Gelsolin was labelled with the sulphydryl-specific fluorescent reagent 6-acryloyl-2-dimethylaminonaphthalene (acrylodan). The degree of labelling using non-denaturing conditions was 1.9 ± 0.5 acrylodans per gelsolin molecule. Circular dichroism and viscosity studies showed no significant effect on gelsolin structure and function on incorporation of the label. Circular dichroism studies did not detect Ca²⁺ effects on aerylodan-labelled gelsolin, but fluorescence studies detected subtle changes in the protein. The presence of Ca²⁺ causes a decrease and red-shift in fluorescence emission, an increase in sensitivity to quenching by I⁻ and a decrease in fluorescence polarisation of the acrylodan-labelled gelsolin. These indicate an increased degree of exposure of the fluorescent label to the solvent environment on interaction of gelsolin with Ca²⁺. Actin binding to gelsolin was evident from a decrease in fluorescence intensity, an increase in sensitivity to quenching and an increase in fluorescence polarisation. Actin binding increases the exposure of the acrylodan label to solvent, as does Ca²⁺ binding. / Science, Faculty of / Chemistry, Department of / Graduate