The effect of spermine on plasmid condensation and dye release observed by fluorescence correlation spectroscopy.

We demonstrate that fluorescence correlation spectroscopy (FCS) can be employed to follow the conformational changes of DNA molecules induced by the addition of a cationic condensing compound (spermine). In our experiments the plasmid pHbetaAPr-1-neo (10 kbp; contour length 3.4 microm) was labeled with propidium iodide (PrIo) and then titrated with spermine to induce its condensation. When spermine was applied at concentrations above 5 microM (spermine/DNAphosphate=0.375), the diffusion time of the labeled plasmid dropped from 15 ms down to 3 ms (its diffusion coefficient, D, increased from 1.0x10(-12) m2/s to 6.0x10(-12) m2/s). The application of spermine was also accompanied by decreasing count rate and particle number, reflecting the dye's dissociation. The data presented show that FCS may become a valuable tool in studying supramolecular aggregate formation, especially when association is followed by a change in the hydrodynamic size of the resulting complex.