Studies on poly( L ‐lysine 50 , L ‐tyrosine 50 )–DNA interaction

Interaction between poly(Lys 50 , Tyr 50 ) and DNA has been studied by absorption, circular dichroism (CD), and fluorescence spectroscopy and thermal denaturation in 0.001 M Tris, pH 6.8. The binding of this copolypeptide to DNA results in an absorbance enhancement and fluorescence quenching on tyrosine. There is also an increase in the tyrosine CD at 230 nm. The CD of DNA above 250 nm is slightly shifted to the longer wavelength which is qualitatively similar to, but quantitatively much smaller than, that induced by polylysine binding. At physiological pH the poly(Lys 50 , Tyr 50 )–DNA complex is soluble until there is one lysine and one tyrosine per nucleotide in the complex. The same ratio of amino acid residues to nucleotide has also been observed in copolypeptide‐bound regions of the complex. The addition of more poly(Lys 50 , Tyr 50 ) to DNA yields a constant melting temperature, T m ′, for bound base pairs at 90°C which is close to that of polylysine‐bound DNA under the same condition. The melting temperature, T m , of free base pairs at about 60°C on the other hand, is increased by 10°C as more copolypeptide is bound to DNA. As the temperature is raised, both absorption and CD spectra of the complexes with high coverage are changed, suggesting structural alteration, perhaps deprotonation, on bound tyrosine. The results in this report also suggest that intercalation of tyrosine in DNA is unlikely to be the mode of binding.