In the present study, we characterize the binding of divalent cations to insulin in aqueous salt solutions by means of capillary electrophoresis and molecular dynamics simulations. The results show a strong pH dependence. At low pH, at which all the carboxylate groups are protonated and the protein has an overall positive charge, all the cations exhibit only weak and rather unspecific interactions with insulin. In contrast, at close to neutral pH, when all the carboxylate groups are deprotonated and negatively charged, the charge-neutralizing effect of magnesium, calcium, and zinc, in particular, on the electrophoretic mobility of insulin is significant. This is also reflected in the results of molecular dynamics simulations showing accumulation of cations at the protein surface, which becomes smaller in magnitude upon effective inclusion of electronic polarization via charge rescaling.

Department of Analytical Chemistry Faculty of Science Charles University Prague Albertov 2030 12840 Prague 2 Czech Republic

Division of Theoretical Chemistry Lund University POB 124 SE 22100 Lund Sweden

Institute of Organic Chemistry and Biochemistry Czech Academy of Sciences Flemingovo nám 542 2 160 00 Prague Czech Republic

J Heyrovský Institute of Physical Chemistry Czech Academy of Sciences v v i Dolejškova 2155 3 182 23 Prague 8 Czech Republic

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