32P-postlabelling and mass spectrometric methods for analysis of bulky, polyaromatic carcinogen-DNA adducts in humans.

There has been significant recent progress toward the development of human carcinogen-DNA adduct biomonitoring methods. 32P-Postlabelling is a technique which has found wide application in human studies. 32P-Postlabelling involves enzymatic preparation and labelling of DNA samples, followed by chromatographic separation of carcinogen-nucleotide adducts from unadducted nucleotides. Thin-layer ion-exchange and high-performance liquid chromatography (HPLC) have been utilized. This paper critically reviews 32P-postlabelling methods for analysis of bulky, polyaromatic carcinogen-DNA adducts and details a strategy to optimize this technique for monitoring human samples. Development of a human carcinogen biomonitoring method requires that the biomarker meet certain criteria: that the biomarker be responsive to exposures known to increase human cancer risk, to reductions in those exposures, and to the influence of metabolic differences. In addition, reliable samples must be available by non-invasive means. The ability of 32P-postlabelling to meet these criteria is traced in the literature and discussed. Identification of specific carcinogen-DNA adducts is a difficult task due to the low (femtomole) levels in human target tissues. Because co-chromatography in thin-layer chromatography (TLC) is generally not considered to be proof of chemical identity, both synchronous fluorescence and HPLC in conjunction with 32P-postlabelling and TLC are used to confirm the identity of specific carcinogen-DNA adducts in human samples. Mass spectrometry is a highly specific method, the sensitivity of which has been improved to the point which may allow its use to confirm the identity of carcinogen-DNA adducts isolated by 32P-postlabelling and other methods. The literature relating to the use of mass spectral techniques in carcinogen-DNA adduct analysis is reviewed.     

DNA topology on an increase in positive writhing number of DNA: conformation changes in the time course of cis-diamminedichloroplatinum(II)-DNA adducts

We show that the topological significance of the gel mobility of cis-diamminedichloroplatinum(II) (DDP)-closed circular DNA (ccDNA) adducts decreases with reaction time, until a point at which it joins relaxed DNA, and that the mobility of the adducts increases again. There is no relationship between the relative length of the adducts and the gel mobility. Although the significance of the decrease of gel mobility is due to the unwinding of cis-DDP-DNA (or trans-DDP-DNA) adducts, the conformational significance of the subsequent increase in mobility is unclear. To elucidate the conformational significance for unwinding of the adducts, we measured the writhing number (Wk) of the adducts using electron microscopy and analyzed the topological states of cis-DDP (or trans-DDP) adducts based on the White rule, Lk=Wk+Tk. Where, Lk and Tk represent the linking and twisting number in the ring, respectively. From the data, we found that the Wk of cis-DDP-ccDNA adducts in comparison with trans-DDP-ccDNA adducts increases from a negative to a positive number with time. This suggests that cis-DDP plays a role in the change of the topological state of ccDNA. In the abstraction of platinum from the adducts with CN- ion, the differences in both topological states may explain why Pt in trans-DDP is abstracted more easily than in cis-DDP. To explain the abstraction of Pt ion, we also discuss the findings based on the thermodynamic cycle in a intermolecular crosslink model Pt(NH3)2(guanine)2(2+)-->Pt(CN)4(2-) using the Pt parametrized PM3 method.     

Synthesis of fluorescently labeled alkylated DNA adduct standards and separation by capillary electrophoresis

DNA adducts are regarded as individual internal dosimeters for the exposure to chemical carcinogens. To date, the most sensitive method for DNA adduct analysis is the radioactive 32P-postlabeling method, which allows the detection of one adduct in 10(10) unmodified nucleotides in microg amounts of DNA. However, this technique suffers from disadvantages such as working with radioactive phosphorus and time-consuming chromatographic separation procedures. In addition, the simultaneous detection of adducts from different classes of carcinogens in a DNA sample is difficult. In order to overcome these drawbacks, we are developing a new detection method, comprising fluorescence labeling of DNA adducts, capillary electrophoretic (CE) separation, and on-line detection by monitoring laser-induced fluorescence (LIF). So far, we have evaluated the separation power and the detection limit of CE with fluorescently labeled standard compounds such as unmodified nucleotides or alkylated thymidines. For this purpose, we developed a universal method for labeling 5'-OH-mononucleosid-3'-dicyanoethyl-phosphates with fluorescent dyes based on the phosphoramidite technology for DNA synthesis. The separation of N3-methylated, N3-, O2- and O4-butylated thymidines from the unmodified nucleotide within a few minutes recommends CE-LIF as a powerful method for DNA adduct analysis.     

