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The incorporation of the nonstandard amino acid para-nitro-L-phenylalanine (pN-Phe) within proteins has been used for diverse applications, including the termination of immune self-tolerance. However, the requirement for the provision of chemically synthesized pN-Phe to cells limits the contexts where this technology can be harnessed. Here we report the construction of a live bacterial producer of synthetic nitrated proteins by coupling metabolic engineering and genetic code expansion. We achieved the biosynthesis of pN-Phe in Escherichia coli by creating a pathway that features a previously uncharacterized nonheme diiron N-monooxygenase, which resulted in pN-Phe titers of 820 ± 130 µM after optimization. After we identified an orthogonal translation system that exhibited selectivity toward pN-Phe rather than a precursor metabolite, we constructed a single strain that incorporated biosynthesized pN-Phe within a specific site of a reporter protein. Overall, our study has created a foundational technology platform for distributed and autonomous production of nitrated proteins. 

The effects of atomic mass in terms of its zero-point vibrational energy, on molar volume, glass transition temperature Tg, and viscosity are studied in glassy and supercooled B2O3 liquids using boron isotope substitutions. The molar volume decreases and Tg and isothermal viscosity increase on the substitution of lighter 10B isotopes with the heavier 11B isotopes. These effects are argued to be a manifestation of the higher zero-point vibrational energy of the lighter isotope, which along with the anharmonicity of the potential well, results in a longer equilibrium inter-atomic distance and larger mean-square displacement with respect to that for the heavier isotope. The isotope effect on viscosity is increasingly enhanced as the temperature approaches Tg, which is shown to be consistent with the prediction of the elastic models of viscous flow and shear relaxation. 

A fundamental and much-debated issue in glass science is the existence and nature of liquid–liquid transitions in glass-forming liquids. Here, we report the existence of a novel reentrant structural transition in a S-rich arsenic sulfide liquid of composition As 2.5 S 97.5 . The nature of this transition and its effect on viscosity are investigated in situ using a combination of differential scanning calorimetry and simultaneous Raman spectroscopic and rheometric measurements. The results indicate that, upon heating significantly above its glass transition temperature (261 K), the constituent [Formula: see text] sulfur chains in the structure of the supercooled liquid first undergo a [Formula: see text] chain-to-ring conversion near ∼383 K, which is exothermic in nature. Further heating above 393 K alters the equilibrium to shift in the opposite direction toward an endothermic ring-to-chain conversion characteristic of the well-known λ-transition in pure sulfur liquid. This behavior is attributed to the competing effects of enthalpy of mixing and conformational entropy of ring and chain elements in the liquid. The existence of reentrant structural transitions in glass-forming liquids could provide important insights into the thermodynamics of liquid–liquid transitions and may have important consequences for harnessing novel functionalities of derived glasses.