The use of p-Iodophenyl[¹²⁵I] isothiocyanate for determination of N-terminal amino acids in nanomole quantities of protein

Abstract

Identification of the N-terminal residue of 2.5-5 nmol of protein was possible by means of a modified Edman reaction and p-iodophenyl[¹²⁵I] isothiocyanate of high specific activity. The second and third residues could also be identified. Yields of the terminal residue were 25-50%, allowing approximate quantitation of the molar content of protein. Use of the isotope allowed certain steps of the degradative cycle to be examined in some detail, and preparation of this relatively non- volatile labelled Edman reagent was safe, cheap and convenient.     The synthesis and properties of the p-iodophenylthiohydantoins of 19 amino acids, and some side products of the coupling reaction, are described.