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Reverse native chemical ligation as a new entry to cyclic peptide therapeutics

Shariff, LS; (2016) Reverse native chemical ligation as a new entry to cyclic peptide therapeutics. Doctoral thesis , UCL (University College London).

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Abstract

Cyclic peptides are of particular therapeutic interest, often exhibiting improved biological activity and resistance to in vivo proteolysis over their linear counterparts, but current production methods can be costly, lengthy and complex. The adaptation of a facile way to join peptide fragments developed by the Macmillan group forms the basis of the investigation and application of a novel method of peptide cyclisation featured within this thesis. A one-pot, tandem reaction of reverse native chemical ligation (retro-NCL via N→S acyl shift) at a C-terminal cysteine, in the presence of a thiol additive to ‘trap’ the generated transient thioester, followed by NCL (S→N) at an N-terminal cysteine on the same peptide chain resulted in the successful cyclisation of various biologically active sequences. Each circular peptide underwent testing for novel therapeutic roles, including a sunflower trypsin inhibitor analogue SFTI(I10H) that was found, upon design of an enzymatic assay, to inhibit human kallikrein 5 (implicated in atopy and epithelial cancers) with IC50 0·14 µM. To facilitate studies on the influence of pKas on reaction rate, synthesis routes to the production of orthogonally protected β-cysteine and γ-cysteine for use in standard solid-phase peptide synthesis were established. In combination with studies using pH variation it was found that the pKa of the amino group participating in the N→S acyl shift does not completely explain variations in reaction rate observed for C-terminal cysteine carboxylates versus carboxamides. Instead it is proposed that the rate of the acyl transfer during retro-NCL is related to stability induced by zwitterion formation for carboxylates but α-amino group pKa values for carboxamides. The importance of the thiol additive identity was also highlighted, with the example captamine being prone to thioester formation in addition to cyclisation under the conditions used.

Type: Thesis (Doctoral)
Title: Reverse native chemical ligation as a new entry to cyclic peptide therapeutics
Event: UCL (University College London)
Language: English
UCL classification: UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Chemistry
URI: https://discovery.ucl.ac.uk/id/eprint/1492947
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