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From Wikipedia article on hyperchromicity (emphasis my own):

"When the bases become unstacked, the wavelength of maximum absorbance does not change, but the amount absorbed increases by 37%."

However, theoretically speaking, any electron delocalization (in this case π-stacking) always lowers the overall energy of the system, hence the wavelength of maximum absorbance should be greater.

Why is this not the case in the hyperchromic effect?

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We are dealing here with the quantities differing by (at least) two orders of magnitude:

  • energy corresponding to 260nm radiation is about 110 kcal/mol (here is a converter)
  • stacking energies of DNA double helix are of the order of 1 kcal/mol

The change in the absorption energy due to electron delocalization when unstacking is thus smaller than 1 kcal mol, and constitutes a negligible correction of less than 1% to the absorption energy.

Update
It is necessary to point out that:

  • The absorption at 260nm does not correspond to a specific electronic transition, but rather to the average of the transition frequencies of different bases (240-270nm) . Thus, the actual position of the peak of absorption is not necessarily at this wave length, but varies depending on the DNA base composition. Moreover, the peak is broader than the stacking energy.
  • The cited Wikipedia article (more precisely its English version) is the only source that I have seen so far, where it is claimed that the position of the absorption peak does not change. In fact, 260nm is not the position of the peak, but the standard wavelength where the absorption is measured and calibrated. In fact, the change in the absorption is so big, that a small shift of the absolute peak does not matter.
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    $\begingroup$ A quick Google search on stacking energy led me to this article (pubmed.ncbi.nlm.nih.gov/9199773), where it says, "the stacking energies in canonical B-DNA base-pair steps vary from -9.5 kcal/mol (GG) to -13.2 kcal/mol (GC)". And according to the converter, this would result in a quite noticeable shift to around 285-290 nm. Am I missing something? $\endgroup$
    – Rusty
    Aug 25, 2021 at 15:29
  • $\begingroup$ @Rusty these values are very different from those that one finds in other sources. $\endgroup$
    – Roger V.
    Aug 25, 2021 at 15:49
  • $\begingroup$ @Rusty my initial answer was a bit simplistic. I have expanded it to include other aspects. $\endgroup$
    – Roger V.
    Aug 25, 2021 at 16:26
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    $\begingroup$ thank you so much, though I'm still uncertain as to the magnitude of the energies (still couldn’t find another reliable source), your updated answer is rather exhaustive! $\endgroup$
    – Rusty
    Sep 9, 2021 at 17:24

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