Abstract
The peptide chain of acetylphenylalanine-L-alanine-phenylalanine-D-alanine methyl ester, C 27 H 30 N 4 O 6 , adopts a 3 10 -helical conformation having right-handed screw sense. The 3 10 -helix is stabilized by intramolecular hydrogen bonds, between CO of the acetyl group and NH of ΔPhe 3 , and between CO of ΔPhe 1 and NH of D-Ala 4 . The hydrogen bonds form two consecutive ten-membered rings whose (φ, ψ) torsion angles are quite close to the standard values for type-III β-turns. In the crystal, the molecules are linked head-to-tail by intermolecular hydrogen bonds to form continuous helical columns. These are aligned along axes parallel to the c axis, with neighbouring columns running in opposite directions. There are no lateral hydrogen bonds between helical columns, but only hydrophobic interactions provided by the interdigitation of apolar side chains of the dehydro-phenylalanine residues, as well as of the C-terminal methyl ester groups.
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