Analysis of benzo[a]pyrene diol epoxide-DNA adducts by capillary zone electrophoresis- electrospray ionization-mass spectrometry in conjunction with sample stacking

Benzo[a]pyrene diol epoxide (BPDE) was reacted in vitro with (2'-deoxy)nucleotides and with calf thymus DNA. The modified DNA was enzymatically hydrolyzed to the 5'-monophosphate nucleotides using deoxyribonuclease I (DNA-ase I), nuclease P1 and snake venom phosphodiesterase (SVP). Most of the unmodified nucleotides were removed using solid phase extraction (SPE) in a polystyrene divinylbenzene copolymer. Three adducts could be detected and identified using capillary zone electrophoresis(negative)-ion electrospray ionization-mass spectrometry (CZE-(-)-ESI-MS) in conjunction with sample stacking. This way, not only a BPDE-dGMP adduct [(M-H)(-) at m/z 648], a well-known nucleotide adduct, could be retrieved, but also a BPDE-dAMP [(M-H)(-) at m/z 632] and a BPDE-dCMP adduct [(M-H)(-) at m/z 608] could be detected for the first time. The presence of the prominent ion at m/z 195 (the deoxyribose-phosphate ion) in the three low-energy collision-activated decomposition (CAD) spectra indicated that the adducts were formed through base-alkylation. CZE-positive ion ESI-MS/MS experiments were performed to obtain further information on base-alkylation. The absence of the loss of NH(3) from the nucleobase in each CAD spectrum points to an alkylated exocyclic NH(2) position of the nucleobase. So, the three adducts could be identified as BPDE-N(2)-dGMP, BPDE-N(6)-dAMP and BPDE-N(4)-dCMP using CZE-ESI-MS and CZE-ESI-MS/MS.     

Improved reversed-phase high-performance liquid chromatographic separation of 32P-labelled nucleoside 3',5'-bisphosphate adducts of polycyclic aromatic hydrocarbons.

32P-Postlabelling is a sensitive technique for the detection and analysis of carcinogen-DNA adducts. In this paper we describe the development of an improved high-performance liquid chromatography (HPLC) method for the separation of 32P-labelled 3',5'-bisphosphates of nucleosides modified by reactive derivatives of carcinogenic polycyclic aromatic hydrocarbons (PAH). Optimal resolution of the major 32P-postlabelled DNA adducts formed by the anti-diol-epoxides of ten PAH was achieved using a phenyl-modified silica gel column with a gradient of methanol in phosphate buffer at low pH and high ionic strength. Use of a radioactivity flow detector coupled to the HPLC apparatus allowed detection of subfemtomole quantities of labelled adducts.     

Transition metal-catalyzed formation of boron-nitrogen bonds: catalytic dehydrocoupling of amine-borane adducts to form aminoboranes and borazines

