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Acid-base and metal ion binding properties of 2-thiocytidine in aqueous solution

Brasuń, Justyna ; Matera, Agnieszka ; Sochacka, Elżbieta ; Swiatek-Kozlowska, Jolanta ; Kozlowski, Henryk ; Operschall, Bert ; Sigel, Helmut

In: JBIC Journal of Biological Inorganic Chemistry, 2008, vol. 13, no. 5, p. 663-674

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    Title
    • Acid-base and metal ion binding properties of 2-thiocytidine in aqueous solution
    Author
    • Brasuń, Justyna. Department of Inorganic Chemistry, Wroclaw Medical University, Szewska 38, 50-139, Wroclaw, Poland
    • Matera, Agnieszka. Department of Inorganic Chemistry, Wroclaw Medical University, Szewska 38, 50-139, Wroclaw, Poland
    • Sochacka, Elżbieta. Department of Organic Chemistry, Technical University of Lodz, Zeromskiego 116, 90-924, Lodz, Poland
    • Swiatek-Kozlowska, Jolanta. Department of Inorganic Chemistry, Wroclaw Medical University, Szewska 38, 50-139, Wroclaw, Poland
    • Kozlowski, Henryk. Faculty of Chemistry, University of Wroclaw, F. Joliot Curie, 50-383, Wroclaw, Poland
    • Operschall, Bert. Department of Chemistry, Inorganic Chemistry, University of Basel, Spitalstrasse 51, 4056, Basel, Switzerland
    • Sigel, Helmut. Department of Chemistry, Inorganic Chemistry, University of Basel, Spitalstrasse 51, 4056, Basel, Switzerland
    Document Type
    • Postprint
    Language
    • English
    Published in
    • JBIC Journal of Biological Inorganic Chemistry, 2008, vol. 13, no. 5, p. 663-674. Springer-Verlag
    Other Version
    OAI-PMH Identifier
    • oai:doc.rero.ch:321347
    Summary
    The thionucleoside 2-thiocytidine (C2S) occurs in nature in transfer RNAs; it receives attention in diverse fields like drug research and nanotechnology. By potentiometric pH titrations we measured the acidity constants of H(C2S)+ and the stability constants of the M(C2S)2+ and M(C2S−H)+ complexes (M2+=Zn2+, Cd2+), and we compared these results with those obtained previously for its parent nucleoside, cytidine (Cyd). Replacement of the (C2)=O unit by (C2)=S facilitates the release of the proton from (N3)H+ in H(C2S)+ (pK a = 3.44) somewhat, compared with H(Cyd)+ (pK a = 4.24). This moderate effect of about 0.8 pK units contrasts with the strong acidification of about 4 pK units of the (C4)NH2 group in C2S (pK a = 12.65) compared with Cyd (pK a≈16.7); the reason for this result is that the amino-thione tautomer, which dominates for the neutral C2S molecule, is transformed upon deprotonation into the imino-thioate form with the negative charge largely located on the sulfur. In the M(C2S)2+ complexes the (C2)S group is the primary binding site rather than N3 as is the case in the M(Cyd)2+ complexes, though owing to chelate formation N3 is to some extent still involved in metal ion binding. Similarly, in the Zn(C2S−H)+ and Cd(C2S−H)+ complexes the main metal ion binding site is the (C2)S− unit (formation degree above 99.99% compared with that of N3). However, again a large degree of chelate formation with N3 must be surmised for the M(C2S−H)+ species in accord with previous solid-state studies of related ligands. Upon metal ion binding, the deprotonation of the (C4)NH2 group (pK a = 12.65) is dramatically acidified (pK a≈3), confirming the very high stability of the M(C2S−H)+ complexes. To conclude, the hydrogen-bonding and metal ion complex forming capabilities of C2S differ strongly from those of its parent Cyd; this must have consequences for the properties of those RNAs which contain this thionucleoside