Analysis of the isomerization of diketopiperazine consisting of proline and aromatic amino acid residues using nuclear magnetic resonance

Abstract Under different concentrations of the base potassium deuteroxide KOD, progress reactions, such as enolization, D‐substitution, isomerization, and conformational changes diketopiperazine cyclo(L‐Pro‐L‐Xxx) cyclo(D‐Pro‐L‐Xxx) (Xxx = Phe, Tyr) in D 2 O solution, was investigated by 1 H nuclear magnetic resonance (NMR). Cyclo(L‐Pro‐L‐Xxx) is mostly isomerized to whereas only slightly even under stronger basic conditions. After adding a deuterated organic solvent (CD 3 COCD , CD SOCD or OD) solution cyclo(L‐Pro‐L‐Xxx), cyclo(D‐Pro‐L‐Xxx), increasing temperature CH‐π interaction between 9 benzene ring cyclo (D‐Pro‐L‐Xxx) than that 8α cyclo(L‐Pro‐L‐Xxx).