Labeling of immunoglobulins with bifunctional, sulfhydryl-selective, and photoreactive coumarins.

4-(Bromomethyl)-6,7-dimethoxy-2-oxo-2H-benzopyran (1a) and 4-(bromomethyl)-7-methoxy-2-oxo-2H-benzopyran (1b) and their iodo analogs (2) reacted selectively with the sulfhydryl groups generated in a limited reduction of the hinge disulfide bonds of an immunoglobulin G (IgG) giving proteins labeled with a fluorescent and/or radioactive moiety. Quantitative alkylation of free thiol groups was obtained with 1 and 125/127I-2. Photodimerization of these adducts produced IgG with covalently joined heavy chains.