Linear oligopeptides. 81. Solid-state and solution conformation of homooligo(.alpha.-aminoisobutyric acids) from tripeptide to pentapeptide: evidence for a 310 helix

Incipient type II β-turn, internal water bridge and head-to-tail sequence in the structure of tri-α-aminoisobutyric acid dihydrate

“…The choice of the particular enantiomorph in the following discussion is purely arbitrary. The bond lengths and valency angles in the structure, given in Table 2, are comparable to those observed in other Aib peptides (Benedetti, Bavoso, Di Blasio, Pavone, Pedone, Crisma, Bonora & Toniolo, 1982;Prasad & Balaram, 1984). It is, however, interesting to note the asymmetry of bond angles around the C ~ of the central residue.…”

“…This unique conformation appears to have been made possible by the absence of terminal protecting groups. All the protected homotripeptides of Aib have a type III or III' fl-turn conformation (Benedetti, Bavoso, Di Blasio, Pavone, Pedone, Crisma, Bonora & Toniolo, 1982;Toniolo, Valle, Bonora, Crisma, Formaggio, Bavoso, Benedetti, Di Blasio, Pavone & Pedone, 1987). The packing diagram of the tripeptide is shown in Fig.…”

Helical l–Leu‐Based Peptides Having Chiral Five‐Membered Carbocyclic Ring Amino Acids with an Ethylene Acetal Moiety

“…The reversal of the signs of the C-terminal residue torsion angles are frequently observed in helical Aib and related peptides, and known as the helix-terminating structure. [34,35] Four intramolecular hydrogen bonds of the i←i+4 type N-H···O=C (α-helix) were observed between H-N(i+4) and C(i)=O(i) (i = 1, 2, 4, 5), and two weak intramolecular hydrogen bonds of the i←i+4 type (α-helix) were observed between H-N(i+4) and C(i)=O(i) (i = 0, 3). In the packing mode, an intermolecular hydrogen bond was observed between the H-N(1) peptide donor and C(8')=O (8') [N(1)···O(8') = 2.85 Å] of a symmetry-related (x, −1+y, z) molecule.…”

Structure of Z-(Aib)9OBu t

“…Aib residues are known as strong helical formers in peptides, also in the presence of common amino acids, favouring or-or 310-helices, depending on chain length, peptide composition and position of the Aib residues. Peptides consisting of only Aib residues show a clear preference for left-and righthanded 310-helices in solution and crystal structures (Benedetti et al, 1982); this is confirmed by recent theoretical studies (Zhang & Hermans, 1994;Hodgkin et al, 1990). The two methyl groups attached to C~ are staggered with the methyl groups of residue n + 3 and N--H...O=C main-chain-main-chain hydrogen bonds are formed between residue n + 3 and n. The helical backbone shows the form of a regular triangular rod.…”