Multivalent counterions (green) bind onto negatively charged phosphates (red) mostly externally for the case of DNA molecule (left) but deep inside the major groove for the case of RNA molecule (right).

From the article:

Opposing Effects of Multivalent Ions on the Flexibility of DNA and RNA
Aleksander V. Drozdetski, Igor S. Tolokh, Lois Pollack, Nathan Baker, and Alexey V. Onufriev
Phys. Rev. Lett. 117, 028101 (2016)

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