A pre-requisite for sexual reproduction is successful unification of the male and female gametes; in externally-fertilising echinoderms the male gamete is brought into close proximity to the female gamete through chemotaxis, the associated signalling and flagellar beat changes being elegantly characterised in several species. In the human, sperm traverse a relatively high-viscosity mucus coating the tract surfaces, there being a tantalising possible role for chemotaxis. To understand human sperm migration and guidance, studies must therefore employ similar viscous in vitro environments. High frame rate digital imaging is used for the first time to characterise the flagellar movement of migrating sperm in low and high viscosities. While qualitative features have been reported previously, we show in precise spatial and temporal detail waveform evolution along the flagellum. In low viscosity the flagellum continuously moves out of the focal plane, compromising the measurement of true curvature, nonetheless the presence of torsion can be inferred. In high viscosities curvature can be accurately determined and we show how waves propagate at approximately constant speed. Progressing waves increase in curvature approximately linearly except for a sharper increase over a distance approximately 20-27 microm from the head/midpiece junction. Curvature modulation, likely influenced by the outer dense fibres, creates the characteristic waveforms of high viscosity swimming, with remarkably effective cell progression against greatly increased resistance, even in high viscosity liquids. Assessment of motility in physiological viscosities will be essential in future basic research, studies of chemotaxis and novel diagnostics.

Relative to other extrinsic factors, the effects of hydrodynamic flow fields on protein stability and conformation remain poorly understood. Flow-induced protein remodeling and/or aggregation is observed both in Nature and during the large-scale industrial manufacture of proteins. Despite its ubiquity, the relationships between the type and magnitude of hydrodynamic flow, a protein's structure and stability, and the resultant aggregation propensity are unclear. Here, we assess the effects of a defined and quantified flow field dominated by extensional flow on the aggregation of BSA, β 2 -microglobulin (β 2 m), granulocyte colony stimulating factor (G-CSF), and three monoclonal antibodies (mAbs). We show that the device induces protein aggregation after exposure to an extensional flow field for 0.36-1.8 ms, at concentrations as low as 0.5 mg mL −1 . In addition, we reveal that the extent of aggregation depends on the applied strain rate and the concentration, structural scaffold, and sequence of the protein. Finally we demonstrate the in situ labeling of a buried cysteine residue in BSA during extensional stress. Together, these data indicate that an extensional flow readily unfolds thermodynamically and kinetically stable proteins, exposing previously sequestered sequences whose aggregation propensity determines the probability and extent of aggregation.extensional flow | aggregation | unfolding | bioprocessing | antibody P roteins are dynamic and metastable and consequently have conformations that are highly sensitive to the environment (1). Over the last 50 y the effect of changes in temperature, pH, and the concentration of kosmatropic/chaotropic agents on the conformational energy landscape of proteins has become well understood (1). This, in turn, has allowed a link to be established between the partial or full unfolding of proteins and their propensity to aggregate (2). The force applied onto a protein as a consequence of hydrodynamic flow has also been observed to trigger protein aggregation and has fundamental (3), medical (4), and industrial relevance, especially in the manufacture of biopharmaceuticals (5-8). Although a wealth of studies have been performed (7,(9)(10)(11)(12)(13), no consensus has emerged on the ability of hydrodynamic flow to induce protein aggregation (7,14,15). This is due to the wide variety of proteins used (ranging from lysozyme, BSA, and alcohol dehydrogenase to IgGs), differences in the type of flow field generated (e.g., shear, extensional, or mixtures of these), and to the presence or absence of an interface (16). A shearing flow field (Fig. 1A, Top) is caused by a gradient in velocity perpendicular to the direction of travel and is characterized by the shear rate (s −1 ). This results in a weak rotating motion of a protein alongside translation in the direction of the flow. An extensional flow field (Fig. 1A, Bottom) is generated by a gradient in velocity in the direction of travel and is characterized by the strain rate (s −1 ). A protein in this type of flow would experien...

This tutorial bridges an important knowledge gap by providing an easily accessible introduction that enables synthetic chemists to explore synthetic electrochemistry.

No abstract

The topic of calcite and aragonite polymorphism attracts enormous interest from fields including biomineralization and paleogeochemistry. While aragonite is only slightly less thermodynamically stable than calcite under ambient conditions, it typically only forms as a minor product in additive-free solutions at room temperature. However, aragonite is an abundant biomineral, and certain organisms can selectively generate calcite and aragonite. This fascinating behavior has been the focus of decades of research, where this has been driven by a search for specific organic macromolecules that can generate these polymorphs. However, despite these efforts, we still have a poor understanding of how organisms achieve such selectivity. In this work, we consider an alternative possibility and explore whether the confined volumes in which all biomineralization occurs could also influence polymorph. Calcium carbonate was precipitated within the cylindrical pores of track-etched membranes, where these enabled us to systematically investigate the relationship between the membrane pore diameter and polymorph formation. Aragonite was obtained in increasing quantities as the pore size was reduced, such that oriented single crystals of aragonite were the sole product from additive-free solutions in 25-nm pores and significant quantities of aragonite formed in pores as large as 200 nm in the presence of low concentrations of magnesium and sulfate ions. This effect can be attributed to the effect of the pore size on the ion distribution, which becomes of increasing importance in small pores. These intriguing results suggest that organisms may exploit confinement effects to gain control over crystal polymorph.

We report a simple, mild, and synthetically clean approach to accelerate the rate of enzymatic oxidation reactions by a factor of up to 100 when compared to conventional batch gas/liquid systems. Biocatalytic decomposition of H O is used to produce a soluble source of O directly in reaction media, thereby enabling the concentration of aqueous O to be increased beyond equilibrium solubility under safe and practical conditions. To best exploit this method, a novel flow reactor was developed to maximize productivity (g product L h ). This scalable benchtop method provides a distinct advantage over conventional bio-oxidation in that no pressurized gas or specialist equipment is employed. The method is general across different oxidase enzymes and compatible with a variety of functional groups. These results culminate in record space-time yields for bio-oxidation.

No abstract

The coalescence of a pair of droplets on a surface is investigated experimentally with images from detailed flow visualisations revealing the morphology of the process. It is found that they merge and evolve to a final state with a footprint that is peanut like in shape, with bulges along the longer sides resulting from the effects of inertia during spreading. The associated dynamics involve a subtle interplay between (i) the motion of the wetting process due to relaxation of the contact angle and (ii) a rapid rise in free-surface height above the point where coalescence began due to negative pressure generated by curvature. During the early stages of the motion, a traveling wave propagates from the point of initial contact up the side of each droplet as liquid is drawn into the neck region, and only when it reaches the apex of each do their heights start to decrease. A further feature of the rapid rise in height of the neck region is that the free surface there overshoots significantly its final equilibrium position; it reaches a height greater than that of the starting droplets, producing a self-excited oscillation that persists long after the system reaches its final morphological state in relation to its footprint.