The hypochromism of helical polynucleotides

Abstract

The hypochromism of the 260 mμ. absorption band of helical DNA is its lower absorbance, per base group, compared to the absorbance of the mononucleotides. The hypochromism of DNA has been calculated as a function pf the base composition and sequence. The oscillator strengths, transition monopoles and transition moments for the π→π* transitions of the bases with wavelengths of 187 to 276 mμ have been calculated from molecular orbital theory and u.v. spectra. The effect of transitions with wavelengths close to 200 mμ, is to make the calculated hypochromism increase with increasing guanine + cytosine content. Experimental measurements of hypochromism, while they seem to show the opposite dependence on base composition, are not yet detailed enough for definite conclusions to be made. The effect of transitions in the vacuum u.v. has been approximated by base polarizabilities and solvent refractive index. The vacuum u.v. transitions inside the helix cause hypochromism; those in the sugar and phosphate groups and the solvent cause hyperchromism. The net hypochromism cannot be accurately calculated but should have little dependence on base composition. The effect of static electric fields is negligible.