Quantitative monitoring of solid-phase peptide synthesis by the ninhydrin reaction
It is a privilege to be able to contribute to this volume in which Professor Zervas’ friends, students, and colleagues have joined together to honor him and his remarkable contributions to custom peptide synthesis.Although I did not know him until after his days in the Bergmann Laboratory at the Rockefeller Institute, I can claim the distinction of now working in those very same rooms that they occupied back in the mid-1930s.Like all peptide chemists, I am greatly indebted to Professor Zervas, having depended so heavily on the carbobenzoxy group and on the various modified urethan protecting groups which have been direct extensions of the revolutionary advance that Bergmann and Zervas made. A rapid and sensitive method for the quantitative determination of free amino groups during custom 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.