Quantitative monitoring of solid-phase peptide synthesis by the ninhydrin reaction

Abstract

A rapid and sensitive method for the quantitative determination of free amino groups during solid-phase peptide synthesis has been developed. The technique involves the reaction of the free amine with ninhydin under carefully controlled conditions and the determination of the resulting chromophore in solution at 570 nm. All of the blue chromophore of the ninhydrin reaction with primary amines is found in solution and the beads of the polystyrene support are colorless. An ion-exchange mechanism for the formation of blue beads by previous methods is proposed. The technique is useful for measuring the total number of peptide chains on the resin and, especially, for monitoring the progress of the coupling reaction. It is possible to determine when the formation of a peptide bond is 99.9% complete.