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Cellular dynamics of the SecA ATPase at the single molecule level
Get PDFIn bacteria, the SecA ATPase provides the driving force for protein secretion via the SecYEG translocon. While the dynamic interplay between SecA and SecYEG in translocation is widely appreciated, it is not clear how SecA associates with the translocon in the crowded cellular environment. We use super-resolution microscopy to directly visualize the dynamics of SecA in Escherichia coli at the single-molecule level. We find that SecA is predominantly associated with and evenly distributed along the cytoplasmic membrane as a homodimer, with only a minor cytosolic fraction. SecA moves along the cell membrane as three distinct but interconvertible diffusional populations: (1) A state loosely associated with the membrane, (2) an integral membrane form, and (3) a temporarily immobile form. Disruption of the proton-motive-force, which is essential for protein secretion, re-localizes a significant portion of SecA to the cytoplasm and results in the transient location of SecA at specific locations at the membrane. The data support a model in which SecA diffuses along the membrane surface to gain access to the SecYEG translocon
Passivation Properties and Formation Mechanism of Amorphous Halide Perovskite Thin Films
Get PDFLead halide perovskites are among the most exciting classes of optoelectronic materials due to their unique ability to form high‐quality crystals with tunable bandgaps in the visible and near‐infrared using simple solution precipitation reactions. This facile crystallization is driven by their ionic nature; just as with other salts, it is challenging to form amorphous halide perovskites, particularly in thin‐film form where they can most easily be studied. Here, rapid desolvation promoted by the addition of acetate precursors is shown as a general method for making amorphous lead halide perovskite films with a wide variety of compositions, including those using common organic cations (methylammonium and formamidinium) and anions (bromide and iodide). By controlling the amount of acetate, it is possible to tune from fully crystalline to fully amorphous films, with an interesting intermediate state consisting of crystalline islands embedded in an amorphous matrix. The amorphous lead halide perovskite has a large and tunable optical bandgap. It improves the photoluminescence quantum yield and lifetime of incorporated crystalline perovskite, opening up the intriguing possibility of using amorphous perovskite as a passivating contact, as is currently done in record efficiency silicon solar cells
A diffusiophoretic mechanism for ATP-driven transport without motor proteins
Get PDFProtein oscillations linked to cell division in Escherichia coli are shown to localize unrelated molecules on the cell membrane via a diffusiophoretic mechanism, in which an effective friction fosters cargo transport along the fluxes set up by the proteins.
The healthy growth and maintenance of a biological system depends on the precise spatial organization of molecules within the cell through the dissipation of energy. Reaction-diffusion mechanisms can facilitate this organization, as can directional cargo transport orchestrated by motor proteins, by relying on specific protein interactions. However, transport of material through the cell can also be achieved by active processes based on non-specific, purely physical mechanisms, a phenomenon that remains poorly explored. Here, using a combined experimental and theoretical approach, we discover and describe a hidden function of the Escherichia coli MinDE protein system: in addition to forming dynamic patterns, this system accomplishes the directional active transport of functionally unrelated cargo on membranes. Remarkably, this mechanism enables the sorting of diffusive objects according to their effective size, as evidenced using modular DNA origami-streptavidin nanostructures. We show that the diffusive fluxes of MinDE and non-specific cargo couple via density-dependent friction. This non-specific process constitutes a diffusiophoretic mechanism, as yet unknown in a cell biology setting. This nonlinear coupling between diffusive fluxes could represent a generic physical mechanism for establishing intracellular organization
Ultrafast Thermal Imprinting of Plasmonic Hotspots
Get PDFPlasmonic photochemistry is driven by a rich collection of near-field, hot charge carrier, energy transfer, and thermal effects, most often accomplished by continuous wave illumination. Heat generation is usually considered undesirable, because noble metal nanoparticles heat up isotropically, losing the extreme energy confinement of the optical resonance. Here it is demonstrated through optical and heat-transfer modelling that the judicious choice of nanoreactor geometry and material enables the direct thermal imprint of plasmonic optical absorption hotspots onto the lattice with high fidelity. Transition metal nitrides (TMNs, e.g., TiN/HfN) embody the ideal material requirements, where ultrafast electron–phonon coupling prevents fast electronic heat dissipation and low thermal conductivity prolongs the heat confinement. The extreme energy confinement leads to unprecedented peak temperatures and internal heat gradients (>10 K nm−1) that cannot be achieved using noble metals or any current heating method. TMN nanoreactors consequently yield up to ten thousand times more product in pulsed photothermal chemical conversion compared with noble metals (Ag, Au, Cu). These findings open up a completely unexplored realm of nano-photochemistry, where adjacent reaction centers experience substantially different temperatures for hundreds of picoseconds, long enough for bond breaking to occur
Multi-layered Barium and Strontium Carbonate Structures Induced by the Small Organic Dye Acid Orange 7
