RESUMO
The parameters characterizing the quenching of fluorescence emitted by the four tryptophans of actin were measured by time resolved techniques in the monomeric (G) and the polymerized (F) forms of the protein. Acrylamide as a neutral and cesium ions as positively charged quenchers were used to characterize the subdomain 1, which contains all the four tryptophan residues. Use of acrylamide did not reveal any difference between the G- and F-forms, cesium, however, did so. The value of the quenching rate constant, k+, is significantly higher for the F-form than that for the G-form. This difference is present independently of the model (discrete or continuous lifetime distribution) used to process the data. These results are compatible with the conclusion that the charge distribution of the microenvironment around at least one of the four tryptophans is changed. This means that this region becomes more attracted to the cesium ion as a result of transition from the G- to the F-form.