Fluorescence Correlation Spectroscopic Studies of a Single Lipopolyamine–DNA Nanoparticle

Abstract

We have studied lipopolyamine–DNA complex formation by fluorescence correlation spectroscopy (FCS). Two lipopolyamines, N 4 , N 9 -dioleoylspermine and N 1 -cholesteryl spermine carbamate, were used to condense linear calf thymus DNA and two plasmid DNAs: pGL3 (5.3 kilobase pairs) and pEGFP (4.7 kilobase pairs). PicoGreen ® (PG), a high-affinity DNA intercalating agent that only fluoresces when intercalated, was used in our FCS study. In this study, the ConfoCor I set-up upgraded with TimeHarp 200 was used. FCS directly visualizes the condensation process by tracking changes in diffusion coefficients and particle numbers. We were able to define the fluorescent signalling behaviour of PG through the process from dye binding to dye release and then dye quenching. Dye release was suggested as the indicator for DNA conformational change, but not for nanoparticle formation. Dye quenching, through the observation of lifetime change, is a more important event accurately and sensitively reporting that a single nanoparticle exists.