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 D(2)O solution, was investigated by (1)H nuclear magnetic resonance (NMR). Cyclo(L-Pro-L-Xxx) is mostly isomerized to cyclo(D-Pro-L-Xxx) in D(2)O 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 (CD(3)COCD(3), CD(3)SOCD(3) or CD(3)OD) to a D(2)O solution of cyclo(L-Pro-L-Xxx), cyclo(D-Pro-L-Xxx), or increasing the temperature of the D(2)O solution, CH-π interaction between H(9) and the benzene ring of cyclo (D-Pro-L-Xxx) was stronger than that between H(8α) and the benzene ring of cyclo(L-Pro-L-Xxx).