Synthesis of 13C/19F/2H labeled indoles for use as tryptophan precursors for protein NMR spectroscopy

Abstract

Synthesis of indoles labeled with ¹³C–¹H and ¹³C–¹⁹F spin pairs is described. All syntheses utilize inexpensive carbon–¹³C dioxide as the ¹³C isotope source. Ruthenium-mediated ring-closing metathesis is the key step in construction of the ¹³C containing indole carbocycle. Fluorine is introduced via electrophilic fluorination at the 7-position and via palladium-mediated cross-coupling at the 4-position. Indole and fluoroindoles are viable tryptophan precursors for in vivo protein expression. We show that they are viable also in in vitro protein synthesis using standard E. coli S30 extracts. Incorporation of the synthesized ¹³C–¹H and ¹³C–¹⁹F spin pair labeled tryptophans into proteins enables high-resolution and high-sensitivity nuclear magnetic resonance (NMR) spectroscopy.