MONOMER UNITS FOR THE BETA-BEND RIBBON STRUCTURE - MEAIB PEPTIDES

A conformational analysis in CDCl3 solution by using i.r. absorption and H-1 nuclear magnetic resonance spectroscopy was performed on the fully blocked dipeptides Z-MeAib-Aib-NHMe, Z-MeAib-L-Ala-NHMe, Z-Aib-MeAib-NHMe, and Z-L-Ala-MeAib-NHMe, representing repeating units of the beta-bend ribbon spiral (an approximate 3(10)-helix, with an intramolecular H-bonding donor every two residues), where Z represents benzyloxycarbonyl, MeAib alpha-methylaminoisobutyric acid, and NHMe methylamino. The molecular and crystal structures of the first three compounds were also assessed by X-ray diffraction. While the -MeAib-Aib-, -MeAib-L-Ala-, and -Aib-MeAib- sequences give stable beta-bend structures, the preferred conformation of the -L-Ala-MeAib- sequence is open. These results indicate that the MeAib residue is a good beta-bend promoter, but less efficient than its unmethylated counterpart at position i + 2.