Thiolysis of O-2,4-dinitrophenyltyrosines. Spectrophotometric monitoring of the reaction and its use in peptide synthesis.

The thiolytic cleavage of O-2,4-dinitrophenyl (Dnp) derivatives of phenols was applied to the synthesis of tyrosine-containing peptides. This paper describes the preparation and properties of starting materials for such syntheses and illustrates their use in the synthesis of some peptides containing tyrosine at either the C- or N-terminus. A spectrophotometric method for following the thiolytic removal of Dnp groups from O-Dnp-tyrosines was developed and used to establish optimal conditions for quantitative deblockage in aqueous and nonaqueous solvents. The method is based on the fact that upon thiolysis, the colorless solution of O-Dnp-tyrosine (λ_max at 298 nm, pH 8.5) becomes yellow due to the formation of a dinitrophenylated thiol (for S-Dnp-2-mercaptoethanol, λmax at 340 nm, pH 8.5). This gives rise to a difference spectrum with a maximum at 354 nm (Δ?_M = + 8680 M^?1 cm^?1), a minimum at 298 nm (Δ?_M = ?5900 M^?1 cm^?1) and a crossover point at 318 nm, which is different (in the 290–320 nm range) from the difference spectrum obtained upon thiolysis of N^Im-Dnp-histidine. This method provides a useful analytical tool in peptide and polypeptide synthesis as well as in protein chemistry.