Assemblage Macromoléculaire Hors-équilibre Compréhension, Contrôle & nouvelles structures – PANORAMA

- The influence of a very fast mixing of on the early stage complexation kinetic is studied with the help of a Stopped-Flow device. Two, three or four solutions are rapidly mixed and final mixture flows in an observation cell. When the flow is stopped, the kinetic is recorded with a spectrophotometer.It is possible to achieve sub microsecond dead times to follow kinetics with a rate constant up to 3500 s-1.
- Isothermal titration calorimetry (ITC) is used to determine the thermodynamic parameters of assembly/complexation in solution. ITC is a quantitative technique that can determine the binding affinity (Ka), enthalpy changes (?H), and binding stoichiometry (n) of the interaction between two or more molecules in solution. From these initial measurements, Gibbs energy changes (?G) and entropy changes (?S) can be determined using the relationship.
- Neutron and light scattering are used to figure-out the overall and local strcure of the complexes formed

i)We have seen a strong influence of the addition order on the complexation thermodynamics of widespread oppositely charged polyelectrolytes comprising Dextran sulfate a strong polyanion and Polyethyleneimine a weak polycation. ITC thermograms exhibit a strong dependency on the titration order with the presence of sharp peaks correlated with neat morphological differences.
ii) The influence of a very fast mixing of on the early stage complexation kinetic with the help of a Stopped-Flow device was used to investigate the kinetics of complex formation between oppositely charged Poly(acrylic) acid and Polydiallyldimethylammonium chloride PEs has been started . PE solutions can be here homogeneously mixed in a few milliseconds. With optimized dead time of about 4 ms, we have shown that initial complexes formed within a few ms. Depending on the PEs molar charge ratio initial complexes followed different evolution pathways. At high charge ratio (z > 0.7), complex aggregation can be identified by an increase of the scattered light with time. Unexpected decay curves pointing out likely complex de-association were also observed at low charge ratio (z < 0.7) below 1 second. The ionic strength was also shown to modulate the kinetic of the complex formation. A bell-shaped kinetic curve is typically observed in the presence 0.2M NaCl.
iii) We have studied the complexations of NPs and PEs as a function of the interaction strength. In presence of a large amount of salt, the interaction between oppositely charged components is fully screened. The salt concentration is then decreased brutally (quench) below the critical ionic strength and the aggregates growth is followed by DLS. The main preliminary result is the appearance of a critical time, below which the aggregates easily dissociate when suffering the dequench; above which they do not dissociate, even in the presence of large amounts of salt. This result demonstrates that the PEs/NPs complexes undergo ageing with time.

In the future, amphiphilic objects (block copolymer) will also be studied with the stopped-Flow approach and the thermodynamics state of the complexes will be studied through thermal degradation
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Further and complementary dequench experiments are planned in order to better understand th ageing phenomenon. In parallel, a special house-made dialysis cell is currently under construction in the lab. This cell will allow to couple dialysis of the samples with the DLS experiment. These experiments are crucial to compare the aggregates size, morphology and stability relatively with those obtained by dilution.

Finally, we will start to study impact of the processing route and interaction strength on the final morphologies of proteins interacting with polymers in order to be able to fabricate innovative nanomaterials using noncovalent binding for nanotechnology and nanoscience applications

1. Evidence of a two-step process and pathway dependency in the thermodynamics of poly(diallyldimethylammonium chloride)/poly(sodium acrylate) complexation. L. Vitorazi, N. Ould-Moussa, S. Sekar, J.-P. Chapel & J.-F. Berret, Soft Matter (2014), 10, 9496-9505.

2. Reusable and recyclable Quartz Crystal Microbalance sensors. Sekar S., Giermanska J. & Chapel J.-P. Sensor Actuat B-Chem (2015), 212, 196-199.

3. Fine-tuning the assembly of highly stable oppositely charged cerium oxide nanoparticles. Sekar S., Giermanska J., Saadaoui H. & Chapel J.-P., J. Col. Int. Sci. in press (2015)

4. Strong influence of the mixing order in the complexation of oppositely charged PEI/DxS polyelectrolyte system as probed through ITC : Enthalpy (aggregation) vs. Entropy (coacervation) driven interaction. Haddou M., J. Giermanska., Schatz Ch. & Chapel J.P. In preparation (2015).

L'assemblage non covalent de macromolécules en solution a été abondamment étudié au cours de la dernière décennie, avec un accent particulier mis sur la relation entre la morphologie finale des agrégats et la chimie des briques élémentaires de construction. Bien que la plupart de ces études supposent des conditions d'équilibre, les applications du monde réel impliquent quant à elles des conditions d’assemblage fortement hors d’équilibre. Il existe donc un réel besoin de concevoir des outils et des méthodologies originales afin de mieux comprendre et contrôler l'assemblage de macromolécules dans diverses conditions thermodynamiques. L’intérêt de cette démarche va bien au-delà dans la science fondamentale avec des retombées technologiques importantes en sciences des matériaux, des polymères et en biotechnologie. Nous proposons dans le projet PANORAMA d'étudier de manière indépendante et systématique l'impact des procédés de mise en œuvre et de l’intensité des interactions entre briques élémentaires sur les morphologies finales et leur(s) état(s) thermodynamique(s) sur des systèmes auto et co assemblés macromoléculaires modèles (agrégats de copolymères à blocs amphiphiles, assemblages électrostatiques de polyélectrolytes, nanoparticules et protéines). Le temps de mélange est choisi ici comme un paramètre clef du procédé de mise en œuvre permettant de faire varier les conditions initiales d’assemblage. En modulant ce temps de mélange (et de réaction) de quelques ms à plus d’une heure, une multitude de systèmes hors d’équilibres peuvent être ainsi générés et étudiés. Les techniques de diffusion (lumière et neutrons) et de microscopie électronique nous aiderons à déterminer la morphologie des agrégats à différentes échelles alors que des mesures de µcalorimétrie (ITC) seront réalisées afin de mieux appréhender la thermodynamique d’assemblage des ces agrégats et autres complexes. La collection systématique de données de type Thermodynamiques vs. Morphologie en fonction des conditions de mélange nous permettra de développer une cartographie prédictive et constituera un excellent point de départ pour des travaux ultérieurs de modélisation qui vont au-delà du but avoué du projet PANORAMA. En outre, la possibilité de réaliser des assemblages proches des conditions d'équilibre sera étudiée en modulant l’intensité de l’interaction (on/off) entre briques élémentaires. Cette démarche pourrait conduire à la formation de nouvelles structures supra macromoléculaires en mettant clairement en évidence l'importance de la voie de formulation pour guider les processus d'assemblages.