Diiodomethane, CH2I2, in a polar solvent undergoes a unique photoinduced reaction whereby I2− and I3− are produced from its photodissociation, unlike for other iodine-containing haloalkanes. While previous studies proposed that homolysis, heterolysis, or solvolysis of iso-CH2I–I, which is a major intermediate of the photodissociation, can account for the formation of I2− and I3−, there has been no consensus on its mechanism and no clue for the reason why those negative ionic species are not observed in the photodissociation of other iodine-containing chemicals in the same polar solvent, for example, CHI3, C2H4I2, C2F4I2, I3−, and I2. Here, using time-resolved X-ray liquidography, we revisit the photodissociation mechanism of CH2I2 in methanol and determine the structures of all transient species and photoproducts involved in its photodissociation and reveal that I2− and I3− are formed via heterolysis of iso-CH2I–I in the photodissociation of CH2I2 in methanol. In addition, we demonstrate that the high polarity of iso-CH2I–I is responsible for the unique photochemistry of CH2I2.

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Many existing protein detection strategies depend on highly functionalized antibody reagents. A simpler and easier to produce class of detection reagent is highly desirable. We designed a single-component, recombinant, luminescent biosensor that can be expressed in laboratory strains of Escherichia coli and Saccharomyces cerevisiae. This biosensor is deployed in multiple homogeneous and immobilized assay formats to detect recombinant SARS-CoV-2 spike antigen and cultured virus. The chemiluminescent signal generated facilitates detection by an unaugmented cell phone camera. Binding-activated tandem split-enzyme (BAT) biosensors may serve as a useful template for diagnostics and reagents that detect SARS-CoV-2 antigens and other proteins of interest.

The incorporation of light-responsive domains into engineered proteins has enabled control of protein localization, interactions, and function with light. We integrated optogenetic control into proximity labeling (PL), a cornerstone technique for high-resolution proteomic mapping of organelles and interactomes in living cells. Through structure-guided screening and directed evolution, we installed the light-sensitive LOV domain into the PL enzyme TurboID to rapidly and reversibly control its labeling activity with low-power blue light. "LOV-Turbo" works in multiple contexts and dramatically reduces background in biotin-rich environments such as neurons. We used LOV-Turbo for pulse-chase labeling to discover proteins that traffick between endoplasmic reticulum, nuclear, and mitochondrial compartments under cellular stress. We also showed that instead of external light, LOV-Turbo can be activated by BRET from luciferase, enabling interaction-dependent PL. Overall, LOV-Turbo increases the spatial and temporal precision of PL, expanding the scope of experimental questions that can be addressed with PL.

The ability to deliver proteins and peptides across the plasma membrane into the cytosol of living mammalian cells would be highly impactful for both basic science and medicine. Natural cell-penetrating protein toxins have shown promise as protein delivery platforms, but existing approaches are limited by immunogenicity, lack of cell-type-specificity, or their multicomponent nature. Here we explore inactivated botulinum neurotoxin (BoNT) as a protein delivery platform. Using split luciferase reconstitution in the cytosol as a readout for endosomal escape and cytosolic delivery, we showed that BoNT chimeras with nanobodies replacing their natural receptor binding domain can be selectively targeted to cells expressing nanobody-matched surface markers. We used chimeric BoNTs to deliver a range of cargo from 1.3 to 55 kDa in size, and demonstrated selective delivery of orthogonal cargoes to distinct cell populations within a mixed culture. These explorations suggest that BoNT may be a versatile platform for targeted protein and peptide delivery into mammalian cells.