The electric linear dichroism spectra of native calf thymus DNA modified to a small extent (1 hydrocarbon residue per 1000 bases) by reaction with (±)-7β,8α-dihydroxy-9α, 10α-epoxy-7,8,9,10-tetrahydrobenzo[a] pyrene (BPDE), which binds covalently mainly to the 2-amino group of guanosine residues, and physical complexes of benzo[a]-pyrene (BP) and proflavin (PF) with DNA were measured. The linear dichroism ΔA of the covalent BPDE-DNA complex in the wavelength region of the absorption of the pyrene-like BPDE chromophore is positive. In contrast, ΔA is negative in the absorption region of the DNA bases, as well as in the absorption region of the BP and PF molecules physically bound to DNA. It is concluded that the orientation of the BPDE moiety is not of the intercalation type as is the case for the physical BP and PF complexes. The reduced linear dichroism is wavelength dependent for the BPDE-DNA and BP·DNA complexes which indicates that there is a heterogeneity of binding sites with different orientations. The quantitative analysis of such results is discussed in detail and it is concluded that there is one major type of oriented BPDE covalently bound to DNA. The long axis of the pyrene-like chromophore of BPDE lies on the surface of a cone whose axis is that of the DNA helix and whose angle, with respect to this axis, is 35° or less. As expected for intercalation-type complexes, the planes of BP and PF are found to be nearly parallel to the planes of the DNA bases, according to this analysis.
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