Optical spectra of wet and dry M -DNA

We present measurements of optical absorption spectra of $M$-DNA, a form of DNA where divalent metal cations are incorporated into the DNA structure by replacing one of the hydrogens from hydrogen bonds in the interior of a double helix. In our experiment we have used ${\mathrm{Zn}}^{2+}$ cations which readily form a complex with DNA in alkaline buffers (pH 9). Absorption spectra of wet and dry Zn-DNA are almost identical and show a redshift of absorption maxima. The red energy shift indicates an increase of the $\ensuremath{\pi}\text{\ensuremath{-}}\ensuremath{\pi}$ overlap of adjacent molecular orbitals in a more compact $M$-DNA structure. However, the effect is smaller $(\ensuremath{\sim}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ than predicted $(\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ which we attribute to sequence randomness of native DNA used in the experiment.