Diketopiperazine-mediated peptide formation in aqueous solution II. Catalytic effect of phosphate

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Abstract
The previous paper (I) reported that DKP (glycine anhydride) spontaneously reacts with glycine (Gly) or oligoglycines (Gly n ) to produce longer oligoglycines (Gly n+2). This paper presents that phosphate catalyzes the condensation reaction quite effectively. Formation of Gly4 from DKP (0.1 M) and Gly2 (0.1 M) in phosphate solution of various concentrations was investigated at a neutral pH at 41 °C. The yields of Gly4 increased almost linearly with the concentration of phosphate from 0.06 M to 0.24 M. The yield in 0.24 M phosphate solution was approximately one hundred times as high as that in the absence of the phosphate, whereas in the case of Gly3 formation from DKP and Gly the effect of the phosphate was of ten times lower than in the former case. Orthophosphate was the most effective catalyst among the various kind of chemicals tried in the present investigation including polyphosphates.