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

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 (M(2+) = Zn(2+), Cd(2+)), 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)NH(2) group in C2S (pK (a) = 12.65) compared with Cyd (pK (a) approximately 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)NH(2) group (pK (a) = 12.65) is dramatically acidified (pK (a) approximately 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.

The copper(II) coordination abilities of three novel cyclic tetrapeptides with -His-Xaa-His- motif.

Three novel cyclic tetrapeptides, containing either l- or d-histidine residues and either Lys or Asp side chains, namely c(HGd-HK) (1), cHGHD (2) and c(HGd-HD) (3), were designed, synthesized, characterized and tested as potential copper(II) ligands. Their pH dependent copper(II) binding properties were analysed in depth by a number of potentiometric and spectroscopic determinations. A rather exhaustive description of the species existing in solution has emerged for each copper(II)/oligopeptide system; solution structures for the individual species are proposed. The specific role of the various side chains in the overall metal coordination process is discussed in comparison with the case of Cu(II)-c(HGHK), previously reported. Data obtained in this study highlight the strong impact of the d-His residue on the metal binding abilities of these cyclic peptides. Remarkably, the cyclic tetrapeptides containing two l-His residues are able to form, at physiologically relevant pH values, a characteristic chromophore where the mononuclear copper(II) centre is simultaneously coordinated by two imidazole nitrogens and two amidic nitrogens of the tetrazadodecane ring. This latter type of copper(II) chromophore has been carefully modelled by computational methods. The potentialities of the applied experimental strategy are stressed.