Kinetics of peptide synthesis studied by fluorescence of fluorophenyl esters

The kinetics of the reaction of Boc‐alanine‐trifluorophenyl, Boc‐alanine‐tetrafluorophenyl, Boc‐alanine‐pentafluoropbenyl, and Boc‐alanine‐ p ‐chlorotetrafluorophenyl esters (BocAlaOTrf, BocAlaOTfp, BocAlaOPfp, and BocAlaTfc, respectively) with leucine amide and with valine methyl ester have been measured using changes in fluorophenyl chromophore emission at 375 nm. The kinetic data cannot be well fit with a simple second‐order reaction scheme. Measurements of the reaction kinetics at different concentrations of the reagents showed that the expression for the reaction rate is in which k is the reaction rate constant, C N is the concentration of either LeuNH 2 or ValOCH 3 , and C AE is the concentration of the fluorophenyl ester. This reaction equation indicates a complex, probably chain‐like, reaction mechanism. The order of reactivity for these active esters with ValOCH 3 is BocAlaOTfc > BocAlaOPfp > BocAlaOTfp > BocAlaTrf. The apparent rate constant, k , for the reaction with LeuNH 2 is higher than that for the reaction with ValOCH 3 .