Linear oligopeptides. Part 227. X-Ray crystal and molecular structures of two α-helix-forming (Aib- L -Ala)sequential oligopeptides, pBrBz-(Aib- L -Ala) 5 -OMe and pBrBz-(Aib- L -Ala) 6 -OMe

A crystal-state structutal analysis of pBrBz-(Aib-LAla)5-OMe tetrahydrate and pBrBz-(Aib-L-Ala)6-OMe dihydrate has been performed by X-ray diffraction. The decapeptide and dodecapeptide molecules are both basically α-helical with five and seven 1 â†� 5 intramolecular H-bonds, respectively. A similarity between the two structures is also seen near the C-terminus, where regularity of the α-helix is disrupted in favour of formation of intramolecular H-bonds of the 1 â†� 4 and 1 â†� 6 types. A brief comparison with parameters and interactions characteristic of the helices present in globular proteins has been made.