Long-Range Fluorescence Quenching Of Ethidium Ion By Cationic Porphyrins In The Presence Of DNA

Robert F. Pasternack, Swarthmore College
M. Caccam
Bartholomew P. Keogh , '88, Swarthmore College
T. A. Stephenson
A. P. Williams
E. J. Gibbs


Binding of ethidium to DNA increases the fluorescence efficiency of the molecular ion by about 1 order of magnitude. Quenching of the fluorescence of such an ethidium.DNA complex by tetrakis(4-N-methylpyridyl)porphine (H2T4), and its nickel(II) and zinc(II) derivatives (NiT4 and ZnT4), was investigated in the present study. Results of absorbance and fluorescence measurements obtained for these systems are consistent with a mechanism for quenching that does not involve appreciable displacement of ethidium from the nucleic acid. Sten-Volmer-type plots are nonlinear and can be fit satisfactorily by an equation of the form[GRAPHICS]in which [Q] is the concentration of the porphyrin quencher and sigma is the minimum number of base pairs between ethidium (E+) and the porphyrin required to permit the excited fluorophore to emit a photon. It can be concluded from these studies that, in the presence of DNA, porphyrins are capable of efficiently quenching the fluorescence of an excited ethidium ion at a distance of 25-30 angstrom.