Strong coupling between adenine nucleobases in DNA single strands revealed by circular dichroism using synchrotron radiation

Circular dichroism (CD) experiments on DNA single strands $\left({\mathrm{dA}}_{\mathrm{n}}\right)$ at the ASTRID synchrotron radiation facility reveal that eight adenine (A) bases electronically couple upon $190\mathrm{nm}$ excitation. After $n=8$, the CD signal increases linearly with $n$ with a slope equal to the sum of the coupling terms. Nearest neighbor interactions account for only 24% of the CD signal whereas electronic communication is limited to nearest neighbors for two other exciton bands observed at 218 and $251\mathrm{nm}$ (i.e., dimer excited states). Electronic coupling between bases in DNA is important for nonradiative deexcitation of electronically excited states since the hazardous energy is spread over a larger spatial region.

Figure 1

(Color online) (a) Absorption per nucleobase of dAMP and ${\mathrm{dA}}_2$.(b) CD spectra of dAMP and ${\mathrm{dA}}_2$. (c) CD spectra of ${\mathrm{dA}}_n$.

Figure 2

(Color online) CD signal of ${\mathrm{dA}}_n$ plotted against $n$. Excitation wavelength 218 (a) and 251 (b) nm. The model fits are linear fits with $\beta =0$ corresponding to the inclusion of only nearest neighbor interactions in the model described in the text [Eq. (2)].

Figure 3

(Color online) (a) CD signal at $190\mathrm{nm}$. The line is a fit to points corresponding to $n>8$ and the dashed line is a fit to a model described in the text. The inset displays the plot for lower $n$ values. (b) Same as (a) with values divided by $n$.