Electrooptical properties of nucleic acids and nucleoproteins II. Study of the deoxyribonucleohistone-proflavine complexes

The interaction between proflavine and the gel-forming deoxyribonucleohistone from calf thymus has been examined by means of the electrooptical method, in connexion with spectrophotometric, equilibrium dialysis and rigidity measurements, in media of very low ionic strenght.

• 1.

  1. The changes in the visible and ultraviolet proflavine spectra were similar to those already mentioned by different authors for DNA and soluble deoxyribonucleohistone-proflavine complexes at higher ionic strenght. Only two important differences were observed: the isosbestic point was absent in the visible spectra and a slight shift (± 5 mμ) of the ultraviolet absorption band of the dye to shorter wavelengths was noticed.

• 2.

  2. Binding curves showed an important interaction between proflavine and the gel-forming deoxyribonucleohistone in spite of the very low net electric charge of the macromolecule.

• 3.

  3. The complexes displayed a negative dichroism in the visible range, due entirely to the dye. The dichroic ratio D increased with decreasing numbers of ligand molecules bound per atom of phosphorus (r) and reached a maximum value of about 1.85 (for r ⩽ 0.10), approximately constant in the whole visible absorption band (field strength: 13–13.5 kV/cm).

• 4.

  4. At the same field strenght, the birefringence (B) at 550 mμ, per unit of gel-forming deoxyribonucleohistone concentration, increased towards a maximum value of about 1.45 dl/mg for r = 0.10.

• 5.

  5. The birefringence relaxation times τ were not noticeably affected by the interaction except for r > 0.10 where a decrease of τ was observed.

The results are consistent with an intercalation of the proflavine cations between adjacent nucleotide pairs, for at most one proflavine molecule per five nucleotide pairs in the gel-forming deoxyribonucleohistone. For higher proflavine contents, the dye molecules “in excess” would be externally attached to the helix.

Some comparative measurements on a soluble-deoxyribonucleohistone fraction were in agreement with this scheme.