Sulfanilation of lysozyme by carbodiimide reaction. Reactivities of individual carboxyl groups

The carboxyl groups of lysozyme were coupled with sulfanilic acid, a chromophoric nucleophile, using 1-ethyl-3-dimethylaminopropylcarbodiimide at pH 5. Other carbodiimides were less effective. Ninety percent of the carboxyl groups were sulfanilated through exhaustive reaction with 1.2 m nucleophile. Isolation and identification of the tryptic peptides from this material showed that all 10 of the carboxyls of lysozyme had reacted. In 0.05 m sulfanilic, Glu-35 and Asp-101 were most reactive while Glu-7, Asp-18, and Asp-66 were least. Change to high concentration of nucleophile (from 0.05 to 1.2 m sulfanilic) altered carboxyl reactivity. Addition of inhibitor reduced reactivity of Asp-101 and Glu-35. Side reactions were not important.