title: Orientation dependence in fluorescent energy transfer between Cy3 and Cy5 terminally attached to double-stranded nucleic acids.
abstract: We have found that the efficiency of fluorescence resonance energy transfer between Cy3 and Cy5 terminally attached to the 5' ends of a DNA duplex is significantly affected by the relative orientation of the two fluorophores. The cyanine fluorophores are predominantly stacked on the ends of the helix in the manner of an additional base pair, and thus their relative orientation depends on the length of the helix. Observed fluorescence resonance energy transfer (FRET) efficiency depends on the length of the helix, as well as its helical periodicity. By changing the helical geometry from B form double-stranded DNA to A form hybrid RNA/DNA, a marked phase shift occurs in the modulation of FRET efficiency with helix length. Both curves are well explained by the standard geometry of B and A form helices. The observed modulation for both polymers is less than that calculated for a fully rigid attachment of the fluorophores. However, a model involving lateral mobility of the fluorophores on the ends of the helix explains the observed experimental data. This has been further modified to take account of a minor fraction of unstacked fluorophore observed by fluorescent lifetime measurements. Our data unequivocally establish that Förster transfer obeys the orientation dependence as expected for a dipole-dipole interaction.
authors.affiliation: Cancer Research United Kingdom Nucleic Acid Structure Research Group, MSI/WTB Complex, The University of Dundee, Dundee DD1 5EH, United Kingdom.
authors.forename: Asif
authors.initials: A
authors.lastname: Iqbal
authors.forename: Sinan
authors.initials: S
authors.lastname: Arslan
authors.forename: Burak
authors.initials: B
authors.lastname: Okumus
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Wilson
authors.forename: Gerard
authors.initials: G
authors.lastname: Giraud
authors.forename: David G
authors.initials: DG
authors.lastname: Norman
authors.forename: Taekjip
authors.initials: T
authors.lastname: Ha
authors.forename: David M J
authors.initials: DM
authors.lastname: Lilley
mesh_terms: D002232:Carbocyanines / Q000737:chemistry*; D004247:DNA / Q000737:chemistry*; D031541:Fluorescence Resonance Energy Transfer; D008956:Models, Chemical; D009690:Nucleic Acid Conformation; D009692:Nucleic Acid Heteroduplexes / Q000737:chemistry*
chemical_list: D002232:Carbocyanines; D009692:Nucleic Acid Heteroduplexes; C089187:cyanine dye 3; C085321:cyanine dye 5; D004247:DNA