The invention provides improved methods to prepare protein conjugates from a ketone modified protein. In one embodiment the protein is prepared by linking two free cysteines by reaction with a 1 3 dihaloacetone or similar ketone containing reactant linking the sulfur atoms of the two cysteines together. The ketone inserted between the sulfur atoms is then used to form an oxime thus conjugating the protein to a payload. In another embodiment two cysteine residues are tied together by reaction with a 1 3 dihaloacetone or similar reactant and the new ketone is used to form an oxime with a suitable payload molecule thus conjugating the protein to a payload. The method provides improved reaction conditions for the oxime formation whereby higher yields and Improved product homogeneity are achieved.
