New Inverse Peptide Synthesis Strategy – NIPS

Our studies were first carried out under solution conditions, and the transposition of the obtained results to solid phase peptide chemistry was one of the final objectives of this project. We have focused our research on developing practical and simple reaction conditions, trying to keep on mind the need for atom economy (compared with the classical use of coupling reagents). This requires the use of simple, cheaper and non-toxic reagents. Moreover, we have established a strong collaboration between our group, specialized in synthetic organic chemistry, and the group of Pr. Gilles Subra, who is expert in supported peptide synthesis. Synergies that arose from the experience of both groups, [in addition of their own synthetic platform which implies the laboratories and all the synthesis and analytical equipment necessary to an ideal working environment and to the project realization (ICGM and IBMM)] have insured the good development of the proposal.

A procedure based on the combination of activated alpha-aminoesters with carboxylic acids has been developed, thanks to the financial support allowed by ANR. The method was successfully applied to the solution synthesis of several dipeptides and tripetides in very good yields. Additionally, a model tetrapeptide could be prepared in the challenging reverse N?C direction, and the use of sensitive cysteine showed encouraging results as no epimerization was detected during the synthesis of a dipeptide bearing such amino acid residue. Transposition to a large-scale synthesis of a dipeptide was also realized.

Our future efforts will cover both, the transposition of our method to solid phase peptide chemistry and also the use of encumbered amino acid residues. Moreover, we also envisage to elucidate the mechanistic pathway of the reaction via NMR 13C studies and in situ IR spectroscopy.

To date, the work achieved within this project gave rise to 3 publications in prestigious international journals (ACIE in 2014; Synthesis in 2014; Organic Syntheses in 2015). Two additional papers are currently under preparation, in which a Chemical Reviews. Moreover, two oral communcations have been done: one during the 3rd SCF-d’Avenir at Montpellier in 2015 (R. Marcia de Figueiredo) and another one during the SECO 52 - France (J.-S. Suppo, NIPS PhD Fellow, oral communication award). Poster presentations (5) were also realized.

Peptides are attracting increasing attention not only as therapeutics (60 peptide drugs are on the market) but also in polymer or material sciences: Artificial silks, hydrogels, supported catalysts, targeting agents for imaging or therapy, are only a few examples of this matter of facts. The main goal of the NIPS project is to radically innovate the way of creating the peptidic bond by activating the N-function instead of the classical carboxylic activation. This New Inverse Peptide Synthesis Strategy might allow the resolution of some problems associated with classical peptide coupling reagents. Although these reagents are very efficient, some drawbacks inherently linked to their mode of activation are well-known:
- The use of these coupling reagents forces the direction of the peptide synthesis. Whereas peptides are naturally built-up in the N?C direction, they are chemically synthesized from the C-terminal to the N-terminal end in order to minimize the epimerization whenever an amino-acid is introduced.
- Some AA are also particularly sensitive to epimerization during their activation. This is the case of histidine and cysteine residues that have the highest susceptibility to racemize during carboxylate activation.
- Moreover, the introduction of sterically hindered AA is known to be inherently more tricky due to the formation of a congested tetrahedric intermediate (SN2t mechanism) during the amide formation. The introduction of these AA usually requires extended time of reactions and/or special peptide coupling reagents. Finally, peptide fragment couplings, and consequently cyclo-peptide synthesis, are usually not allowed due to unpredictable levels of racemisation of the C-terminal AA residue due, again, to non-predictable epimerization levels, thus demanding a great deal of optimization studies.
In conclusion the NIPS project aims on developing a new innovative strategy to construct peptides through the activation of the ‘amino’ function. With this tool in hands, and according to preliminary encouraging results, we hope to be able to palliate, at least part of, the main problems associated with current classical peptide coupling reagents.