Modulation of properties in analogues of Zeise's anion on changing the ligand trans to ethene. X-ray crystal structures of trans-[PtCl2(OH)(η2-C2H4)]- and trans-[PtCl2(η1-CH2NO2)(η2-C2H4)]-

To get further insight in the reaction of nucleophilic substitution upon changing the ligand trans to a η(2)-olefin, the reactivity of some monoanionic platinum(II) complexes (trans-[PtCl(2)X(η(2)-C(2)H(4))](-), X = Cl(-), 1, OH(-), 2, and CH(2)NO(2)(-), 3) towards pyridines with different steric hindrance (py, 4-Mepy, and 2,6-Me(2)py) has been tested. All crystallographic (2 and 3 reported for the first time) and spectroscopic data are in accord with a platinum-olefin interaction decreasing in the order 2 > 1 > 3, paralleling the decreasing electronegativity of the donor atom (O > Cl > C). Not only the platinum-olefin bond but also the bond between platinum and the ligand trans to the olefin appear to be strongest in 2 (Pt-O distance at the lower limit for this type of bond). In the reaction with py, the ligand trans to the olefin is displaced in 1 and 2. Moreover the reaction is in equilibrium in the case of sterically hindered 2,6-Me(2)py, the equilibrium being shifted moderately or prevalently toward the reagents in the case of 1 and 2, respectively. In the case of 3, the reaction with pyridines leads to substitution of the olefin instead of the carbanion. This is in accord with the observation that carbanions strongly weaken the trans Pt-olefin bond.     

A rare example of three abundant conformers in one retro model of the cisplatin-DNA d(GpG) intrastrand cross link. Unambiguous evidence that guanine O6 to carrier amine ligand hydrogen bonding is not important. possible effect of the Lippard base pair step adjacent to the lesion on carrier ligand hydrogen bonding in DNA adducts

Guanine O6 to carrier ligand hydrogen bonding is a central feature of many hypotheses advanced to explain the anticancer activity of cis-type anticancer drugs, cis-PtA(2)X(2) (A(2) = diamine or two amines). Early structural evidence suggested that cis-Pt(NH(3))(2)(d(GpG)) (the cross-link model for the key cisplatin-DNA adduct) and other cis-PtA(2)(d(GpG)) adducts exist exclusively or mainly as the HH1 conformer with head-to-head (HH) bases. The dynamic motion of the d(GpG) in these adducts is too rapid to permit definitive characterization of both the conformation and the H-bonding. Hence, we use retro models having A(2) ligands designed to slow the motion. Here, we employ Me(2)ppz (N,N'-dimethylpiperazine), which lacks NH groups. Me(2)ppz is unique in having sp(3) N-methyl groups directly in the coordination plane, allowing the coexistence of multiple conformers but hindering dynamic motion in Me(2)ppzPt(d(GpG)) and Me(2)ppzPt(GpG) retro models. Dynamic processes are decreased enough in Me(2)ppzPt(d(GpG)) to permit HPLC separation of three abundant forms. After HPLC separation, the three re-equilibrate, proving that the three forms must be conformers and that Me(2)ppz has little influence on conformer distribution. This marks the first reported characterization of three abundant conformers for one cis-PtA(2)(d(GpG)) adduct. From NMR evidence, the Me(2)ppzPt(d(GpG)) HH1 conformer has uncanted bases. Another conformer, one of two recently discovered conformer types, has head-to-tail (HT) bases with Delta chirality. For this Delta HT1 form, several lines of evidence establish that the dinucleotide moieties have essentially identical structures in d(GpG) (and GpG) adducts of Me(2)ppzPt and other cis-PtA(2) complexes. For example, the shifts of the highly structure-sensitive G H8 NMR signals are almost identical for the Delta HT1 form of all adducts. In previous models, the stabilization of the Delta HT1 form could be attributed to G O6 H-bonding to A(2) NH groups. Such H-bonds are not possible for Me(2)ppz. The unambiguous conclusions are that G O6 H-bonding is weak and that neither canting nor H-bonding is essential in HH forms. These two features are present in almost all other small models but are essentially absent in the cross-link base pair (bp) step in duplexes. We conclude from our work that the forces favoring canting and H-bonding are weak, and we hypothesize that steric effects within the Lippard bp step adjacent to this cross-link bp step easily overcome these forces.     

PtCl(PHCy2)[(PCy2O)2H]: a hydrogen bonded phosphinito complex obtained by direct reaction of molecular oxygen with a terminal phosphido complex

Exposure of solid trans-[Pt(PHCy2)2(PCy2)Cl] (1) to dry oxygen unexpectedly leads to [PtCl(PHCy2)[(PCy2O)2H]] (2) as the major product, the formation of which has been followed by NOESY 1H NMR techniques.     

The mononuclear metal center of type-I dihydroorotase from aquifex aeolicus

Background

Dihydroorotase (DHO) is a zinc metalloenzyme, although the number of active site zinc ions has been controversial. E. coli DHO was initially thought to have a mononuclear metal center, but the subsequent X-ray structure clearly showed two zinc ions, α and β, at the catalytic site. Aquifex aeolicus DHO, is a dodecamer comprised of six DHO and six aspartate transcarbamoylase (ATC) subunits. The isolated DHO monomer, which lacks catalytic activity, has an intact α-site and conserved β-site ligands, but the geometry of the second metal binding site is completely disrupted. However, the putative β-site is restored when the complex with ATC is formed and DHO activity is regained. Nevertheless, the X-ray structure of the complex revealed a single zinc ion at the active site. The structure of DHO from the pathogenic organism, S. aureus showed that it also has a single active site metal ion.

Results

Zinc analysis showed that the enzyme has one zinc/DHO subunit and the addition of excess metal ion did not stimulate catalytic activity, nor alter the kinetic parameters. The metal free apoenzyme was inactive, but the full activity was restored upon the addition of one equivalent of Zn²⁺ or Co²⁺. Moreover, deletion of the β-site by replacing the His180 and His232 with alanine had no effect on catalysis in the presence or absence of excess zinc. The 2.2 Å structure of the double mutant confirmed that the β-site was eliminated but that the active site remained otherwise intact.

Conclusions

Thus, kinetically competent A. aeolicus DHO has a mononuclear metal center. In contrast, elimination of the putative second metal binding site in amidohydrolyases with a binuclear metal center, resulted in the abolition of catalytic activity. The number of active site metal ions may be a consideration in the design of inhibitors that selectively target either the mononuclear or binuclear enzymes.