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Mesoscopic modelling of synthetic and biological ionic macromolecular systems

Final Report Summary - POLION (Mesoscopic modelling of synthetic and biological ionic macromolecular systems)

Mesoscopic modelling of synthetic and biological ionic macromolecular systems

Ionic polymers or polyelectrolytes constitute an important class of water-soluble macromolecules and find an increasing number of industrial applications, ranging from biomedicine, food industry, home and personal care products to oil recovery, agro-chemistry and water treatment. The great majority of biopolymers (proteins, nucleic acids and polysaccharides) carry ionisable groups and are also included in the class of polyelectrolytes. Therefore, profound understanding of the relation between architectural complexity and properties of synthetic and natural ionic macromolecules is crucial for innovative technological approaches as well as for molecular biology and biomedicine.
Because of the high complexity of macromolecular and biopolymer systems, this understanding can be achieved only on the basis of interdisciplinary approach in which theory and numerical simulations are brought together in close relation with experiment. The POLION exchange program aims at building long-term partnership and exploiting highly complementary expertises of participating institutions in the domain of physical chemistry, theoretical and computational science. The main objectives of POLION are:
1. Develop a multidisciplinary theoretical research platform, that enables establishing systematic relationship between increasingly complex topology of branched ionic macromolecules and their conformational and stimuli-responsive properties;
2. Investigate structure of interfacial monolayers formed by hierarchically branched ionic macromolecules and give valuable insights in their mechanical properties;
3. Reach in-depth understanding of mechanisms of uptake and complexation of guest macromolecules, nano-colloids and proteins as well as interaction of multivalent ions with branched ionic macromolecules;
4. Understand physical-chemical basis of in vivo organization and (dis)function of biomacromolecular assemblies;
5. Develop new time-efficient computational algorithms which enable to study architecturally-complex macromolecular systems with account of multitude of competing interactions.
Solution properties of branched polyelectrolytes: A detailed analysis of conformations and stimuli-induced conformational transitions in starlike weak and strong polyelectrolytes was performed by means of grand canonical Monte Carlo simulations combined with numerical Self-Consistent Field (SCF) theoretical approach. Both cases of good and poor solvent were investigated. It was shown that the interplay of long-range electrostatic and short-range solvophobic interactions with branched architecture of the macroions leads to specific patterns in the intramolecular nanoscale organization.
Conformational properties, and, in particular, induced bending rigidity, of dendronized polymers with various architectures of lateral branches decorating the main backbone chain were studied by combining analytical and numerical SCF approaches. The most striking result concerns a decrease in the induced rigidity of the dendronized polymer as a function of increasing degree of branching of the lateral chains. This property is manifested in a decrease in the overall dimensions of the macromolecule that might have an impact for its ability to overcome biological barriers (in function of the therapeutics carrier).
Conformations of branched macromolecules at interfaces: Combination of analytical theory and numerical SCF modelling was applied for the study of dendron brushes, which are molecular structures built up of treelike macromolecules, with multiple generations of branches, grafted with a root segment to a surface (or to a colloidal particle) with a sufficiently high grafting density so that neighbouring dendrons strongly interact. The developed theory enabled us further to get a deep insight into intrinsic structure of brushes formed by branched macromolecules of arbitrary branched and cyclic topology. This approach is further coupled to the Poisson-Boltzmann approximation for analysis of structural properties of brushes formed by ionic branched macromolecules. It was demonstrated that compared to brushes of linear chains, the dendron brushes provide a sharper increase in the repulsive force when the brushes pushed to overlap. However, dendron brushes do not provide better stability for colloidal dispersions as compared to the linear ones because of smaller brush thickness. At the same time the dendron brushes decorating apposing surfaces exhibit weaker interpenetration than brushes of linear chains. As a result branching improves tribological properties of the brush-decorated surface in terms of lowering of the friction forces. The impact of macromolecular architecture (branching, dendronization, cyclization) on bending rigidity of polymer-decorated nano-membranes was investigated. It was proven that an increase in the degree of branching provokes a decrease in the absolute values of bending moduli. An inherent property of the dendron brushes in the non-linear stretching regime is vertical stratification manifested in segregation of dendrons in weaker and stronger stretched populations. We demonstrated how this stratification is developed upon an increase in gradating density or/and interactions strength.
Theory of interpolyelectrolyte complexation in aqueous solutions of oppositely charged weak polyelectrolytes was advanced by combining scaling approach and Random Phase Approximation, complete phase diagram of the system was constructed and effect of variable external conditions on structural and thermodynamic properties of the complexes was investigated; possibility of microphase separation in the coacervate phase was predicted.
Biomacromolecular assemblies: Structural organization of brushes formed by mixture of branched polyelectrolytes of various topologies grafted to planar or cylindrical surfaces as a model of aggrecan- hyaluronic acid complexes was studied by means of analytical theory and numerical SCF calculation. The SCF method enabled us to analyse structure of mixed brushes formed by macromolecules which differ in their degree of branching and even topology. We demonstrated that branched and linear macromolecules with selected molecular weights and architectures can distribute their free ends all over the volume of a mixed brush and produce unified polymer density profiles which results in equal availability of terminal functional groups. This property is controlled solely by topology of the brush-forming chains irrespectively of the specific nature of intermolecular interactions. Structural properties of asymmetric poly(L-lysine) dendrimers were studied by numerical SCF modelling.
Development of new computational approaches: The method of grand canonical Monte Carlo simulation in the reaction ensemble was developed and validated by the study of solution properties of starlike weak polyelectrolytes. A hybrid SCF - Monte Carlo computational method that enables time-efficient modelling of complex macromolecular systems with up to three-dimensional gradient of local properties was developed. The method has been applied for the study of interaction and complexation of model nanocolloids with polymer brushes in complex geometries.
As a final scientific result we achieved substantial progress in modelling of properties of architecturally complex hierarchically branched macromolecular systems, including dendronized surfaces, colloidal and molecular polyelectrolyte brushes. This was achieved by systematic and proper exploitation of analytical theory and numerical modelling techniques enabling us to unravel the structural organization of macromolecular systems with a complex topology that experience both long-range ionic and short-range interactions.
The generated within the project knowledge will have a positive impact for the European industry. New classes of rationally designed branched ionic macromolecules are capable of providing novel properties to the materials that outperform state of the art products. Such new materials will play an important role in various fields ranging from e.g. drug delivery systems, crop protection agents with enhanced bioavailability to cosmetic agents on skin or hair and water treatment. The nano-catalytic systems formed by binding enzymes in the hydrated intra-molecular compartments of branched macroions will lead to a real breakthrough in converting traditional processes into energy- and recourse-saving ones with concomitant environmental impact. Finally, in-depth understanding of physical mechanisms of (dis)functions of biomacromolecular assemblies network, may provide a rational for the development of novel therapeutics that are more efficient to fight against, e.g. neurodegenerative diseases, thus contributing to improving health-care and quality of life worldwide.