Enhanced Diffusion of Molecular Catalysts is Due to Convection

Pair dynamics of active force dipoles in an odd-viscous fluid

We discuss the lateral dynamics of two active force dipoles, which interact with each other via hydrodynamic interactions in a thin fluid layer that is active and chiral. The fluid layer is modeled as a two-dimensional (2D) compressible fluid with an odd viscosity, while the force dipole (representing an active protein or enzyme) induces a dipolar flow. Taking into account the momentum decay in the 2D fluid, we obtain analytically the mobility tensor that depends on the odd viscosity and includes nonreciprocal hydrodynamic interactions. We find that the particle pair shows spiral behavior due to the transverse flow induced by the odd viscosity. When the magnitude of the odd viscosity is large as compared with the shear viscosity, two types of oscillatory behaviors are seen. One of them can be understood as arising from closed orbits in dynamical systems, and its circular trajectories are determined by the ratio between the magnitude of the odd viscosity and the force dipole. In addition, the phase diagrams of the particle dipolar angles are obtained numerically. Our findings reveal that the nonreciprocal response leads to complex dynamics of active particles embedded in an active fluid with odd viscosity.

Study Enhanced Enzyme Diffusion with High-Speed Single Nanoparticle Rotational and Translational Tracking

Catalytic enzymes exhibiting enhanced motion have drawn extensive attention over the past decade; nevertheless, little is known about the effect on the environment induced by enzymes. Herein, we studied the active urease system by simultaneously monitoring the diffusion of single anisotropic gold nanorods (AuNRs) with high speed dark-field imaging. We found both the translational and the rotational diffusion coefficients of AuNRs were enhanced but with inconsistent degrees, indicating the catalytic reaction had a minor effect on the physiochemical properties of the environment according to the Stokes-Einstein equation. With the increase of substrate concentration, the diffusion of AuNRs showed increased spatial but decreased temporal heterogeneity. Additionally, high speed imaging revealed AuNRs could experience intermittent ballistic motion for tens of milliseconds. These results imply inhomogeneous distribution of enzymes in free solution induced by active enzymatic reactions.

Interconnectivity between Surface Reactivity and Self-Assembly of Kemp Elimination Catalyzing Nanorods

Understanding the fundamental facts behind dynamicity of catalytic processes has been a longstanding quest across disciplines. Herein, we report self-assembly of catalytically active gold nanorods that can be regulated by tuning its reactivity towards a proton transfer reaction at different pH. Unlike substrate-induced templating and co-operativity, the enhanced aggregation rate is due to alteration of catalytic surface charge only during reactivity as negatively charged transition state of reactant (5-nitrobenzisoxazole) is formed on positively charged nanorod while undergoing a concerted E2-pathway. Herein, enhanced diffusivity during catalytic processes might also act as an additional contributing factor. Furthermore, we have also shown that nanosized hydrophobic cavities of clustered nanorods can also efficiently accelerate the rate of an aromatic nucleophilic substitution reaction, which also demonstrates a catalytic phenomenon that can lead to cascading of other reactions where substrates and products of the starting reactions are not directly involved.

Using NMR to Test Molecular Mobility during a Chemical Reaction

We evaluate critically the use of pulsed gradient spin-echo nuclear magnetic resonance to measure molecular mobility during chemical reactions. With raw NMR spectra available in a public depository, we confirm the boosted mobility during the click chemical reaction (Wang et al. Science 2020 369, 537-541) regardless of the order of magnetic field gradient (linearly increasing, linearly decreasing, random sequence). We also confirm boosted mobility for the Diels-Alder chemical reaction. The conceptual advantage of the former chemical system is that a constant reaction rate implies a constant catalyst concentration, whereas that of the latter is the absence of a paramagnetic catalyst, precluding paramagnetism as an objection to the measurements. The data and discussion in this paper show the reliability of experiments when one avoids convection, allows decay of nuclear spin magnetization between successive pulses and recovery of its intensity between gradients, and satisfies quasi-steady state during the time window to acquire each datum. Especially important is to make comparisons on the time scale of the actual chemical reaction kinetics. We discuss possible sources of mistaken conclusions that are desirable to avoid.

Boosted molecular mobility during common chemical reactions

Mobility of reactants and nearby solvent is more rapid than Brownian diffusion during several common chemical reactions when the energy release rate exceeds a threshold. Screening a family of 15 organic chemical reactions, we demonstrate the largest boost for catalyzed bimolecular reactions, click chemistry, ring-opening metathesis polymerization, and Sonogashira coupling. Boosted diffusion is also observed but to lesser extent for the uncatalyzed Diels-Alder reaction, but not for substitution reactions SN1 and SN2 within instrumental resolution. Diffusion coefficient increases as measured by pulsed-field gradient nuclear magnetic resonance, whereas in microfluidics experiments, molecules in reaction gradients migrate “uphill” in the direction of lesser diffusivity. This microscopic consumption of energy by chemical reactions transduced into mechanical motion presents a form of active matter.