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

Under different concentrations of the base potassium deuteroxide KOD, the progress of reactions, such as enolization, D-substitution, isomerization, and conformational changes of diketopiperazine cyclo(L-Pro-L-Xxx) and cyclo(D-Pro-L-Xxx) (Xxx = Phe, Tyr) in D2O solution, was investigated by 1H nuclear magnetic resonance (NMR). Cyclo(L-Pro-L-Xxx) is mostly isomerized to cyclo(D-Pro-L-Xxx) in D2O solution, whereas cyclo(D-Pro-L-Xxx) is only slightly isomerized to cyclo(L-Pro-L-Xxx) even under stronger basic conditions. After adding a deuterated organic solvent (CD3COCD3, CD3SOCD3 or CD3OD) to a D2O solution of cyclo(L-Pro-L-Xxx), cyclo(D-Pro-L-Xxx), or increasing the temperature of the D2O solution, CH-π interaction between H9 and the benzene ring of cyclo (D-Pro-L-Xxx) was stronger than that between H8α and the benzene ring of cyclo(L-Pro-L-Xxx).