Protonation Equilibria of Hoechst 33258 in Aqueous Solution

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State: Public
Version: Final published version
Serval ID
serval:BIB_2C1662BCE55B
Type
Article: article from journal or magazin.
Collection
Publications
Title
Protonation Equilibria of Hoechst 33258 in Aqueous Solution
Journal
Helvetica Chimica Acta
Author(s)
Ladinig Manfred, Leupin Werner, Meuwly Markus, Respondek Michal, Wirz Jakob, Zoete Vincent
ISSN
0018-019X
Publication state
Published
Issued date
24/01/2005
Peer-reviewed
Oui
Volume
88
Number
1
Pages
53-67
Language
english
Abstract
The bis‐benzimidazole derivative Hoechst 33258 (2′‐(4‐hydroxyphenyl)‐5‐(4‐methylpiperazin‐1‐yl)‐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.
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