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

Abstract

Abstract 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 m μ ) 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.