Method of Acylating a Peptide or Protein

A method for selectively acylating one amino group in a peptide or protein which has two or more reactive nucleophilic functional groups, the method comprising:

• a) reacting in an aqueous media a peptide or protein having at least two reactive nucleophilic functional groups with an acylating agent of the general formula I
• wherein
• n is 1-6
• m is 1-2
• w is 4-20
• R1 is a leaving group which when reacting said acylating agent with a free amine forms an amide bond between the carbonyl group attached to R1 and said amine,
• wherein pH in the aqueous media is between from pH 8 to pH 14;
• and
• b) isolating the N-acylated peptide or protein,
• wherein an N-acylated peptide or protein is obtained comprising at least one reactive nucleophilic functional group which is not acylated or only partially acylated.