Environment‐dependent conformation of Boc‐Pro‐Ser‐NHCH 3

The conformation of Boc‐Pro‐Ser‐NHCH 3 ( 1 ) was studied in the solid state and in solution. In the crystal, the steric structure of 1 is characterized by an E ( cis ) urethane tertiary amide bond and the lack of intramolecular H bonds. Four‐hundred megahertz 1 H‐ and 101‐MHz 13 C‐nmr studies in CDCl 3 clearly show the presence of two conformers differing in the rotameric state of the tertiary‐amide bond. Selective 1 H‐ 13 C nuclear Overhauser enhancement experiments at −20°C as well as 1 H‐nmr and ir data indicate that the major trans conformer (84% in CDC1 3 ) may adopt a type I β‐turn conformation with a possible O γ H‐N interaction, similarly to Piv‐Pro‐Ser HCH 3 (2) [A. Aubry, N. Ghermani, and M. Mar‐raud (1984) Int. J. Peptide Protein Res. 23 , 113‐122]. Molecular mechanics calculations on the cis rotamer show that the β‐pleated‐like backbone conformation of serine in the crystal of 1 is not a low‐energy state for the isolated molecule; it is caused by packing forces, particularly by the helical network of intermolecular H bonds.