A mild, catalytic dehydrocoupling route to aminoboranes and borazine derivatives from either primary or secondary amine-borane adducts has been developed using late transition metal complexes as precatalysts. The adduct Me(2)NH.BH(3) thermally eliminates hydrogen at 130 degrees C in the condensed phase to afford [Me(2)N-BH(2)](2) (1). Evidence for an intermolecular process, rather than an intramolecular reaction to form Me(2)N=BH(2) as an intermediate, was forthcoming from "hot tube" experiments where no appreciable dehydrocoupling of gaseous Me(2)NH.BH(3) was detected in the range 150-450 degrees C. The dehydrocoupling of Me(2)NH.BH(3) was found to be catalyzed by 0.5 mol % [Rh(1,5-cod)(mu-Cl)](2) in solution at 25 degrees C to give 1 quantitatively after ca. 8 h. The rate of dehydrocoupling was significantly enhanced if the temperature was raised or if the catalyst loading was increased. The catalytic activity of various other transition metal complexes (Ir, Ru, Pd) for the dehydrocoupling of Me(2)NH.BH(3) was also demonstrated. This new catalytic method was extended to other secondary adducts RR'NH.BH(3) which afforded the dimeric species [(1,4-C(4)H(8))N-BH(2)](2) (2) and [PhCH(2)(Me)N-BH(2)](2) (3) or the monomeric aminoborane (i)Pr(2)N=BH(2) (4) under mild conditions. A new synthetic approach to the linear compounds R(2)NH-BH(2)-NR(2)-BH(3) (5: R = Me; 6: R = 1,4-C(4)H(8)) was developed and subsequent catalytic dehydrocoupling of these species yielded the cyclics 1 and 2. The species 5 and 6 are postulated to be intermediates in the formation of 1 and 2 directly from the catalytic dehydrocoupling of the adducts R(2)NH.BH(3). The catalytic dehydrocoupling of NH(3).BH(3), MeNH(2).BH(3), and PhNH(2).BH(3) at 45 degrees C to give the borazine derivatives [RN-BH](3) (10: R = H; 11: R = Me; 12: R = Ph) was demonstrated. TEM analysis of the contents of the reaction solution for the [Rh(1,5-cod)(mu-Cl)](2) catalyzed dehydrocoupling of Me(2)NH.BH(3) together with Hg poisoning experiments suggested a heterogeneous catalytic process involving Rh(0) colloids.     

Styrene oxide DNA adducts: quantitative determination using <Superscript>31</Superscript>P monitoring

The reaction of styrene oxide, a potential carcinogen in humans, with DNA constituents has been used to develop an improved method for quantification of DNA adducts. To enable monitoring of DNA adducts caused by xenobiotics at physiological relevant levels, a robust, reliable and powerful method based on monitoring of phosphorus in nucleotides is described. An efficient enzymatic digestion step and a sample-preconcentration procedure are essential, and enable separation of alkylated nucleotides from the large excess of native nucleotides. The adducts are detected by means of the phosphorus signal measured at mass m/z=31 with an inductively-coupled-plasma mass spectrometer. Bis(4-nitrophenyl)phosphate (BNPP) serves as internal standard for quantification of the adducts. The absolute limit of detection, 45 fmol, corresponds to detection of three modified nucleotides among 10⁷ native nucleotides (the calculation is based on use of 50 g calf thymus DNA). An adduct formation ratio at the DNA of 3.6 adducts per 1000 nucleotides was measured, which is 75% lower than for reaction with monomeric 2-deoxy-nucleotides. In addition, a substantial amount of phosphate adducts were detected, but in DNA the rate of phosphate formation was lower than with monomeric nucleotides. Most probably these adducts escaped unnoticed when ³¹P-post-labelling was employed.     

One-milliliter wet-digestion for inductively coupled plasma mass spectrometry (ICP-MS): determination of platinum-DNA adducts in cells treated with platinum(II) complexes

Platinum (Pt)–DNA adducts formed by the anti-tumor agent cisplatin are recognized by the DNA mismatch repair (MMR) system. To investigate the involvement of MMR proteins including hMLH1 in the removal of these adducts, we developed a mL-scale wet-digestion method for inductively coupled plasma mass spectrometry (ICP-MS). The detection limit was 0.01 ng mL^–1 Pt, which corresponded to 2 pg Pt/g DNA when 10 g of DNA was used. The mean relative errors were 5.4% or better for a dynamic range of 0.01–10 ng mL^–1 Pt. DNA (~500 g) had no matrix effect. To improve the accuracy, DNA preparations were treated with ribonuclease and the apparent reduction in the concentration of Pt was corrected using cellular DNA levels, which were determined with Hoechst 33258. No significant differences were observed, in terms of the formation of Pt–DNA adducts or their removal over 6 h, between hMLH1-deficient HCT116 cells, a human colorectal cancer cell line, and hMLH1-complemented HCT116+ch3 cells (n=5; P>0.05), indicating that the hMLH1-dependent DNA repair systems contribute to neither the formation nor the removal of the adducts at detectable levels. In addition, approximately 19% of the adducts were removed within 6 h in both cell lines. A time course analysis (~24 h) suggested that the removal of cisplatin-generated Pt–DNA adducts follows first-order kinetics (t_1/2=32 h). The amount of Pt–DNA adduct formed by oxaliplatin in 1 h was 56% (ratio of means) of that generated by an equimolar concentration of cisplatin in HCT116. The proposed procedure could be useful for determining Pt–DNA adducts formed by Pt(II) complexes.     

