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The incorporation of noncanonical amino acids (ncAAs) into fluorescent proteins is promising for red-shifting their fluorescence and benefiting tissue imaging with deep penetration and low phototoxicity. However, ncAA-based red fluorescent proteins (RFPs) have been rare. The 3-aminotyrosine modified superfolder green fluorescent protein (aY-sfGFP) represents a recent advance, yet the molecular mechanism for its red-shifted fluorescence remains elusive while its dim fluorescence hinders applications. Herein, we implement femtosecond stimulated Raman spectroscopy to obtain structural fingerprints in the electronic ground state and reveal that aY-sfGFP possesses a GFP-like instead of RFP-like chromophore. Red color of aY-sfGFP intrinsically arises from a unique "double-donor" chromophore structure that raises ground-state energy and enhances charge transfer, notably differing from the conventional conjugation mechanism. We further developed two aY-sfGFP mutants (E222H and T203H) with significantly improved (∼12-fold higher) brightness by rationally restraining the chromophore's nonradiative decay through electronic and steric effects, aided by solvatochromic and fluorogenic studies of the model chromophore in solution. This study thus provides functional mechanisms and generalizable insights into ncAA-RFPs with an efficient route for engineering redder and brighter fluorescent proteins. [ABSTRACT FROM AUTHOR]

Mitochondrial membrane potential (MMP) is a frequently used indicator for mitochondrial function. Herein, we report a photostable near-infrared (NIR) fluorescent dye for monitoring MMP. This new probe, named NIMAP, is non-fluorescent in aqueous solution and can be activated by cell membranes, providing high fluorescence contrast and low background fluorescence. NIMAP has been validated for monitoring MMP in living mammalian cells and in mice. Due to the large fluorescence response, low fluorescence background, high photostability, and excellent tissue penetration resulting from red-shifted excitation and emission in the “optical window” above 600 nm, broad applications of this new probe are expected. [ABSTRACT FROM AUTHOR]

A mutant pyrrolysyl-tRNA synthetase/tRNA pair was used to genetically encode allylcarbamoyl methyllysine in bacteria. This amino acid can be converted to methyllysine with a ruthenium catalyst, providing a straightforward approach for site-specifically introducing methyllysine residues into proteins. [ABSTRACT FROM AUTHOR]

We hereby report a mild and efficient coupling reaction between alkyl aldehydes and aryl diamines. In the presence of a Cu(2+) or a Zn(2+) ion, oxygen (O2) in air is able to promote the oxidative condensation of the two readily preparable functional groups, forming stable benzimidazole linkages in neutral aqueous solution at room temperature (RT). We demonstrated that the reaction could be utilized to label a T4 lysozyme protein containing a chemically installed aryl diamine group with a fluorescent aldehyde dye molecule at 37 °C.