Labeling and Natural Post-Translational Modification of Peptides and Proteins via Chemoselective Pd-Catalyzed Prenylation of Cysteine

Abstract

The prenylation of peptides and proteins is an important post-translational modification observed in vivo. We report that the Pd-catalyzed Tsuji–Trost allylation with a Pd/BIPHEPHOS catalyst system allows the allylation of Cys-containing peptides and proteins with complete chemoselectivity and high n/i regioselectivity. In contrast to recently established methods, which use non-native connections, the Pd-catalyzed prenylation produces the natural n-prenylthioether bond. In addition, a variety of biophysical probes such as affinity handles and fluorescent tags can be introduced into Cys-containing peptides and proteins. Furthermore, peptides containing two cysteine residues can be stapled or cyclized using homobifunctional allylic carbonate reagents.

Pd/BIPHEPHOS is an Efficient Catalyst for the Pd‐Catalyzed S‐Allylation of Thiols with High n‐Selectivity

“…The n/i ratio, however, was found to constantly improve over time to afford the pure n-thioether 3 aa after 5 h (Scheme 3C, Table S5) although complete conversion was already reached. [9] This clearly indicates that the S-allylation is reversible under Pd-catalyzed conditions. Importantly, the isomerization takes place only in the presence of the Pd catalyst, whereas an intermolecular S N 2' attack by thiolate can be excluded (Scheme 3D).…”

“…[4] Other metals such as Ir, [5] Fe, [6] and Rh [7] have been demonstrated to serve as effective catalysts for this transformation, showing different regioselectivity. As we did not only envision its application for the synthesis of small molecules but also intended its use for the posttranslational modification of Cys-containing peptides and proteins, [9] we were aiming for a catalyst which would favour the unbranched ("linear") n-isomer. [8] The limited use of Pd-catalyzed S-allylation can be associated with two intrinsic difficulties: a) S-nucleophiles can also function as efficient ligands for Pd and poison the catalyst, and b) thiols are easily oxidized and the reactions have to be carried out under exclusion of air.…”

“…[9] The reversible character of this catalytic reaction offers The outstanding n-regioselectivity of this system results from a combination of intrinsic ligand properties together with the reversible nature of this reaction.…”

“…In particular we were interested if we could observe any trends regarding the influence of the natural bite angle and the electronic properties of the ligands on the reaction outcome. [10] Phosphine ligands with large bite angles ( [9][10][11], which unfortunately did not result in any increased regioselectivity. To our delight, we were instead able to identify the electron-deficient bidentate phosphite BIPHEPHOS ( Table 1, entry 12) as our ligand of choice, affording the thioether 3 aa with full conversion and complete n-selectivity.…”

“…Indeed, we could identify several solvents that are suitable alternatives without affecting conversion or selectivity of the reaction, encompassing polar aprotic substitutes such as propylene carbonate, NMP, DMSO, DME and EtOAc ( (Table 1, entry 23) were also tolerated by our catalyst system. [9] It is noteworthy that these solvents cover an extremely broad range of polarities, which should enable an easy adjustment of our methodology to any thiol substrate addressing solubility issues. In total 31 solvents were considered in this screening (Table S4).…”