The gut microbiota synthesize hundreds of molecules, many of which are known to impact host physiology. Among the most abundant metabolites are the secondary bile acids deoxycholic acid (DCA) and lithocholic acid (LCA), which accumulate at ~500 µM and are known to block C. difficile growth 1 , promote hepatocellular carcinoma 2 , and modulate host metabolism via the GPCR TGR5 3 . More broadly, DCA, LCA and their derivatives are a major component of the recirculating bile acid pool 4 ; the size and composition of this pool are a target of therapies for primary biliary cholangitis and nonalcoholic steatohepatitis. Despite the clear impact of DCA and LCA on host physiology, incomplete knowledge of their biosynthetic genes and a lack of genetic tools in their native producer limit our ability to modulate secondary bile acid levels in the host. Here, we complete the pathway to DCA/LCA by assigning and characterizing enzymes for each of the steps in its reductive arm, revealing a strategy in which the A-B rings of the steroid core are transiently converted into an electron acceptor for two reductive steps carried out by Fe-S flavoenzymes. Using anaerobic in vitro reconstitution, we establish that a set of six enzymes is necessary and sufficient for the 8-step conversion of cholic acid to DCA. We then engineer the pathway into Clostridium sporogenes, conferring production of DCA and LCA on a non-producing commensal and demonstrating that a microbiome-derived pathway can be expressed and controlled heterologously. These data establish a complete pathway to two central components of the bile acid pool, and provide a road map for deorphaning and engineering pathways from the microbiome as a critical step toward controlling the metabolic output of the gut microbiota.

We report a bis-naphthopyran mechanophore that exhibits force-dependent changes in visible absorption. A series of polymers incorporating a chain-centered bis-naphthopyran mechanophore are activated using ultrasonication. By varying the length of the polymer chains, the force delivered to the mechanophore is modulated systematically. We demonstrate that the relative distribution of two distinctly colored merocyanine products is altered predictably with different magnitudes of applied force, resulting in gradient multicolor mechanochromism. The mechanochemical reactivity of bis-naphthopyran is supported by DFT calculations and described by a theoretical model that provides insight into the force-color relationship.

Mechanochromic molecular force probes conveniently report on stress and strain in polymeric materials through straightforward visual cues. We capitalize on the versatility of the naphthopyran framework to design a series of mechanochromic mechanophores that exhibit highly tunable color and fading kinetics after mechanochemical activation. Structurally diverse naphthopyran crosslinkers are synthesized and covalently incorporated into silicone elastomers, where the mechanochemical ring-opening reactions are achieved under tension to generate the merocyanine dyes. Strategic structural modifications to the naphthopyran mechanophore scaffold produce dramatic differences in the color and thermal electrocyclization behavior of the corresponding merocyanine dyes. The color of the merocyanines varies from orange-yellow to purple upon the introduction of an electron donating pyrrolidine substituent, while the rate of thermal electrocyclization is controlled through electronic and steric factors, enabling access to derivatives that display both fast-fading and persistent coloration after mechanical activation and subsequent stress relaxation. In addition to identifying key structure-property relationships for tuning the behavior of the naphthopyran mechanophore, the modularity of the naphthopyran platform is demonstrated by leveraging blends of structurally distinct mechanophores to create materials with desirable multicolor mechanochromic and complex stimuliresponsive behavior, expanding the scope and accessibility of force-responsive materials for applications such as multimodal sensing.Scheme 1 Reaction of naphthopyran in PDMS materials generates colored merocyanine dyes with substituent-dependent mechanochromic properties.

We introduce the concept of mechanochemically gated photoswitching. Mechanical regulation of a photochemical reaction is exemplified using a newly designed mechanophore based on a cyclopentadiene− maleimide Diels−Alder adduct. Ultrasound-induced mechanical activation of the photochemically inert mechanophore in polymers generates a diarylethene photoswitch via a retro-[4 + 2] cycloaddition reaction that photoisomerizes between colorless and colored states upon exposure to UV and visible light. Control experiments demonstrate the thermal stability of the cyclopentadiene−maleimide adduct and confirm the mechanical origin of the "unlocked" photochromic reactivity. This technology holds promise for applications such as lithography and stress-sensing, enabling the mechanical history of polymeric materials to be recorded and read ondemand.

