Macrocyclic β-Sheet Peptides that Mimic Protein Quaternary Structure through Intermolecular β-Sheet Interactions

摘要

This paper reports the design, synthesis, and characterization of a family of cyclic peptides that mimic protein quaternary structure through β-sheet interactions. These peptides are 54-membered-ring macrocycles comprising an extended heptapeptide β-strand, two Hao β-strand mimics [JACS >2000, 122, 7654] joined by one additional α-amino acid, and two δ-linked ornithine β-turn mimics [JACS >2003, 125, 876]. Peptide >3a, as the representative of these cyclic peptides, contains a heptapeptide sequence (TSFTYTS) adapted from the dimerization interface of protein NuG2 [PDB ID: 1mio]. ¹H NMR studies of aqueous solutions of peptide >3a show a partially folded monomer in slow exchange with a strongly folded oligomer. NOE studies clearly show that the peptide self-associates through edge-to-edge β-sheet dimerization. Pulsed-field gradient (PFG) NMR diffusion coefficient measurements and analytical ultracentrifugation (AUC) studies establish that the oligomer is a tetramer. Collectively, these experiments suggest a model in which cyclic peptide >3a oligomerizes to form a dimer of β-sheet dimers. In this tetrameric β-sheet sandwich, the macrocyclic peptide >3a is folded to form a β-sheet, the β-sheet is dimerized through edge-to-edge interactions, and this dimer is further dimerized through hydrophobic face-to-face interactions involving the Phe and Tyr groups. Further studies of peptides >3b–>3n, which are homologues of peptide >3a with 1–6 variations in the heptapeptide sequence, elucidate the importance of the heptapeptide sequence in the folding and oligomerization of this family of cyclic peptides. Studies of peptides >3b–>3g show that aromatic residues across from Hao improve folding of the peptide, while studies of peptides >3h–>3n indicate that hydrophobic residues at positions R3 and R5 of the heptapeptide sequence are important in oligomerization.