Rapid Catalyst-Free Hydrazone Ligation: Protein-Pyridoxal Phosphoramides

Pyridoxal-5′-phosphate (PLP) represents an active form of Vitamin B₆ that shows relatively fast imine formation with hydrazines under physiological conditions without the need of a catalyst. A convenient phosphate/amine conjugation protocol was developed to covalently link PLP to proteins, affording proteins capable of hydrazone formation with bioorthogonal hydrazinyl functional groups. Thus, the lectin Concanavalin A (Con A) was labeled with PLP. Pretreatment with fluorescein hydrazide gave dye-labeled Con A that labeled cell surfaces efficiently. Alternatively, pretargeting was achieved by labeling cells with Con A-PLP, then treatment in vitro with Alexa Fluor 488 hydrazide

A Redox-Reconfigurable, Ambidextrous Asymmetric Catalyst

A redox-reconfigurable catalyst derived from l-methionine and incorporating catalytic urea groups has been synthesized. This copper complex catalyzes the enantioselective addition of diethyl malonate to <i>trans</i>-β-nitrostyrene. Either enantiomer of the product can be predetermined by selection of the oxidation state of the copper ion. Enantiomeric excesses of up to 72% (S) and 70% (R) were obtained in acetonitrile. The ability of the catalyst to invert enantiomeric preference was reproduced with several different solvents and bases. Facile interconversion between the Cu²⁺ and Cu⁺ redox states allowed easy access to both active helical forms of the complex and, therefore, dial-in enantioselectivity