Di ketopiperazines

S. Seki, K. Suzuki, and T. Koide. J. Chem. Soc. Japariy Pure Chem. Sect. 77, 346-9 (1956). Vapor pressure A/f, from heat of sublimation, dimcthylgloximc, di-ketopiperazine. 

The observed relative rates of isoleucine epimerization in heating experiments are N-terminal and di-ketopiperazine groups s> C-terminal and interior groups free isoleucine (in aqueous solution). However, the relative extents of epimerization observed in real samples is diketopiperazine > N-terminal > C-terminal > free s> interior (Mitterer Kriausakul 1984).The apparent dilemma of why C-terminal and free isoleucine can exhibit extensive epimerization but their rates of epimerization are slow is explained by the conversion of some of the more highly epimerized isoleucine from quickly (N-terminal and diketopiperazine) to slowly (C-terminal and free isoleucine) epimerizing units via diketopiperazine and hydrolysis (Fig. 5.39 Kriausakul Mitterer 1983). 

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