KINETICS OF PEPTIDE-SYNTHESIS STUDIED BY FLUORESCENCE OF FLUOROPHENYLESTERS

The kinetics of the reaction of Boc-alanine-trifluorophenyl, Boc-alani ne-tetrafluorophenyl, Boc-alaninepentafluorophenyl, and Boc-alanine-p- chlorotetrafluorophenyl esters (BocAlaOTrf, BocAlaOTfp, BocAlaOPfp, an d BocAlaTfc, respectively) with leucine amide and with valine methyl e ster have been measured using changes in fluorophenyl chromophore emis sion at 375 nm. The kinetic data cannot be well fit with a simple seco nd-order reaction scheme, Measurements of the reaction kinetics at dif ferent concentrations of the reagents showed that the expression for t he reaction rate is V = kC(N)(0.5)C(AE)(1.5), in which k is the reacti on rate constant, C-N is the concentration of either LeuNH(2) or ValOC H(3), and C-AE is the concentration of the fluorophenyl ester, This re action equation indicates a complex, probably chain-like, reaction mec hanism. The order of reactivity for these active esters with ValOCH(3) is BocAlaOTfc > BocAlaOPfp > BocAlaOTfp > BocAlaTrf. The apparent rat e constant, k, for the reaction with LeuNH(2) is higher than that for the reaction with ValOCH(3). (C) Munksgaard 1994.