Modification of cysteine residues with N-methyl iodoacetamide.

Cysteine residues derivatized with N-methyl iodoacetamide (MIAA) can be analyzed by the Edman sequencing with a high degree of reliability. By HPLC, the phenylthiohydantoin (PTH) derivative of MIAA-modified cysteine eluted between dimethylphenylthiourea and PTH-Ala--a wide gap which is not occupied or interfered with by any other by-products or PTH amino acids. During extended Edman degradation, the recovery of PTH derivative of MIAA-modified Cys was as quantitative and reproducible as that of other stable PTH derivatives such as Ala, Val, and Leu.