Manfred Ladinig, Werner Leupin, Markus Meuwly, Michal Respondek, Jakob Wirz and Vincent Zoete, "Protonation Equilibria of Hoechst 33258 in Aqueous Solution", Helv. Chim. Acta 2005, 88, 53-67.
Abstract: The widely used bis-benzimidazole derivative Hoechst 33258 (2'-(4-hydroxyphenyl)-5-(4-methyl-1-piperazinyl)-2,5'-bi-1H-benzimidazole) binds to the minor groove of DNA duplexes and is widely used as fluorescent cytological stain for DNA. The neutral compound, 1, is amphiphilic with four basic and three acidic sites. We have determined all seven acidity constants by spectrophotometric titration to define the pH-dependent distribution of species, from the fully protonated tetracation 14+ to the fully deprotonated trianion 13-, in aqueous solution. The structures of the intermediate protonation states were assigned with the aid of density functional calculations. Electrostatic interaction free energies were calculated to adjust the acidity constants of the molecular subunits of 1 to their environment in the species 14+ to 13-. The experimental and theoretical pKa values agree well, but they differ substantially from previous estimates given in the literature.