Addition of a peptide fragment on an alpha-helical depsipeptide induces alpha/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 alpha/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 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/3(10)- and alpha-helices appear in these CD spectra.