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ABSTRACT CRISPR-Cas12a is widely used for genome editing and biomarker detection since it can create targeted double-stranded DNA breaks and promote non-specific DNA cleavage after identifying specific DNA. To mitigate the off-target DNA cleavage of Cas12a, we previously developed aFrancisella novicidaCas12a variant (FnoCas12a^KD2P) by introducing double proline substitutions (K969P/D970P) in a conserved helix called the bridge helix (BH). In this work, we used cryogenic electron microscopy (cryoEM) to understand the molecular mechanisms of BH-mediated activation of Cas12a. We captured five structures of FnoCas12a^KD2Pat different states of conformational activation. Comparison with wild-type (FnoCas12a^WT) structures unravels a mechanism where BH acts as a trigger that allosterically activates REC lobe movements by tracking the number of base pairs in the growing RNA-DNA hybrid to undergo a loop-to-helical transition and bending to latch onto the hybrid. The transition of the BH is coupled to the previously reported loop-to-helix transition of the “lid”, essential for opening RuvC endonuclease, through direct interactions of residues of the BH and the lid. We also observe structural details of cooperativity of BH and “helix-1” of RuvC for activation, a previously proposed interaction. Overall, our study enables development of high-fidelity Cas12a and Cas9 variants by BH-modifications. 

Abstract Herein, a new family of hybrid metal halides, (DMAP)₂MBr₄(M = Cu, Zn), featuring zero‐dimensional (0D), pseudo‐layered crystal structures containing isolated molecular 4‐dimethylaminopyridinium (DMAP, C₇H₁₁N₂⁺) cations and MBr₄²⁻tetrahedral anions are reported. (DMAP)₂MBr₄show remarkable long‐term stability, with no signs of degradation after one year of ambient air exposure. The reported solution synthesis affords large crystals measuring up to 1 cm, which showed significant response to soft 8 keV X‐ray photons when implemented into X‐ray detectors. Furthermore, (DMAP)₂ZnBr₄demonstrates tunable color light emission properties, which is attributed to the organic molecular units based on our combined experimental and computational results. The measured photoluminescence quantum yield (PLQY) for (DMAP)₂ZnBr₄is 7.35 %, a remarkable enhancement of emission efficiency as compared to a weak emission from the organic precursor. The inexpensive and earth‐abundant chemical compositions and ease of preparation of the new hybrid metal halides make them promising candidates for optical and electronic applications.