Macrocyclic β-sheet peptides that mimic protein quaternary structure through intermolecular β-sheet interactions

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.