European Biophysics Journal

, Volume 36, Issue 6, pp 609–620 | Cite as

Liposome complexation efficiency monitored by FRET: effect of charge ratio, helper lipid and plasmid size

  • Catarina Madeira
  • Luís M. S. Loura
  • Manuel Prieto
  • Aleksander Fedorov
  • M. Raquel Aires-Barros
Original Paper


Cationic lipid/DNA complexes (lipoplexes) are promising vehicles for DNA vaccines or gene therapy. In these systems, transfection efficiency is highly related to lipoplex charge ratio, since lipoplexes with charge ratios (±) lower than electroneutrality have most DNA uncovered by the liposomes, and thus are unprotected from enzyme degradation. However, a large excess of cationic lipids is undesirable because of eventual cytotoxicity. The aim of this work was to determine the minimum charge ratio from which all DNA molecules are complexed by the liposomes varying the lipid formulation and plasmid size, using a new FRET (fluorescence resonance energy transfer) methodology. The similarity of FRET results, fluorescence intensity data and fluorescence decays of several charge ratios above (±) ≥ 4 or 5 confirmed that once all DNA is covered by the liposomes, additional lipid molecules do not affect the lipoplex multilamellar repeat distance. It was also verified by FRET that the presence of helper lipid reduces the amount of cationic lipid required for DNA protection but does not affect the lipoplex multilamellar repeat distance. This distance varies with the plasmid size when supercoiled plasmid is used, being apparently larger when longer plasmids are used. Our study indicates that, despite the complexity of these systems not being totally described by our model, FRET is an informative technique in lipoplex characterization.


FRET Lipoplex BOBO-1 Cationic lipid Plasmid DNA Gene delivery 



Benzothiazolium, 2,2′-[1,3- propanediylbis[(dimethyliminio)-3,1- propanediyl-1(4H)-pyridinyl-4- ylidenemethylidyne]]bis[3-methyl]-, tetraiodide




1,1′-[1,3- propanediylbis[(dimethyliminio)-3,1- propanediyl]]bis[4-[(3-methyl-2(3H)- benzoxazolylidene)methyl]]-, tetraiodide



C. M. acknowledges financial support from FCT, PRAXIS XXI (BD/21476/1999), Portugal. L. M. S. L., A. F. and M. P. acknowledge financial support from POCTI projects (FCT).


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Copyright information

© EBSA 2007

Authors and Affiliations

  • Catarina Madeira
    • 1
  • Luís M. S. Loura
    • 2
  • Manuel Prieto
    • 3
  • Aleksander Fedorov
    • 3
  • M. Raquel Aires-Barros
    • 1
  1. 1.Centro de Engenharia Biológica e QuímicaInstituto Superior TécnicoAv. Rovisco PaisPortugal
  2. 2.IBB-Institute for Biotechnology and Bioengineering, Centre for Biological and Chemical EngineeringUniversidade de ÉvoraÉvoraPortugal
  3. 3.Centro de Química-Física Molecular, Complexo IInstituto Superior TécnicoAv. Rovisco PaisPortugal

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