A surprisingly stable macrochelate formed from the reaction of cis dinuclear platinum antitumor compounds with reduced glutathione

The structurally unique macrochelate [{Pt(en)}2-mu-{H2N(CH2)6 NH2}-mu-(SG)] (I) is the principal product of the reaction of the dinuclear compound [{PtCl(en)}2-mu-{H2N(CH2)6 NH2}]Cl2 (1) with reduced glutathione (GSH) in a stoichiometric 1:1 ratio in phosphate buffered saline (PBS) (pH 7.35). The macrochelate is formed through simultaneous bridging of the hexanediamine linker and glutathione thiolate. This represents a novel structure for glutathione adducts of platinum. At higher (1:4) ratios of Pt complex to GSH, an interesting interchange between bridged Pt-(SG)-Pt and terminal Pt-SG species is observed with the diamine linker still remaining intact in all cases. The integrity of I is further evident when reaction ratios are increased to 1:4 (Pt complex/GSH), and additional minor products are identified as [{Pt(en)SG}2-mu-{NH2(CH2)6 NH2}] (II), which transforms to [{Pt{NH2(CH2)2 NH2}(SG)}2-mu-{H2N(CH2)6 NH2}-mu-(SG)] (III), where the chelate ring is broken to produce a dangling monodentate ethylenediamine. The chemical shifts of the Pt-NH2 linker in all compounds are explained by consideration of the enhanced rigidity of the macrochelate (I) leading to shielding in comparison to the "open" monodentate structures (II, III). The remarkable stability of I is discussed in terms of possible biological implications.     

High resolution mass spectrometry for studying the interactions of cisplatin with oligonucleotides

Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been used to probe the interaction of the anticancer drug cisplatin with oligonucleotides. The binding kinetics, the nature of the adducts formed, and the location of the binding site within the specifically designed double-stranded DNA oligonucleotides, ds(GTATTGGCACGTA) and ds(GTACCGGTGTGTA), were determined by recording mass spectra over time and/or employing tandem mass spectrometry (MS/MS). The FT-ICR MS studies show that binding to DNA takes place via a [Pt(NH 3) 2Cl] (+) intermediate prior to formation of bifunctional [Pt(NH 3) 2] (2+) adducts. Tandem MS reveals that the major binding sites correspond to GG and GTG, the known preferred binding sites for cisplatin, and demonstrates the preference for binding to guanosine within the oligonucleotide. The obtained results are discussed and compared to published data obtained by other mass spectrometric techniques, NMR spectroscopy and X-ray crystallography.     

