The invention provides methods to prepare protein conjugates from proteins having at least two cysteines. In one embodiment, a protein with a disulfide linkage is reduced to provide two free cysteines for reaction with a 1,3-dihaloacetone or similar reactant, linking the sulfur atoms of the two cysteines together. The ketone inserted between the sulfur atoms is then used to form a Schiff base to an aminated payload molecule, 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. The linkage between the sulfur atoms in each case holds the protein or peptide in a constrained conformation, while also providing a convenient place for attaching a payload with good specificity and efficiency.
