2-Thiouracil metal complexes

Garrett ER and Weber DJ, Metal complexes of thiouracils I. Stability constants by potentiometric titration studies and structures of complexes,... 

Hemideprotonation of a metalated nucleobase and the resulting selfcomplementarity is also the key in rationalizing the triple hydrogen-bonding pattern seen in an associate of a neutral and an anionic purine-6-thione Ru(III) complex (130), which is analogous to that seen with 2-thiouracil [(84), see Section II. D]. 

For the above reason the open species of the thiouracilate and phosphate groups were individually evaluated by considering also a potential chelate formation [105]. The result is that for Cd[(U4S - H)MP] the complex is to 99% present as [Cd-(U4S - H)MP] (base-bound) and only 1% exists as [(U4S - H)MP-Cd] (phosphate-bound), hence, the Cd -thiouracilate isomer dominates strongly no indicatirMi for chelate formation was found. This is understandable because the structure of [(U4S - H)MP] is such that a metal ion bound at the (C4)S/(N3) unit can hardly reach the phosphate group and vice versa (see Figure 8). Of course, the two open species are also in equilibrium with each other. 

For Cd[(U2S - H)MP] exactly the same isomer distribution may be surmised [105] because for the chelate effect [164] only 0.11 0.15 log unit were derived [105]. Hence, within the error limits the chelate effect is zero, meaning that no chelate forms. However, based on the mentioned 0.11 log unit it could be that about 20% chelate, M[(U2S - H)MP] [, exist and that the remaining 80% of the open species are then present to ca. 79% with Cd " at the thiouracilate residue and about 1% bound at the phosphate group [105]. Of course, chelate formation of a metal ion coordinated at the (C2)S/(N3) site with the phosphate group (or vice versa) is only possible if the nucleotide is transformed into the syn confoimer (Figure 8). In any case, it is evident that replacement of (QO by (C)S in the nucleobase residue enhances the stability of the Cd " complexes considerably. 

---