A spectrophotometric study on the interaction of neutral red with double-stranded DNA in large excess

With the measurement of molecular absorption, the interaction of neutral red (NR) with double-stranded DNA in large excess was investigated. It was found that the interaction of NR, existing in the acidic state (HNR) at pH 4.56, with double-stranded structure DNA displays different spectral features depending on the molar ratio of HNR/DNA, R. If R>1.33, the binding process is characterized by a binding constant at the 10(6) level with each nucleotide residue of double-stranded DNA binding one HNR molecule. If R<0.67, the binding constant is reduced to the 10(4) level, and the binding number for each nucleotide residue of double-stranded DNA to HNR is less than one.