We study the phase behavior of suspensions of DNA-based molecules, which either consist of combinations of DNA fragments, or DNA in combination with synthetic colloids and polymers.
Direct visualization of conformation and dense packing of DNA-based soft colloids
We introduce novel DNA-coated colloids with thick brushes of surface-grafted DNA (up to a thickness of 10 micron). These DNA-coated colloids allow the direct visualization of the internal brush structure and arm’s free-end locations with confocal fluorescent microscopy, as a function of grafting density, brush thickness, and ionic strength. The highly packed two-dimensional hexagonal lattice formed by these macromolecular assemblies reveal an unexpected resistance to mutual interpenetration of their charged corona at pressures approaching the MPa (see the figure). We also find that this lattice is surprisingly tolerant to particle size variation. These DNA-synthetic hybrids could aid to obtain a deeper insight in a wide range of problems in soft matter, including the study and design of biomimetic lubricated surfaces [Zhang2014].
A New Smectic Phase in Suspensions of DNA Duplexes
Stiff, double stranded DNA fragments are know not to form a smectic phase. Introducing a single stranded spacer in the middle of a double stranded DNA, thus increasing flexibility, unexpectedly leads to the formation of a smectic phase. X-ray scattering and simulations show that the stiff parts of the duplexes reside within the same smectic layer (see the figure). We named this phase the “smectic-fA phase”, where the “f” stands for “folding” [Salamonczyk2016].
The patchy interactions between proteins give rise to a phase behavior that is not found in relatively simple colloidal systems. We develop and apply theoretical methods that account for these patchy interactions on the basis of a minimal model for the pair-interaction potential, using advanced statistical physics tools, and we assess to what extent the simplifying theory can explain experimental observations and computer simulations.
Phase Behavior of Globular Proteins: Effects of Patchiness and Additives
Solutions of globular proteins such as lysozyme and BSA have a rich equilibrium and non-equilibrium phase behaviour, triggered by competing attractive and repulsive interactions [Gögelein2008, Gögelein2012] and the surface patchiness of the proteins. We have successfully analysed the phase behaviour and microstructure and dynamics [Gapinski2005, Heinen2012, Riest2015] of protein solutions using colloid-based theoretical models, in comparison with experimental results and computer simulations.
Quasi–two–dimensional protein dispersions
In many biological systems, the diffusional motion of phospholipids and membrane-proteins takes place under quasi-two-dimensional (Q2D) conditions, with proteins organized in finite-sized domains such as lipid rafts and clusters. Clustering occurs in neuronal signal transduction, membrane sorting, protein processing, and virus trafficking. The formation of membrane-protein clusters is triggered by competing short-range attractive (SA) and long-range repulsive (LR) interactions [Riest2015, Riest2018]. Using state-of-the-art mesoscale simulations such as MPC simulations with hydrodynamic interactions included [Tan2021 , Das2018], we explore the phase behavior and microstructure of SALR Brownian particle dispersions in Q2D confinement. We find salient differences and similarities in the clustering and phase behavior of Q2D and three-dimensional SALR systems.
Z. Tan and G. Nägele
Filamentous viruses are used as model systems for very long and thin colloidal rods. The native viruses are highly charged, their charge density can be chemically modified, and they can be coated with quite thick polymer brushes. Moreover, the micron-sized length allows for imaging single particles. A variety of different viruses is used, with different persistence- and contour lengths.
Glass Transition in Suspensions of Very Long and Thin Colloidal Rods
Suspensions of very long and thin rods (fd-virus particles) at low ionic strength (corresponding to a Debye length of 27 nm) exhibit a (Wigner) glass transition well within the chiral-nematic state. It is shown that at the glass-transition concentration, the single-particle dynamics is arrested, as well as the dynamics of the chiral-nematic orientational texture. The dynamics of the domain texture is probed by video time-correlation spectroscopy. The single-particle dynamics continuously slows down on approach of the glass transition by increasing the fd-concentration, while the time scale on which the domain texture freezes exhibits a discontinuity, as shown in the figure [Kang2013, Kang2013].