The electronic structures and spectra of proflavine,acridine orange,and their DNA complexes

The electronic structure of the proflavine cation is studied by the SCF –ASMO –CI method using the Pariser–Parr–Pople approximations. It is shown that the band at 445 mμ may be assigned to the ¹A₁ → ¹B₁, transition polarized along the long axis of the molecule. The bands in the neighbourhood of 260 mμ, which are composed of three absorption bands, are tentatively assigned to the ¹A₁ → ¹B₁, ¹A₁ → ¹B₁, and ¹A₁ → ¹A₁ transitions, respectively, in order of decreasing wavelength. The spectrum of the acridine orange cation may be understood to have the same assignment as that of the proflavine cation. The acridine dye cations are well known for their dimerization with concentration. The intermolecular distances in these dimers are estimated from the band shifts due to the formation of dimers, using the exciton theory. The main contribution to the molecular interaction is shown to be the electrostatic dipole–dipole interaction. Since the first excitation band of the dye molecule which exhibits a remarkable change due to the formation of the DNA–acridine dye complex, is suggested to be polarized along the long axis, preference of the outside stacking or the intercalation model is qualitatively discussed from the spectral shift of the acridine dye bound to the DNA, assuming simple models.