dsDNA persistence length with divalent ions

Finding a theoretical formula for the persistence length of polyelectrolytes for the whole experimental range of salt concentration is a long standing challenge. Using the Tethered Particle Motion technique, the double-stranded DNA persistence length is measured for four monovalent and divalent salts on a three-decade concentration range. The formula proposed by Trizac and Shen [EPL, 116 18007 (2016)] and extended to divalent ions fits the data. This formula mixes the high salt limit solution of the Poisson-Boltzmann equation together with the DNA charge renormalisation. Magnesium ions induce a fitted DNA radius smaller than the geometrical one, consistent with a site-specific binding.