Phosphonate and Thiasugar Analogues of Glucosamine-6-phosphate: Activation of the glmS Riboswitch and Antibiotic Activity

The glmS riboswitch is a motif found in 5'-untranslated regions of bacterial mRNA that controls the synthesis of glucosamine-6-phosphate (GlcN6P), an essential building block for the bacterial cell wall, by a feedback mechanism. Activation of the glmS riboswitch by GlcN6P mimics interferes with the ability of bacteria to synthesize its cell wall. Accordingly, GlcN6P mimics acting as glmS activators are promising candidates for future antibiotic drugs that may overcome emerging bacterial resistance against established antibiotics. We describe the synthesis of a series of phosphonate mimics of GlcN6P as well as the thiasugar analogue of GlcN6P. The phosphonate mimics differ in their pKa value to answer the question of whether derivatives with a pKa matching that of GlcN6P would be efficient glmS activators. We found that all derivatives activate the riboswitch, however, less efficiently than GlcN6P. This observation can be explained by the missing hydrogen bonds in the case of phosphonates and is valuable information for the design of future GlcN6P mimics. The thiasugar analogue of GlcN6P on the other hand turned out to be a glmS riboswitch activator with the same activity as the natural metabolite GlcN6P. The nonphosphorylated thiasugar displayed antimicrobial activity against certain bacilli. Therefore, the compound is a promising lead structure for the development of future antibiotics with a potentially novel mode of action.

Solvent-Assisted Crystallization of an α-Fe2O3 Electron Transport Layer for Efficient and Stable Perovskite Solar Cells Featuring Negligible Hysteresis

Inorganic-organic metal halide perovskite solar cells (PSCs) show power conversion efficiency values approaching those of state-of-the-art silicon solar cells. In a quest to find suitable charge transport materials in PSCs, hematite (α-Fe₂O₃) has emerged as a potential electron transport layer (ETL) in n-i-p planar PSCs due to its low cost, UV light stability, and nontoxicity. Yet, the performance of α-Fe₂O₃-based PSCs is far lower than that of state-of-the-art PSCs owing to the poor quality of the α-Fe₂O₃ ETL. In this work, solvent-assisted crystallization of α-Fe₂O₃ ETLs was carried out to examine the impact of solvents on the optoelectronic properties of α-Fe₂O₃ thin films. Among the various solvents used in this study (deionized water, ethanol, iso-propanol, and iso-butanol), optimized ethanol-based α-Fe₂O₃ ETLs lead to champion device performance with a power conversion efficiency of 13% with a reduced hysteresis index of 0.04 in an n-i-p-configured PSC. The PSC also exhibited superior long-term inert and ambient stabilities compared to a reference device made using a SnO₂ ETL. Through a series of experiments spanning structural, morphological, and optoelectronic properties of the various α-Fe₂O₃ thin films and their devices, we provide insights into the reasons for the improved photovoltaic performance. It is noted that the formation of a pinhole-free compact morphology of ETLs facilitates crack-free surface coverage of the perovskite film atop an α-Fe₂O₃ ETL, reduces interfacial recombination, and enhances charge transfer efficiency. This work opens up the route toward novel ETLs for the development of efficient and photo-stable PSCs.

Increasing the Modulation Depth of GdIII-Based Pulsed Dipolar EPR Spectroscopy (PDS) with Porphyrin-GdIII Laser-Induced Magnetic Dipole Spectroscopy

Distance determination with pulsed EPR has become an important technique for the structural investigation of biomacromolecules, with double electron-electron resonance spectroscopy (DEER) as the most important method. Gd^III-based spin labels are one of the most frequently used spin labels for DEER owing to their stability against reduction, high magnetic moment, and absence of orientation selection. A disadvantage of Gd^III-Gd^III DEER is the low modulation depth due to the broad EPR spectrum of Gd^III. Here, we introduce laser-induced magnetic dipole spectroscopy (LaserIMD) with a spin pair consisting of Gd^III(PymiMTA) and a photoexcited porphyrin as an alternative technique. We show that the excited state of the porphyrin is not disturbed by the presence of the Gd^III complex and that herewith modulation depths of almost 40% are possible. This is significantly higher than the value of 7.2% that was achieved with Gd^III-Gd^III DEER.