Characterization of Beryllium Ion Complexation in the Presence of Biological Buffers Using Isothermal Titration Calorimetry

Document Type

Conference Proceeding

Publication Date

8-25-2019

Publication Title

Fall 2019 ACS National Meeting

Publisher

American Chemical Society

Publisher Location

San Diego, CA

Abstract

Beryllium salt is biologically active. Be2+ is a potent inhibitor of GSK3 kinases, and addition of BeSO4 to cell culture medium initiates a signaling cascade that produces cytostatic effects in some types of human cancer cells. Isothermal titration calorimetry (ITC) is a potential approach to study the interaction of beryllium ion with biomolecules. Compared to other Group 2 elements, beryllium ion exhibits unique properties. Aqueous solubility is limited above pH 5.5 and quite limited above pH 6. Be2+ is likely to interact with the buffer molecules that must unavoidably be present during biochemical analyses. These factors affect ITC observations profoundly, and the present study aims to improve our understanding of the interactions of Be2+ with buffers in the near-physiological pH range. Bis-Tris, piperazine, and MES are three commonly used buffers appropriate for the pH region of interest. When Be2+ in Bis-Tris buffer at pH 5.5 was titrated into buffer alone, an entropy-driven endothermic dissociation process was observed by ITC. Piperazine buffer at the same pH produced similar results. However, when the experiment was repeated using MES buffer under similar conditions, no metal-buffer dissociation was observed. Although the MES-Be2+ interaction was too weak to be observed directly, the equilibrium constant could be inferred from the apparent K values observed when Be2+ chelation by EDTA was assayed in various buffers. The observed KITC reflects contributions from the intrinsic KM-EDTA, which is invariant, and the buffer-specific KM-B, which depends on the strength of metal-buffer interaction for the buffer species under consideration. These types of indirect measurements confirmed that Be2+ interacts more strongly with Bis-Tris and piperazine than MES. This conclusion is perhaps counterintuitive from an electrostatic perspective: due to the presence of a sulfonate group, the unprotonated form of MES carries a -1 charge, whereas the unprotonated forms of Bis-Tris and piperazine are neutral and +1, respectively.

Disciplines

Chemistry

Language

English


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