Quenching of the fluorescence of actin modified at lysine-61

The fluorescence of actin modified with fluorescein isothiocyanate at Lys-61 is quenched in the presence of iodide. The approximate second order rate constant for quenching is 9 X 10(-8) M-1 s-1, which approaches that for a diffusion-limited process. However, a measurable degree of protection is provided to the fluorescein group by the protein structure around it. A tryptic digest of the modified actin is more susceptible to quenching than is the native protein. Free fluorescein is even more vulnerable. The addition of deoxyribonuclease I to the modified actin reduces the susceptibility of the fluorescence to quenching by only a small degree.