Addition of a peptide fragment on an α‐helical depsipeptide induces α/3 10 ‐conjugated helix: Synthesis, crystal structure, and CD spectra of Boc–Leu–Leu–Ala–(Leu–Leu–Lac) 3 –Leu–Leu–OEt

The depsipeptide Boc 1 –Leu 2 –Leu 3 –Ala 4 –Leu 5 –Leu 6 –Lac 7 –Leu 8 –Leu 9 –Lac 10 –Leu 11 –Leu 12 –Lac 13 –Leu 14 –Leu 15 –OEt 16 ( 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/3 10 ‐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/3 10 ‐ and α‐helices appear in these CD spectra. © 2004 Wiley Periodicals, Inc. Biopolymers, 2004