Spectroscopic and kinetic characterization of eosin-5-maleimide.

Eosin-5-maleimide (EM) is an increasingly important and widely used probe in the study of membrane protein structure and function. Yet little is known about its spectral properties in hydrophobic and hydrophilic environments. Furthermore, EM is hydrolyzed faster than the traditional N-ethylmaleimide. To offer a more solid foundation for the use of EM in studies of membrane protein structure and function, we have undertaken a detailed study of the absorbance and fluorescence spectra of EM, eosin-5-maleic acid, and the L-cysteine and beta-mercaptoethanol adducts of EM in aqueous and hydrophobic environments; we have studied the kinetics of hydrolysis of EM in various environments, and we have investigated the reaction kinetics of EM with L-cysteine.