The invention provides methods to prepare protein conjugates from proteins having at least four accessible cysteine residues. In one embodiment, an antibody with four reducible disulfide linkages is reduced to provide four pairs of free cysteines. Each pair of free cysteines is reacted with a 1,3-dihaloacetone or similar reactant, linking the sulfur atoms of each pair together though a 3-carbon tether with a reactive ketone. A pair of these reactive ketones are linked together with and used to attach a single payload molecule, thus conjugating the antibody to two payload groups. This method gives a stabilized antibody conjugate with high selectivity for a ratio of two payloads per antibody.