Binding of DNA purine sites to dirhodium compounds probed by mass spectrometry

The adducts formed between the antitumor active compounds [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](BF(4))(2), Rh(2)(O(2)CCH(3))(4), and Rh(2)(O(2)CCF(3))(4) with DNA oligonucleotides have been assessed by matrix-assisted laser desorption ionization (MALDI) and nanoelectrospray (nanoESI) coupled to time-of-flight mass spectrometry (TOF MS). A series of MALDI studies performed on dipurine (AA, AG, GA, and GG)-containing single-stranded oligonucleotides of different lengths (tetra- to dodecamers) led to the establishment of the relative reactivity cis-[Pt(NH(3))(2)(OH(2))(2)](2+) (activated cisplatin) approximately Rh(2)(O(2)CCF(3))(4) > cis-[Pt(NH(3))(2)Cl(2)] (cisplatin) > [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](BF(4))(2) > Rh(2)(O(2)CCH(3))(4) approximately Pt(C(6)H(6)O(4))(NH(3))(2) (carboplatin). The relative reactivity of the complexes is associated with the lability of the leaving groups. The general trend is that an increase in the length of the oligonucleotide leads to enhanced reactivity for Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](BF(4))(2) and Rh(2)(O(2)CCH(3))(4) (except for the case of [Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](2+), which reacts faster with the GG octamers than with the dodecamers), whereas the reactivity of Rh(2)(O(2)CCF(3))(4) is independent of the oligonucleotide length. When monitored by ESI, the dodecamers containing GG react faster than the respectiveAA oligonucleotides in reactions with Rh(2)(O(2)CCF(3))(4) and Rh(2)(O(2)CCH(3))(2)(CH(3)CN)(6)](BF(4))(2), whereas AA oligonucleotides react faster with Rh(2)(O(2)CCH(3))(4). The mixed (AG, GA) purine sequences exhibit comparable rates of reactivity with the homopurine (AA, GG) dodecamers in reactions with Rh(2)(O(2)CCH(3))(4). The observation of initial dirhodium-DNA adducts with weak axial (ax) interactions, followed by rearrangement to more stable equatorial (eq) adducts, was achieved by electrospray ionization; the Rh-Rh bond as well as coordinated acetate or acetonitrile ligands remain intact in these dirhodium-DNA adducts. MALDI in-source decay (ISD), collision-induced dissociation (CID) MS-MS, and enzymatic digestion studies followed by MALDI and ESI MS reveal that, in the dirhodium compounds studied, the purine sites of the DNA oligonucleotides interact with the dirhodium core. Ultimately, both MALDI and ESI MS proved to be complementary, valuable tools for probing the identity and stability of dinuclear metal-DNA adducts.     

Topology effect for DNA structure of cisplatin: topological transformation of cisplatin-closed circular DNA adducts by DNA topoisomerase I.

The reaction of cis-Pt(NH3)2Cl2(cis-DDP)-closed circular DNA adducts with DNA topoisomerse I(topo I) were studied by electron microscopy. We identified unique topoisomers such as a singly-linked catenane (2(1)2), trefoil (3(1), and dimetric catenane (2(1)2), etc., by analysis with electron micrographs. These unique recombination products resulted from cis-DDP-intra-twisting looped DNA adducts by DNA topo I, and the products could be explained a new mechanism based on an odd-even number rule. Our results suggest a new model on the working mechanisms for DNA topology of cis-DDP which enhances the recombination of DNA. Based on our results, we propose the topological idea that the yields of a mini closed circular DNA and pseudo trefoil DNA, etc., can be expected by reaction of cis-DDP-DNA-histone complexes with DNA topo I in the body.     

Reversible and selective amine interactions of [Cd(mu2-N,O-p-NH2C6H4SO3)2(H2O)2]n

[Cd(mu2-N,O-p-NH2C6H4SO3)2(H2O)2]n (1) is a layered coordination compound. The solid-vapor reactions between crystalline 1 and a series of volatile amines were investigated and the corresponding amine adducts were characterized by EA, TGA, PXRD and IR. Among them, the C2H5NH2 and C3H7NH2 adducts, namely [Cd(C2H5NH2)4(H2O)2](H2NC6H4SO3)2 (3) and [Cd(C3H7NH2)4(O-p-H2NC6H4SO3)2].C3H7NH2 (4), grew into single crystals in situ from the solid-vapor reaction processes and their crystal structures were characterized. In both cases, 4 mol equiv. of amine molecules coordinate to Cd(II) via replacing the N,O-p-NH2C6H4SO3 ligands or coordinated water molecules. The single-phase product suggests that the solid-vapor reaction between the metal sulfonate and volatile alkylamines could be used as a green process to synthesize monoamine-coordinated Cd(II) complexes without any solvent and routine separation. Finally, the substitution reaction is reversible at room conditions and selective for primary alkylamines.     

