p-maleimidophenyl isocyanate: a novel heterobifunctional linker for hydroxyl to thiol coupling.

p-Maleimidophenyl isocyanate (PMPI, 1) is a heterobifunctional cross-linking agent useful for thiol to hydroxyl coupling. Several maleimide-activated compounds were prepared and characterized and then shown to be reactive with thiol-containing proteins. Examples include activation of vitamin B12, digoxigenin, digitoxigenin, estradiol, progesterone, and some serine-containing peptides.