Optimization of critical parameters to obtain difficult peptide sequences using induction-heat energy on solid phase peptide synthesizer PurePep®Chorus

Lorenzo Pacinia,^b, Luísa Aguiar^c, Łukasz Frankiewicz^c, Florence Genet^c, Federico Pratesi^d, Justyna Brasuń^e, Paolo Roveroa,^f, and Anna Maria Papinia,^b

^a Interdepartmental Research Unit of Peptide and Protein Chemistry and Biology - Peptlab, MoD&LS Laboratory, University of Florence, 50019 Sesto Fiorentino, Italy
^b Department of  Chemistry “Ugo Schiff”, University of Florence, 50019 Sesto Fiorentino, Italy
^c Gyros Protein Technologies Inc., 4675 South Coach Drive, Tucson, Arizona, 85714, USA
^d Department of Clinical and Experimental Medicine, University Hospital of Pisa, 56126 Pisa, Italy
^e Department of Inorganic Chemistry, Wroclaw Medical University, Borowska 211A, 50-556 Wroclaw, Poland
^f Department of Neurosciences, Psychology, Drug Research and Child Health Section of Pharmaceutical Sciences and Nutraceutics, University of Florence, 50019 Sesto Fiorentino, Italy

Introduction
In the last decade, more and more difficult syntheses of peptides in terms of
sequence length and complexity are requested to peptide chemists.
An increasing number of peptides that are used as Active Pharmaceutical
Ingredients (APIs) or as diagnostic tools are based on over 40 residues,
multi-branched systems, cyclic structures, etc. Moreover, their syntheses
can require incorporating synthetic strategies such as stapling and/or
introduction of post-translational modifications, such as disulfide bridges
and glycosylated residues.

Herein, two syntheses of difficult peptides that were carried out on
are reported as a proof of concept:

1. A 72mer peptide covering the receptor-binding interface (receptor
   binding motif, RBM436–507) of the SARS-CoV-2 Spike protein.
   Ac-WNSNNLDSKVGGNYNYLYRLFRKSNLKPFERDISTEIYQAGSTP[CNGVEGFNC]YFPLQSYGFQPTNGVGYQP-NH2
2. A head-to-tail disulfide bridged bicyclopeptide (BCL):
   a peptide with marked metal binding properties that can form mono and dinuclear complexes because of the presence of two His moieties.
   BCL: c(PKKHP-c(CFWKTC)-PKKH)

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