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 (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 thionucleosid

Acid–base and metal ion-binding properties of thiopyrimidine derivatives

The thionucleoside 2-thiocytidine (C2S) as well as the thiouridines (US) occur in Nature, especially in transfer RNAs, and they also receive attention in diverse fields like nanotechnology and drug research. If (C2)O in cytidine (Cyd) is replaced by (C2)S to give the thio analogue C2S, the release of H + from (N3)H in H(C2S) + (p K a = 3.44) is facilitated somewhat [H(Cyd) + ; p K a = 4.24], yet, the deprotonation of the (C4)NH 2 group is much more affected: the p K a decreases from ca. 16.7 in Cyd to 12.65 in C2S. This is because the amino-thione tautomer dominating in the neutral C2S, transfers into the imino-thioate form, which has the charge largely localized on (C2)S − . As a consequence, the M(C2S) 2+ species (M 2+ = Zn 2+ or Cd 2+ ) transfer very easily into their deprotonated M(C2S − H) + forms. This reaction is extremely facilitated by M 2+ coordination at (C2)S − and occurs already at a pH slightly above 3. It is shown that the (C2)S M 2+ coordination dominates to more than 99% in both the M(C2S) 2+ and the M(C2S − H) + complexes; their structures, including chelate formation with the participation of N3, are evaluated. In 2-thiouridine (U2S), 4-thiouridine (U4S), and 2,4-dithiouridine (U2S4S), the release of H + from (N3)H, compared to Urd (p K a = 9.18), is facilitated by ca. 1 to 2 p K units, the charge being largely localized on the (C)S sites; this leads with (U4S − H) − and (U2S4S − H) − to the reduction of Cu(II) to Cu(I), transforming the thiolate into a disulfide. In Cu(U2S − H) + Cu(II) is stable, most likely due to steric constraints inhibiting disulfide formation. The stability of the M(US − H) + complexes with Ni 2+ , Cu 2+ or Cd 2+ is enhanced by about 1.3 to 2 log units compared to the corresponding uridinate complexes. The properties of the biologically relevant Zn(US − H) + are expected to be between those with Ni 2+ and Cd 2+ . The relatively high affinity of the (C)S sites for these M 2+ is reflected in the 2-thiouridine 5′-monophosphate (U2SMP 2− ) and 4-thiouridine 5′-monophosphate (U4SMP 2− ) complexes, M 2+ being located to more than 99% at the thiouracil residue and only traces are coordinated at the phosphate group. In the N3-deprotonated Cu[(U2S − H)MP] − species the anti conformer is partly turned into the syn one allowing thus a formation degree of about 60% of the macrochelate formed by (C2)S − and the phosphate group. The corresponding coordination pattern also seems to hold for Cd[(U2S − H)MP] − , though the formation degree of the macrochelate is lower. No macrochelate formation is detected for Ni[(U2S − H)MP] − , as well as for Ni[(U4S − H)MP] − and Cd[(U4S − H)MP] − . The reasons for the indicated coordination patterns are discussed, as well as the biological implications of the summarized results, especially with regard to tRNAs

Short-chain oligopeptides with copper(II) binding properties: the impact of specific structural modifications on the copper(II) coordination abilities

A series of linear tetrapeptides containing two histidyl residues in position 2 and 4, namely DHGH, DHGDH, KHGH, KHGdH, Ac-DHGH-NH2, Ac-DHGdH-NH2, Ac-KHGH-NH2, and Ac-KHGdH-NH2, were syn- thesized and characterised. Their copper(II) binding properties were investigated in depth through a vari- ety of physicochemical methods. Potentiometric titrations were first carried out to establish the stoichiometry and the stability of the resulting copper(II)–peptide complexes. The copper(II) chromoph- ores that are formed in the various cases in dependence of pH were subsequently characterised by exten- sive spectroscopic analysis (UV–Vis, EPR, CD) in strict correlation with potentiometric data. The effects of the nature of the first amino acid (Lys versus Asp) and of N-terminal amino group protection on copper(II) binding were specifically addressed. On turn, the careful comparison of the copper(II) coordination abil- ities of the linear peptides with those of their cyclic analogs provided insight into the effects of cyclization on the overall metal binding properties

Extent of metal ion-sulfur binding in complexes of thiouracil nucleosides and nucleotides in aqueous solution

Previously published stability constants of several metal ion (M2+) complexes formed with thiouridines and their 5`-monophosphates, together with recently obtained log K-M(U)(M) versus pK(U)(H) plots for M2+ complexes of uridinate derivatives (U-) allowed now a quantitative evaluation of the effect that the exchange of a (C)O by a (C)S group has on the stability of the corresponding complexes. For example, the stability of the Ni2+, Cu2+ and Cd2+ complexes of 2-thiouridinate is increased by about 1.6, 2.3, and 1.3 log units, respectively, by the indicated exchange of groups. Similar results were obtained for other thiouridinates, including 4-thiouridinate. The structure of these complexes and the types of chelates formed (involving (N3)(-) and (C)S) are discussed. A recently advanced method for the quantification of the chelate effect allows now also an evaluation of several complexes of thiouridinate 5`-monophosphates. In most instances the thiouracilate coordination dominates the systems, allowing only the formation of small amounts of phosphate-bound isomers. Among the complexes studied only the one formed by Cu2+ with 2-thiouridinate 5`-monophosphate leads to significant amounts of the macrochelated isomer, which means that in this case Cu2+ is able to force the nucleotide from the anti to the syn conformation, allowing thus metal ion binding to both potential sites and this results in the formation of about 58 isomer. The remaining 42 coordinated to the thiouracilate residue; Cu2+ binding to the phosphate group occurs in this case only in trace amounts. (c) 2007 Elsevier Inc. All rights reserved

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 (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) approximate to 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 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) approximate to 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