Addition of a peptide fragment on an )alpha) -helical depsipeptide induces (alpha)/310-conjugated helix: Synthesis, crystal structure, and CD spectra of Boc-Leu-LeuAla-(Leu-Leu-Lac)3-Leu-Leu-OEt

The depsipeptide Boc1-Leu2-Leu3-Ala4-Leu5-Leu6-Lac7-Leu8-Leu9-Lac10-Leu11-Leu12-Lac13-Leu14-Leu15-OEt16 (1) (Boc = tert-butyloxycarbonyl, Lac = Llactic acid residue) has been synthesized from the peptide Boc-Leu-Leu-Ala-OEt (2) and a depsipeptide, Boc-(Leu-Leu-Lac)3-Leu-Leu-OEt (3). Single crystals of 1 were successfully obtained and the structure has been solved by direct methods (such as Sir2002 and Shake-and-Bake). Interestingly, 1 adopts an (alpha)/310conjugated helix containing a kink at the junction of peptide and depsipeptide segments, Leu3-Lac7. This is significantly different from the conformation of 3, which has a straight (alpha)-helical structure with standard (phi) and (psi) angles. Microcrystalline CD spectra were also studied to compare structural properties of 1 and 3. The differences between (alpha)/310- and (alpha)-helices appear in these CD spectra.