Spotlights From The Journal of Physical Chemistry Letters: Volume 8, Issue 6

Each issue of The Journal of Physical Chemistry Letters includes a special section for Spotlights –accessible, plain-language summaries of selected papers from the journal.
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General Method to Determine the Flux of Charged Molecules through Nanopores Applied to β-Lactamase Inhibitors and OmpF

Gram-negative bacteria are responsible for many well-known diseases, including cholera, chlamydia, and pneumonia. This group of bacteria is so-named because they do not retain Gram staining, which is used to differentiate bacteria. Incidents of antibiotic-resistant Gram-negative bacteria are on the rise, and much work is being done to find out why. It is known that nonpermeable or poorly permeable biological porins in the outer membrane of Gram-negative bacteria are key players in the development of antibiotic resistance. Although target-based screening provides many new potential antibiotics, their lack of membrane permeability necessitates high doses of the drugs, which can lead to cell toxicity and problematic side effects. To overcome the low antibiotic sensitivity and the increase in resistance, there is a need for robust, high-sensitivity assays that can verify the permeability of bacterial outer membrane porin nanopores for newly developed antibiotics. In their Letter, Ghai et al. describe the development and successful application of a generally applicable electrophysiological permeability assay for large ions through nanopores. They used transmembrane bi- and tri-ionic gradients of charged large antimicrobial ions and determined relative ion permeabilities. Using the relative permeabilities and the conductance of the compounds, they calculated the transport number for the flow of the macro-ions driven solely by the chemical potential. The authors used the electrophysiological permeability assay to investigate the permeability of three pharmacologically highly relevant ß-lactamase inhibitors through OmpF, a classical nanometer-sized porin playing essential roles in the antibiotics uptake into bacteria. They show that three ß-lactamase inhibitor anions—avibactam, sulbactam, and tazobactam—reveal a surprisingly high permeability through OmpF that is even slightly above the one for the small chloride anion, indicating that the low uptake of β-lactamase inhibitors observed by OmpF-expressing bacteria cannot be wholly attributed to a low transport capacity of OmpF for these compounds. Rather, additional regulatory processes are likely to control the permeation of the β-lactamase inhibitors through the pore of OmpF by protein–protein interactions.

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Decreasing Radiative Recombination Coefficients via an Indirect Band Gap in Lead Halide Perovskites

In the field of photovoltaics, lead-halide perovskites are known to possess a unique combination of exceptionally long lifetimes and high absorption coefficients, and recent work claims that this is due to a direct band gap with an indirect band gap just below it. The idea is that the direct band gap may be responsible for the strong absorption, while the indirect band gap provides the long lifetimes. Kirchartz and Rau used numerical simulations to show that the achievement of high absorption and slow recombination is indeed possible in the radiative limit, but relevant only when charge collection is an issue. At open circuit, the addition of an indirect band gap below a direct one cannot improve the photovoltage relative to the situation without the indirect gap because any enhancement of lifetime has to be paid for by a reduction in band gap and thereby an increase in equilibrium-charge-carrier concentration. While the lifetimes can increase, the recombination rates at a given voltage cannot go down. This distinction is important for a better understanding of the effect. For the special case of MAPI (CH3NH3PbI3), the authors show that most likely the indirect band gap transition is quite strong and actually dominates radiative recombination, meaning that the addition of the indirect gap in MAPI decreases open-circuit voltage relative to the situation where the indirect gap does not exist.

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Influence of the Anomeric Conformation in the Intermolecular Interactions of Glucose

Sugars are fundamental biomolecules that are well-known for storing energy, but they also play a central role in the immune systems of multicellular organisms. The extracellular side of a cell membrane is decorated with glycans that are probed by the cells of the immune system; if the sequence of the glycan is not recognized, an immune response is triggered. There is enormous variability of such glycans, and the receptors in the immune system are able to recognize tiny differences, such as the axial/equatorial position of a single hydroxyl group in a given sugar. Usabiaga et al. studied the interaction between sugar units using a combination of mass-resolved laser spectroscopy and quantum-chemical calculations. They formed molecular aggregates of glucose using a laser desorption setup coupled with a supersonic expansion. Using a combination of ultraviolet and infrared lasers, the authors recorded the resonance-enhanced multiphoton ionization and mass-resolved infrared spectra of such aggregates. They found that the interaction between different anomers of the same sugar molecule is significantly different. While the structure of the beta anomers allows the molecules to establish a hydrogen bond network that embraces both molecules, such symmetric structure cannot be reached in β–α or α–α interactions. Thus, the small difference between α and β anomers, the axial/equatorial position of the hydroxyl substituent of the anomeric carbon, is amplified by cooperative hydrogen bond networks that extend from one molecule to another during aggregation, demonstrating that such a subtle difference results in very different interaction energy values. This observation is in good agreement with the higher selectivity of the synthetic receptors for a given anomer of a sugar molecule or the differences in solubility between starch (formed by α anomers) and cellulose (formed by β anomers of the same sugar molecule).

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