How are mobility and friction related in viscoelastic fluids?

The motion of a colloidal probe in a viscoelastic fluid is described by friction or mobility, depending on whether the probe is moving with a velocity or feeling a force. While the Einstein relation describes an inverse relationship valid for Newtonian solvents, both concepts are generalized to time-dependent memory kernels in viscoelastic fluids. We theoretically and experimentally investigate their relation by considering two observables: the recoil after releasing a probe that was moved through the fluid and the equilibrium mean squared displacement (MSD). Applying concepts of linear response theory, we generalize Einstein's relation and, thereby, relate recoil and MSD, which both provide access to the mobility kernel. With increasing concentration, however, MSD and recoil show distinct behaviors, rooted in different behaviors of the two kernels. Using two theoretical models, a linear two-bath particle model, and hard spheres treated by mode coupling theory, we find a Volterra relation between the two kernels, explaining differing timescales in friction and mobility kernels under variation of concentration.

Gas-liquid phase transition in a binary mixture with an interaction that creates constant density profiles.

If, in a hard sphere fluid, a single (test) particle is fixed, the other particles display a density profile that possesses long-ranged oscillations. Surprisingly, one can show via classical density functional theory that it takes a simple, purely repulsive (external) potential with a finite range in addition to the fixed hard sphere that forces these oscillations to vanish completely. This can give rise to interesting phenomena; however, it gained little attention in the past. In this work, we use the potential in question as an inter-component interaction in a binary hard-sphere mixture, where it is shown that the effective interaction induced by one component resembles qualitatively the well-known Asakura-Oosawa-Vrij potential and can lead to a liquid-gas phase transition in the other component.

Insights into the functional properties of the marneral oxidase CYP71A16 from Arabidopsis thaliana.

The Arabidopsis thaliana gene encoding CYP71A16 is part of the gene cluster for the biosynthesis and modification of the triterpenoid marneral. Previous investigations of A. thaliana have revealed that CYP71A16 catalyzes marneral oxidation, while it also can accept marnerol as substrate. The aim of the present study was to investigate functional properties of CYP71A16 in vitro. For this purpose, heterologous expression of a N-terminally modified version of CYP71A16 was established in Escherichia coli, which yielded up to 50mgL-1 recombinant enzyme. The enzyme was purified and activity was reconstituted in vitro with different redox partners. A heterologous bacterial redox partner system consisting of the flavodoxin YkuN from Bacillus subtilis and the flavodoxin reductase Fpr from E. coli clearly outperformed the cytochrome P450 reductase ATR2 from A. thaliana in supporting the CYP71A16-mediated hydroxylation of marnerol. Substrate binding experiments with CYP71A16 revealed a dissociation constant KD of 225µM for marnerol. CYP71A16 catalyzed the hydroxylation of marnerol to 23-hydroxymarnerol with a KM of 142µM and a kcat of 3.9min-1. Furthermore, GC/MS analysis revealed an as of yet unidentified overoxidation product of this in vitro reaction. This article is part of a Special Issue entitled: Cytochrome P450 biodiversity and biotechnology, edited by Erika Plettner, Gianfranco Gilardi, Luet Wong, Vlada Urlacher, Jared Goldstone.