Chemical modification of methionine residues in azurin.

Azurin from $\text{Pseudomonas}\text{aeruginosa}$ has been treated with bromoacetate at low pH to alkylate methionine residues. Two classes of methionine side chains are observed as a result of these reactions — four of the six methionines are reactive at pH 4, whereas all six are reactive at pH 3.2. The product containing four alkylated methionines maintains a significant portion of the blue color and spectroscopic characteristics of the native protein. The product which has been fully modified at the methionine residues, on the other hand, has lost all blue color and appears to be largely in a random coil form.