The protein dynamics of bovine and caprine ß-lactoglobulin differ as a function of pH.

The properties of milk proteins differ between mammalian species. ß-Lactoglobulin (ßlg) proteins from caprine and bovine milk are sequentially and structurally highly similar, yet their physicochemical properties differ, particularly in response to pH. To resolve this conundrum, we compared the dynamics of both the monomeric and dimeric states for each homologue at pH 6.9 and 7.5 using hydrogen/deuterium exchange experiments. At pH 7.5, the rate of exchange is similar across both homologues, but at pH 6.9 the dimeric states of the bovine ßlg B variant homologue have significantly more conformational flexibility compared with caprine ßlg. Molecular dynamics simulations provide a mechanistic rationale for the experimental observations, revealing that variant-specific substitutions encode different conformational ensembles with different dynamic properties consistent with the hydrogen/deuterium exchange experiments. Understanding the dynamic differences across ßlg homologues is essential to understand the different responses of these milks to processing, human digestion, and differences in immunogenicity.

A numerical investigation of the hydrodynamic dispersion in triply periodic chromatographic stationary phases.

3D printed custom chromatographic stationary phases have recently been demonstrated. Using the Lattice Boltzmann Method, we compared the model-predicted chromatographic performance of random packing of monodisperse spheres, open tubular columns (OTC) and stationary phases based on three triply periodic minimal surfaces (TPMS): Schwarz Diamond (SD), Schoen Gyroid (SG) and Schwarz Primitive (SP). Three performance metrics were employed in this comparison: i) reduced plate height, ii) Darcy number, iii) kinetic performance factor. Each simulated geometry was unconfined with an impermeable stationary phase to remove wall effects and pore diffusion. The performance was studied for macro-porosities in the range 0.2 to 0.8, depending on the geometry. OTCs were found to have superior permeability to both random sphere packing and TPMS structures across the entire porosity range. At porosity greater than 0.366, the Schwarz Diamond medium achieved the lowest levels of band broadening and greatest kinetic performance. The reduced plate height of all stationary phase geometries was shown to increase with bed porosity. The kinetic performance was found to increase with porosity for TPMS structures, decrease with porosity for random packing and be independent of porosity for OTCs. This work illustrates that chromatographic stationary phase geometries based on TPMS structures are theoretically competitive with random packing and open tubular columns and their feasibility for practical chromatography should continue to be explored.

Capillaric field effect transistors.

Controlling fluid flow in capillaric circuits is a key requirement to increase their uptake for assay applications. Capillary action off-valves provide such functionality by pushing an occluding bubble into the channel using a difference in capillary pressure. Previously, we utilized the binary switching mode of this structure to develop a powerful set of fundamental fluidic valving operations. In this work, we study the transistor-like qualities of the off-valve and provide evidence that these structures are in fact functionally complementary to electronic junction field effect transistors. In view of this, we propose the new term capillaric field effect transistor to describe these types of valves. To support this conclusion, we present a theoretical description, experimental characterization, and practical application of analog flow resistance control. In addition, we demonstrate that the valves can also be reopened. We show modulation of the flow resistance from fully open to pinch-off, determine the flow rate-trigger channel volume relationship and demonstrate that the latter can be modeled using Shockley's equation for electronic transistors. Finally, we provide a first example of how the valves can be opened and closed repeatedly.

New flow control systems in capillarics: off valves.

Capillary systems are a promising technology for point-of-care microfluidics, since they are pre-programmable and self-powered. This work introduces "off valves" as a key building block for capillaric circuits, providing easy-to-use, multi-purpose valving functionality and autonomous flow control. To this end we present a set of switching valve designs that use trigger channels and liquid input alone to close or open connections between channels in a highly controllable fashion. The key element of all these valve designs is a new off trigger valve, which is characterised in detail here and holds the potential for transistor-like switching and resistance tuning. As an example for the potential applications of switching valves, we demonstrate how they can be used for flow resistance control in a complex microfluidic circuit and for sequential chemical loading into a reaction chamber. Use of the switching valves for the latter in particular allowed for the tuning of incubation times and volumetric measurement, thus confirming applicability of the valves for automated and self-powered immunoassays in point-of-care environments.

Numerical Elucidation of Flow and Dispersion in Ordered Packed Beds: Nonspherical Polygons and the Effect of Particle Overlap on Chromatographic Performance.

Spherical particles are widely considered as the benchmark stationary phase for preparative and analytical chromatography. Although this has proven true for randomly packed beds in the past, we challenge this paradigm for ordered packings, the fabrication of which are now feasible through additive manufacturing (3D printing). Using computational fluid dynamics (Lattice Boltzmann Method) this work shows that nonspherical particles can both reduce mobile-phase band broadening and increase permeability compared with spheres in ordered packed beds. In practice, ordered packed beds can only remain physically stable if the particles are fused to form a contiguous matrix, thus creating a positional overlap at the points of fusion between what would otherwise be discrete particles. Overlap is shown to decrease performance of ordered packed beds in all observed cases, thus we recommend it should be kept to the minimum extent necessary to ensure physical stability. Finally, we introduce a metric to estimate column performance, the mean deviated velocity, a quantitative description of the spread of the velocity field in the column. This metric appears to be a good indicator of mobile-phase dispersion in ordered packed bed media, including overlapped beds, and is a useful tool for screening new stationary-phase morphologies without having to perform computationally expensive simulations.

Modelling ordered packed beds of spheres: The importance of bed orientation and the influence of tortuosity on dispersion.

Ordered packing has previously been considered for porous media applications in the industrial and analytical worlds, with implementation constrained only by the lack of feasible fabrication methods. Additive manufacturing now provides the answer to this limitation, which leads to the novel domain of customized ordered packing and a variety of optimized geometries. In this work, the chromatographic behaviour of ordered configurations of particles was described using computational fluid dynamics methods based on the Lattice Boltzmann Model. The model was first validated by matching van Deemter trends for ordered and random packings shown in previous research. The influence of rotations of the ordered configurations was then considered, indicating that orientational changes with respect to the main flow axis can strongly affect minimum plate height. In particular, it is demonstrated that targeted rotations of ordered packings can reduce axial dispersion while improving transverse dispersion, thus improving chromatographic performance. This principle is clearly shown in a strong linear correlation between tortuosity and plate height, offering an additional parameter to enable a priori control of the performance of ordered packings. Furthermore, rotation of the packing does not change porosity or surface area and has a relatively small effect on permeability. Thus, highly permeable packings with poor dispersion can be improved in terms of chromatographic impedance by simple rotation of the packing orientation. This work further demonstrates the advantages of ordered packings over randomly packed beds, and introduces new perspectives on the development of chromatographic structures with improved performance.