Abstract
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).
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