Fluorescence Correlation Spectroscopic Studies of a Single Lipopolyamine–DNA Nanoparticle

We have studied lipopolyamine–DNA complex formation by fluorescence correlation spectroscopy(FCS). Two lipopolyamines,

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4

,

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9

-dioleoylspermineand

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-cholesteryl spermine carbamate, wereused to condense linear calf thymus DNA and two plasmid DNAs: pGL3 (5.3 kilobase pairs) and pEGFP(4.7 kilobase pairs). PicoGreen

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(PG), a high-affinity DNA intercalating agent that only fluoresceswhen intercalated, was used in our FCS study. In this study, the ConfoCor I set-up upgraded with TimeHarp 200was used. FCS directly visualizes the condensation process by tracking changes in diffusion coefficientsand particle numbers. We were able to define the fluorescent signalling behaviour of PG through the processfrom dye binding to dye release and then dye quenching. Dye release was suggested as the indicator forDNA conformational change, but not for nanoparticle formation. Dye quenching, through the observation oflifetime change, is a more important event accurately and sensitively reporting that a singlenanoparticle exists.