title: Synthesis and conformational analysis of a cyclic peptide obtained via i to i+4 intramolecular side-chain to side-chain azide-alkyne 1,3-dipolar cycloaddition.
abstract: Intramolecular side-chain to side-chain cyclization is an established approach to achieve stabilization of specific conformations and a recognized strategy to improve resistance toward proteolytic degradation. To this end, cyclizations, which are bioisosteric to the lactam-type side-chain to side-chain modification and do not require orthogonal protection schemes, are of great interest. Herein, we report the employment of Cu(I)-catalyzed 1,3-dipolar cycloaddition of side chains modified with azido and alkynyl functions and explore alternative synthetic routes to efficiently generate 1,4-disubstituted [1,2,3]triazolyl-containing cyclopeptides. The solid-phase assembly of the linear precursor including epsilon-azido norleucine and the propargylglycine (Pra) in positions i and i+4, respectively, was accomplished by either subjecting the resin-bound peptide to selective on-resin diazo transformation of a Lys into the Nle(epsilon-N3) or the incorporation of Fmoc-Nle(epsilon-N3)-OH during the stepwise build-up of the resin-bound peptide 1b. Solution-phase Cu(I)-catalyzed 1,3-dipolar cycloaddition converts the linear precursor Ac-Lys-Gly-Nle(epsilon-N3)-Ser-Ile-Gln-Pra-Leu-Arg-NH2 (2) into the 1,4-disubstituted [1,2,3]triazolyl-containing cyclopeptide [Ac-Lys-Gly-Xaa(&(1))-Ser-Ile-Gln-Yaa(&(2))-Leu-Arg-NH2][(&(1)(CH2)4-1,4-[1,2,3]triazolyl-CH2&(2))] (3). The conformational preferences of the model cyclopeptide 3 (III), which is derived from the sequence of a highly helical and potent i to i+4 side-chain to side-chain lactam-containing antagonist of parathyroid hormone-related peptide (PTHrP), are compared to the corresponding lactam analogue Ac[Lys(13)(&(1)),Asp(17)(&(2))]hPTHrP(11-19)NH2 (II). CD and NMR studies of 3 and II in water/hexafluoroacetone (HFA) (50:50, v/v) revealed a high prevalence of turn-helical structures involving in particular the cyclic regions of the molecule. Despite a slight difference of the backbone arrangement, the side-chains of Ser, Gln, and Ile located at the i+1 to i+3 of the ring-forming sequences share the same spatial orientation. Both cyclopeptides differ regarding the location of the turn-helical segment, which in II involves noncyclized residues while in 3 it overlaps with residues involved in the cyclic structure. Therefore, the synthetic accessibility and conformational similarity of i to i+4 side-chain to side-chain cyclopeptide containing the 1,4-disubstituted [1,2,3]triazolyl moiety to the lactam-type one may result in similar bioactivities.
authors.affiliation: Laboratory for Translational Research, Harvard Medical School, One Kendal Square, Building 600, Cambridge, Massachusetts 02139, USA.
authors.forename: Sonia
authors.initials: S
authors.lastname: Cantel
authors.forename: Alexandra Le Chevalier
authors.initials: Ale C
authors.lastname: Isaad
authors.forename: Mario
authors.initials: M
authors.lastname: Scrima
authors.forename: Jay J
authors.initials: JJ
authors.lastname: Levy
authors.forename: Richard D
authors.initials: RD
authors.lastname: DiMarchi
authors.forename: Paolo
authors.initials: P
authors.lastname: Rovero
authors.forename: Jose A
authors.initials: JA
authors.lastname: Halperin
authors.forename: Anna Maria
authors.initials: AM
authors.lastname: D'Ursi
authors.forename: Anna Maria
authors.initials: AM
authors.lastname: Papini
authors.forename: Michael
authors.initials: M
authors.lastname: Chorev
mesh_terms: D000480:Alkynes / Q000737:chemistry*; D000595:Amino Acid Sequence; D001386:Azides / Q000737:chemistry*; D002942:Circular Dichroism; D003500:Cyclization; D008958:Models, Molecular; D019906:Nuclear Magnetic Resonance, Biomolecular; D010456:Peptides, Cyclic / Q000138:chemical synthesis* / Q000737:chemistry; D017434:Protein Structure, Tertiary
chemical_list: D000480:Alkynes; D001386:Azides; D010456:Peptides, Cyclic