We report the discovery of a 2H-naphtho[1,2-b]pyran mechanophore that produces a permanent merocyanine dye

The ability to accurately and quantitatively characterize structure−mechanochemical activity relationships is important for informing the fundamental understanding of mechanochemical reactivity and, in turn, the successful advancement of the rapidly growing field of polymer mechanochemistry. Ultrasound-induced mechanical activation of polymers remains one of the most general methods for studying mechanophore reactivity; however, the activation rates of scissile mechanophores are still routinely deduced from changes in polymer size using gel permeation chromatography (GPC) that indirectly report on mechanophore activation with questionable accuracy. Here, the rates of ultrasound-induced mechanochemical activation of two distinct scissile and fluorogenic mechanophores are measured using photoluminescence spectroscopy and compared directly to rates determined using various methods for analyzing chain scission kinetics from GPC measurements. This systematic study confirms that the conventional method for analyzing chain scission kinetics is inaccurate and that it provides a misleading picture of mechanophore activity. Instead, time-dependent changes in the GPC refractive index response closely reproduce the rates of mechanophore activation determined spectroscopically. These results expand on prior work by providing a systematic evaluation of the methods used to characterize mechanophore activation kinetics and emphasize the need for a unified approach to kinetic analysis in the field of polymer mechanochemistry. Moreover, analysis of mechanophore activation efficiency reveals an important insight into the consequences of molecular weight dispersity on the characterization of mechanophore reactivity.

23The gut microbiota synthesize hundreds of molecules, many of which are known to impact 24 host physiology. Among the most abundant metabolites are the secondary bile acids deoxycholic 25 acid (DCA) and lithocholic acid (LCA), which accumulate at ~500 µM and are known to block C. 26 difficile growth 1 , promote hepatocellular carcinoma 2 , and modulate host metabolism via the GPCR 27 TGR5 3 . More broadly, DCA, LCA and their derivatives are a major component of the recirculating 28 bile acid pool 4 ; the size and composition of this pool are a target of therapies for primary biliary 29 cholangitis and nonalcoholic steatohepatitis. Despite the clear impact of DCA and LCA on host 30 physiology, incomplete knowledge of their biosynthetic genes and a lack of genetic tools in their 31 native producer limit our ability to modulate secondary bile acid levels in the host. Here, we 32 complete the pathway to DCA/LCA by assigning and characterizing enzymes for each of the steps 33 in its reductive arm, revealing a strategy in which the A-B rings of the steroid core are transiently 34 converted into an electron acceptor for two reductive steps carried out by Fe-S flavoenzymes. 35Using anaerobic in vitro reconstitution, we establish that a set of six enzymes is necessary and 36 sufficient for the 8-step conversion of cholic acid to DCA. We then engineer the pathway into 37 Clostridium sporogenes, conferring production of DCA and LCA on a non-producing commensal 38 and demonstrating that a microbiome-derived pathway can be expressed and controlled 39 heterologously. These data establish a complete pathway to two central components of the bile 40 acid pool, and provide a road map for deorphaning and engineering pathways from the microbiome 41 as a critical step toward controlling the metabolic output of the gut microbiota. 42

Vibrio bacteria are pathogens of fish, shellfish, coral, and humans due to contaminated seafood consumption. Vibrio virulence factors are controlled by the cell-to-cell communication called quorum sensing, thus this signaling system is a promising target for therapeutic design. We screened a compound library and identified nine compounds, including several 2thiophenesulfonamides, that inhibit the master quorum sensing transcription factor LuxR in Vibrio campbellii but do not affect cell growth. We synthesized a panel of 50 thiophenesulfonamide compounds to examine the structure-activity relationship effects on quorum sensing in vivo. The most potent molecule identified, PTSP (3-phenyl-1-(thiophen-2ylsulfonyl)-1H-pyrazole), specifically inhibits LuxR homologs in multiple strains of Vibrio vulnificus, Vibrio parahaemolyticus, and V. campbellii with sub-micromolar concentrations.PTSP efficacy is driven by amino acid conservation in the binding pocket, which is accurately predicted using in silico modeling of inhibitors. Our results underscore the potential for developing thiophenesulfonamides as specific quorum sensing-directed treatments for Vibrio infections.