substrates used allows to calculate flux of alanine carbon through the various metabolic steps taken into account in the model. The validity of this model is indicated by the agreement found between calculations and measurement of the 14CO2 released from [1-14C]alanine as well as between the values of flux through pyruvate carboxylase calculated in two different ways. It is shown that the oxaloacetate synthesized by pyruvate carboxylase enters into the Krebs cycle and into the pathway of phosphoenolpyruvate synthesis in about equal proportions and that about 40% of the oxaloacetate synthesized as a result of alanine metabolism is derived from the Krebs cycle operation. These results, together with the conclusion that flux of alanine carbon through pyruvate dehydrogenase is negligible, are in agreement with known characteristics of hepatic alanine metabolism in the fasted state and, therefore, provide further evidence for the validity of the model proposed in the present study.

Vitamin K1 diffusion across the placental barrier in the gravid female rat.

The hemorrhagic disease of the newborn can be prevented by administration of vitamin K1. The modes of utilization of the vitamin need to be studied in the mother and child. An experimental pharmacokinetic study was conducted, using the gravid female rat. Results showed rapid intestinal absorption of phylloquinone. The fetus plasma concentration rose from 8.6 micrograms/l at time 0 to 0 to 44.3 micrograms/l at 8 h. Such an increase is an evidence of phylloquinone diffusion across the placenta. The phenomenon requires a high gradient. On the other hand, maximum plasma concentrations are not reached until the 8th hour.