The distance separating Cys-10 from the high-affinity metal binding site in actin

The fluorescence resonance energy transfer between 5-[2-[iodoacetyl)amino)ethyl]aminonaphthalene-1-sulphonic acid (1,5-IAEDANS) attached to Cys-10 residue and Co2+ bound to the high affinity metal site was measured. The resonance energy transfer efficiency was 8 +/- 2%, which corresponds to a distance of 2.1 nm using the absorption spectrum of Co-EDTA and 3.0 nm using the more relevant absorption spectrum of Co2+ bound to carboxypeptidase as a model spectrum of Co2+ bound to actin, respectively.