Phase behavior and structure of semiflexible polymers in spherical confinement

• Applicants: Professor Dr. Arash Nikoubashman; Dr. Peter Virnau, since 10/2022
• Subject Area: Experimental and Theoretical Physics of Polymers
• Term: from 2014 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 261177998

Semiflexible biopolymers, such as nucleic acids (e.g. DNA) and proteins, can self-assemble into liquid crystals with nematic ordering at sufficiently high concentrations. These systems exhibit a wide range of structural and functional properties, which are of fundamental interest for questions regarding morphogenesis and the evolution of living organisms, as well as for biomedical applications.To elucidate the behavior of these complex systems, we studied model systems of semiflexible chains in lyotropic solution under confinement using molecular dynamics (MD) simulations and density functional theory (DFT) calculations. To cover a representative range of length scales (in practice, radii of capsids and vesicles vary between 10nm and 50µm), we focused on two limiting cases, i.e. spherical confinement where the sphere radii are comparable to the chain (or persistence) length, as well as planar confinement, corresponding to infinitely large radii. Here, we investigated both purely repulsive and attractive wall-polymer interactions. For strong attractions, we identified novel ordered states at the walls, resembling a liquid-crystalline "smectic C" arrangement. When the radius of curvature was comparable to the chain length, we found a thin nematically ordered layer of polymers at the confining membrane with topological defects with a "tennis-ball" or bipolar texture. As the polymer concentration was increased, we found a non-trivial competition between the local ordering in the interior and on the surface of the sphere.In order to rationalize these phenomena, it is also required to study in detail the ordered states and phase transitions of this model in the bulk. So far, we found that the standard theory (based on the wormlike chain model in combination with the second virial coefficient) is inaccurate, due to the non-trivial role of the ratio between the persistence length and chain diameter. This finding has motivated the development of two new versions of DFT for these systems by Prof. Egorov (Virgina) and by Prof. Doyle (Oxford). However, despite their improvements, these theories cannot describe the emergence of collective bending fluctuations (on the scale of the so-called deflection length). Since for many of the applications listed above elastic deformation of the confining surface of the cell or vesicle is relevant, now also confining membranes that are not rigid but deformable will be included in the study. Also elastic properties of the relevant liquid-crystalline phases will be studied. Finally, the interplay between nematic order and phase separation under confinement will be investigated.

• DFG Programme: Research Grants
• International Connection: Bulgaria
• Cooperation Partner: Professor Dr. Andrey Milchev
• Ehemaliger Antragsteller: Professor Dr. Kurt Binder, until 10/2022 (†)