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

AbstractThe depsipeptide Boc1–Leu2–Leu3–Ala4–Leu5–Leu6–Lac7–Leu8–Leu9–Lac10–Leu11–Leu12–Lac13–Leu14–Leu15–OEt16 (1) (Boc = tert‐butyloxycarbonyl, Lac = L‐lactic 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 α/310‐conjugated 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 α‐helical structure with standard ϕ and ψ angles. Microcrystalline CD spectra were also studied to compare structural properties of 1 and 3. The differences between α/310‐ and α‐helices appear in these CD spectra. © 2004 Wiley Periodicals, Inc. Biopolymers, 2004