The depletion thickness in solutions of semi-flexible polymers near colloidal surfaces: analytical approximations

Coil dimensions of macromolecules in the presence of crowding colloids: Impact of crowder size

HYPOTHESIS

The size of a macromolecule in solution is strongly influenced by the size and concentration of added colloidal particles. Previous experimental and computer simulation studies have shown conflicting results regarding the influence of colloid size on coil compaction. We suggest the coil size depends on the Kuhn segment / nanoparticle size ratio and argue its subtle influence on the shrinking and expansion of a polymer chain.

METHODS

Based upon the work of van der Schoot (1998) [42] we propose theory that predicts how the colloid size mediates the compaction of macromolecules in crowded environments. The theoretical predictions are compared to self-consistent field (SCF) lattice computations and scattering experiments on polymer solutions exposed to crowders.

FINDINGS

The theoretical approach predicts that the shrinking of a polymer coil upon adding colloidal particles varies with the size of the colloids. We find coil shrinking is weakest when the colloidal particles are approximately the same size as the Kuhn segment length. The extent of coil shrinking passes a minimum at a specific colloid size relative to the Kuhn segment length, which is confirmed by SCF computations. Comparison with scattering experiments reveals that these experiments corroborate the extent of polymer shrinking at a given volume fraction of colloids. Our theoretical approach reproduces the functional dependence of the collapse on the crowder volume fraction.

Magnesium alginate as a low-viscosity (intramolecularly cross-linked) system for the sustained and neuroprotective release of magnesium

The administration of Mg ions is advantageous in pathological scenarios such as pre-enclampsia and forms of neuroinflammation (e.g. stroke or injury); yet, few systems exist for their sustained delivery. Here, we present the (static light scattering and diffusing-wave spectroscopy) characterization of magnesium alginate (MgAlg) as a potentially injectable vehicle ifor the delivery of Mg. Differently from other divalent cations, Mg does not readily induce gelation: it acts within MgAlg coils, making them more rigid and less prone to entangle. As a result, below a threshold concentration (notionally below 0.5 % wt.) MgAlg are inherently less viscous than those of sodium alginate (NaAlg), which is a major advantage for injectables; at higher concentrations, however, (stable, Mg-based) aggregation starts occurring. Importantly, Mg can then be released e.g. in artificial cerebrospinal fluid, via a slow (hours) process of ion exchange. Finally, we here show that MgAlg protects rat neural stem cells from the consequence of an oxidative insult (100 μM H2O2), an effect that we can only ascribe to the sustained liberation of Mg ions, since it was not shown by NaAlg, MgSO4 or the NaAlg/MgSO4 combination. Our results therefore indicate that MgAlg is a promising vehicle for Mg delivery under pathological (inflammatory) conditions.

Depletion interaction mediated by semiflexible polymers

We present a simple mean-field theory to describe the polymer-mediated depletion attraction between colloidal particles that accounts for the polymer's chain stiffness. We find that for fixed polymer radius of gyration and volume fraction, the strength of this attraction increases with increasing chain stiffness in both dilute and semidilute concentration regimes. In contrast, the range of attraction monotonically decreases with chain stiffness in the dilute regime, while it attains a maximum in the semidilute regime. The obtained analytical expressions for the depletion interaction were compared with numerical self-consistent field lattice computations and shown to be in quantitative agreement. From the interaction potential between two spheres, we calculated the second osmotic virial coefficient B₂, which appears to be a convex function of chain stiffness. A minimum of B₂ as a function of chain stiffness was observed both in the numerical self-consistent field computations and the analytical theory. These findings help explain the general observation that semiflexible polymers are more effective depletants than flexible polymers and give insight into the phase behavior of mixtures containing spherical colloids and semiflexible polymers.