Environment‐dependent conformation of Boc‐Pro‐Ser‐NHCH3

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Abstract
The conformation of Boc‐Pro‐Ser‐NHCH3 (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 1H‐ and 101‐MHz 13C‐nmr studies in CDCl3 clearly show the presence of two conformers differing in the rotameric state of the tertiary‐amide bond. Selective 1H‐13C nuclear Overhauser enhancement experiments at −20°C as well as 1H‐nmr and ir data indicate that the major trans conformer (84% in CDC13) may adopt a type I β‐turn conformation with a possible Oγ  H‐N interaction, similarly to Piv‐Pro‐Ser HCH3 (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.