Fluorescence spectroscopic studies of pyrene-actin adducts

Reaction kinetic studies of the sulfhydryl-directed fluorescent probes N-(1-pyrene)maleimide (PM) and N-(1-pyrenyl)iodoacetamide with actin from rabbit skeletal muscle showed that there were three accessible sulfhydryl groups in actin. Fluorescence spectral studies showed energy transfer from aromatic amino acid residues to fluorophore reacted at Cys-373, as well as weak excimer fluorescence probably due to doubly labeled molecules at Cys-10 and Cys-373. These results provide further evidence that trytophan and tyrosine residues are located near the probe attached to Cys-373 or Cys-10 and the latter two thiols are in close proximity. In age PM-labeled F-actin, the succinimido ring of PM underwent intramolecular aminolysis, resulting in large emission spectral changes and increased excimer fluorescence. Solvent perturbation studies indicate that the probes were located in a hydrophobic environment; their quantum yield and spectrum properties were very sensitive to changes in the microenvironment. Nanosecond-pulse fluorimetry studies revealed complex fluorescence emission decays with three intrinsic lifetimes in adducts with low molecular weight thiols as well as in labeled proteins. Fluorescence lifetimes were 17, 48 and 111 ns for the pyrenemaleimide adduct of actin, and 3, 14 and 60 ns for the pyrenyliodoacetamide adduct. Supporting evidence is given for the argument that multiple fluorescence lifetimes are an intrinsic property of the pyrene derivatives and are not due to the presence of impurity or heterogeneity in the protein reaction sites. Because of their high sensitivity and long lifetimes, pyrene derivatives are extremely useful.