Get PDFThe crystal growth behavior induced by small molecular additives is commonly assumed to be far less complex and rich in comparison to that obtained when using macromolecules. Herein, we demonstrate that the small organic molecule Acid Orange 7 can induce a large diversity of multi-layered barium carbonate structures. These multi-layered structures stem from the small molecule imperfectly blocking the fastest growing crystal face. By tuning the balance of growth and inhibition, we control the layer shape and thickness of the structures. Extending these strategies to strontium carbonate enables the precipitation of large quasi two-dimensional multi-layer sheets. Collectively, these findings highlight the unforeseen potential for using small organic molecules to induce the formation of complex inorganic structures
Counteracting Enantiospecific Behavior of Tailor‐Made Additives During Chiral Symmetry Breaking: Growth Inhibition versus Solid‐Solution Formation
Get PDFAll biological systems are composed of molecules of a single chirality. Although many different scenarios can lead to chiral symmetry breaking, the transmission of the absolute configuration of one compound to another one remains challenging. We here demonstrate that during crystallization-induced chiral symmetry breaking by Viedma ripening, a cascade of different and counteracting processes can occur: solid-solution formation and dissolution inhibition favor symmetry breaking towards the same absolute configuration, while enantioselective growth inhibition favors symmetry breaking towards the opposite absolute configuration. These insights offer a new playground for controlling symmetry breaking processes that are of fundamental and practical importance
Localization-limited exciton oscillator strength in colloidal CdSe nanoplatelets revealed by the optically induced stark effect
Get PDF2D materials are considered for applications that require strong light-matter interaction because of the apparently giant oscillator strength of the exciton transitions in the absorbance spectrum. Nevertheless, the effective oscillator strengths of these transitions have been scarcely reported, nor is there a consistent interpretation of the obtained values. Here, we analyse the transition dipole moment and the ensuing oscillator strength of the exciton transition in 2D CdSe nanoplatelets by means of the optically induced Stark effect (OSE). Intriguingly, we find that the exciton absorption line reacts to a high intensity optical field as a transition with an oscillator strength F-Stark that is 50 times smaller than expected based on the linear absorption coefficient. We propose that the pronounced exciton absorption line should be seen as the sum of multiple, low oscillator strength transitions, rather than a single high oscillator strength one, a feat we assign to strong exciton center-of-mass localization. Within the quantum mechanical description of excitons, this 50-fold difference between both oscillator strengths corresponds to the ratio between the coherence area of the exciton's center of mass and the total area, which yields a coherence area of a mere 6.1 nm(2). Since we find that the coherence area increases with reducing temperature, we conclude that thermal effects, related to lattice vibrations, contribute to exciton localization. In further support of this localization model, we show that F-Stark is independent of the nanoplatelet area, correctly predicts the radiative lifetime, and lines up for strongly confined quantum dot systems
Non-flipping DNA glycosylase AlkD scans DNA without formation of a stable interrogation complex
Get PDFThe multi-step base excision repair (BER) pathway is initiated by a set of enzymes, known as DNA glycosylases, able to scan DNA and detect modified bases among a vast number of normal bases. While DNA glycosylases in the BER pathway generally bend the DNA and flip damaged bases into lesion specific pockets, the HEAT-like repeat DNA glycosylase AlkD detects and excises bases without sequestering the base from the DNA helix. We show by single-molecule tracking experiments that AlkD scans DNA without forming a stable interrogation complex. This contrasts with previously studied repair enzymes that need to flip bases into lesion-recognition pockets and form stable interrogation complexes. Moreover, we show by design of a loss-of-function mutant that the bimodality in scanning observed for the structural homologue AlkF is due to a key structural differentiator between AlkD and AlkF; a positively charged β-hairpin able to protrude into the major groove of DNA
Information advantage from polarization-multiplexed readout of nanophotonic scattering overlay sensors
Get PDFNanophotonic structures are powerful tools for sensing, with the goal of retrieving parameters accurately at maximum speed and minimum photon budget. As information on those parameters can be distributed over multiple output scattering channels that propagate to the far field, considering well-chosen combinations of far-field optical degrees of freedom could benefit measurement precision. We explore how multiplexing readout across different polarization channels enhances parameter retrieval in nanophotonic overlay sensors. We measure the relative position between layers with nanoscale scattering structures, known as overlay in semiconductor metrology, and show that multiplexing either incident or analyzed polarization leads to improved parameter retrieval in the systems studied. At fixed photon budget, we extract additional information equivalent to more than 7 dB in signal level. These results demonstrate that significant advantages in measurement performance of nano-optical sensors can be gained by exploiting the vectorial nature of optical fields
Accelerated Vibrational Energy Relaxation of Water in Alkaline Environments
Get PDFWe observe that hydrated hydroxide ions introduce an additional relaxation channel for the vibrational relaxation of the OD vibrations of HDO molecules in aqueous NaOH solutions. This additional relaxation path involves resonant (Förster) vibrational energy transfer from the excited OD vibration to OH stretch vibrations of hydrated OH– complexes. This energy transfer constitutes an efficient mechanism for dissipation of the OD vibrational energy, as the accepting OH stretch vibrations show an extremely rapid subsequent relaxation with a time constant of <200 fs. We find that the Förster energy transfer is characterized by a Förster radius of 2.8 ± 0.2 Å