Diffusion-enhanced lanthanide energy-transfer study of DNA-bound cobalt(III) bleomycins: comparisons of accessibility and electrostatic potential with DNA complexes of ethidium and Acridine Orange

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Abstract
Energy transfer in the rapid-diffusion limit reflects the equilibrium properties of a donor-acceptor system. Rates of energy transfer from freely diffusing Tb chelates to DNA-binding chromophores change dramatically when DNA is added; energy transfer from an electrically neutral chelate is reduced because the energy acceptor becomes partially buried in DNA, while energy transfer from a positive chelate is increased because of electrostatic attraction. The rate constants for energy transfer to DNA-bound chromophores from a positively charged Tb chelate, relative to those from a neutral chelate, were used to estimate the following values for the electrostatic potential near the surface of each DNA-bound acceptor at 298.degree. K in the presence of 1.0 mM added salt (in units of -e/kT): acridine orange, 4.54 .+-. 0.11; ethidium, 4.66 .+-. 0.07; green Co(III) bleomycin A2, 4.06 .+-. 0.11; orange Co(III) bleomycin A2, 3.11 .+-. 0.10. Smaller numbers indicate less negative potentials; these can be due to a combination of positive charge on the chromophore, location of the chromophore [particularly Co(III) bleomycin] away from the DNA phosphates, and/or separation of DNA phosphate negative charges by an intercalator. The magnitudes of the individual rate constants indicate that all the DNA-bound chromophores can be directly encountered by the Tb probes. Energy-transfer rate constants from a neutral Tb chelate to DNA-bound and free acceptors can provide a measure of the accessibility of the Tb chelate to DNA-bound and free acceptors can provide a measure of the accessibility of the Tb probe to each bound chromophore. The ratios of these rate constants were as follows: acridine orange, 0.17 .+-. 0.01; ethidium, 0.27 .+-. 0.02; green form of Co(III) bleomycin A2, 0.48 .+-. 0.06; orange form of Co(III) bleomycin A2, 0.71 .+-. 0.06. These results are consistent with the probable differences in binding mechanisms for the intercalating chromophores (ethidium and acridine orange) as compared to the Co(III) bleomycins (in which the relevant chromophores are nonintercalating metal centers). All the results imply that the green Co(III) bleomycin chromophore binds closer to DNA than the orange; this provides a first step toward understanding the structural basis for the different biological properties of these metallobleomycins. Control experiments and theoretical considerations necessary to establish the validity of the results are also presented.