Protecting group free radical C-H trifluoromethylation of peptides

Abstract

Two radical-based approaches have been developed to effect the trifluoromethylation of aryl C-H bonds in native peptides either using stoichiometric oxidant or visible light photoredox catalysis. The reported methods are able to derivatize tyrosine and tryptophan sidechains under biocompatible conditions, and a number of examples are reported involving fully unprotected peptides with up to 51 amino acids. The development of this chemistry adds to the growing array of chemical methods for selectively modifying amino acid residues in the context of complex peptides. The direct incorporation of trifluoromethyl groups into biopolymers enables the study of a range of biological and biochemical systems, and preliminary results indicate this method can be extended to the incorporation of other fluoroalkyl groups for bioconjugation applications.

Fig. 1. Radical trifluoromethylation of aromatic residues relevant to peptide functionalization.

Fig. 2. Amino acid compatibility of trifluoromethylation of tyrosine-containing dipeptides^a,b,c,d,e [^aconditions: substrate (0.03 mmol), Zn(SO₂CF₃)₂ (3 equiv.), TBHP (5 equiv.) in 10% AcOH (aq) buffer (0.2 M) for 16 h at 23 or 37 °C. Yield was determined by ¹⁹F-NMR spectroscopy using α,α,α-trifluorotoluene as an internal standard. Each entry is an average of at least two data points. ^bTrifluoromethylation occurred at the 2-position of Trp indole in Trp–Tyr. ^cTyr–Thr–NH₂ was used. ^dTrifluoromethylated Cys–Tyr was obtained as the corresponding disulfide dimer. ^eThe thioether sidechain in trifluoromethylated Met–Tyr was oxidized].

Fig. 3. Comparison of stoichiometric and photoredox catalytic approaches to radical trifluoromethylation of peptides.

Fig. 4. (a) Comparison of SOFAST ¹³C methyl HMQC NMR spectra of native unmodified insulin (black) and CF₃-Tyr-A19 insulin 18 (red); (b) ribbon diagram of insulin showing proximity of Tyr-A19 and Ile-A2.

Fig. 5. Installation of azidoalkyl groups through radical tyrosine C–H alkylation^a [^asubstrate (0.10 mmol), ZnCl₂ (1.5 equiv., 16 only), DAAS-Na (3 equiv.), TBHP (5 equiv.), DMSO : H₂O (2 : 1) at 45 °C, 20 h].