Insights into the reaction of transplatin with DNA and proteins: methionine-mediated formation of histidine-guanine trans-Pt(NH3)2 cross-links

Simultaneous exposure of transplatin to polypeptides and DNA was mimicked by using a model peptide-oligonucleotide conjugate. Initially formed methionine-guanine chelates evolved into adducts with histidine-guanine trans-Pt(NH3)2 cross-links that differed in constitution and stability from those formed by reaction of the same conjugate with the anticancer drug cisplatin. This finding may be due to different capacities of the two diamminedichloroplatinum(II) complexes to interfere with biological processes and may explain their differing cytotoxicities.     

Investigation relevant to the conformation of the 17-membered Pt(d(GpG)) macrocyclic ring formed by Pt anticancer drugs with DNA: Pt complexes with a Goldilocks carrier ligand

Platinum anticancer drug DNA intrastrand cross-link models, LPt(d(G*pG*)) (G* = N7-platinated G residue, L = R(4)dt = bis-3,3'-(5,6-dialkyl)-1,2,4-triazine), and R = Me or Et), undergo slow Pt-N7 bond rotation. NMR evidence indicated four conformers (HH1, HH2, ΔHT1, and ΛHT2); these have different combinations of guanine base orientation (head-to-head, HH, or head-to-tail, HT) and sugar-phosphodiester backbone propagation relative to the 5'-G* (the same, 1, or opposite, 2, to the direction in B DNA). In previous work on LPt(d(G*pG*)) adducts, Pt-N7 rotation was too rapid to resolve conformers (small L with bulk similar to that in active drugs) or L was too bulky, allowing formation of only two or three conformers; ΛHT2 was not observed under normal conditions. The (R(4)dt)Pt(d(G*pG*)) results support our initial hypothesis that R(4)dt ligands have Goldilocks bulk, sufficient to slow G* rotation but insufficient to prevent formation of the ΛHT2 conformer. Unlike the (R(4)dt)Pt(5'-GMP)(2) adducts, ROESY spectra of (R(4)dt)Pt(d(G*pG*)) adducts showed no EXSY peaks, a result providing clear evidence that the sugar-phosphodiester backbone slows conformer interchange. Indeed, the ΛHT2 conformer formed and converted to other conformers slowly. Bulkier L (Et(4)dt versus Me(4)dt) decreased the abundance of the ΛHT2 conformer, supporting our initial hypothesis that steric crowding disfavors this conformer. The (R(4)dt)Pt(d(G*pG*)) adducts have a low abundance of the ΔHT1 conformer, consistent with the proposal that the ΔHT1 conformer has an energetically unfavorable phosphodiester backbone conformation; its high abundance when L is bulky is attributed to a small d(G*pG*) spatial footprint for the ΔHT1 conformer. Despite the Goldilocks size of the R(4)dt ligands, the bases in the (R(4)dt)Pt(d(G*pG*)) adducts have a low degree of canting, suggesting that the ligand NH groups characteristic of active drugs may facilitate canting, an important aspect of DNA distortions induced by active drugs.     

High resolution mass spectrometry based profiling of diet-related deoxyribonucleic acid adducts

Exposure of DNA to endo- and exogenous DNA binding chemicals can result in the formation of DNA adducts and is believed to be the first step in chemically induced carcinogenesis. DNA adductomics is a relatively new field of research which studies the formation of known and unknown DNA adducts in DNA due to exposure to genotoxic chemicals. In this study, a new UHPLC-HRMS(/MS)-based DNA adduct detection method was developed and validated. Four targeted DNA adducts, which all have been linked to dietary genotoxicity, were included in the described method; O⁶-methylguanine (O⁶-MeG), O⁶-carboxymethylguanine (O⁶-CMG), pyrimidopurinone (M1G) and methylhydroxypropanoguanine (CroG). As a supplementary tool for DNA adductomics, a DNA adduct database, which currently contains 123 different diet-related DNA adducts, was constructed. By means of the newly developed method and database, all 4 targeted DNA adducts and 32 untargeted DNA adducts could be detected in different DNA samples. The obtained results clearly demonstrate the merit of the described method for both targeted and untargeted DNA adduct detection in vitro and in vivo, whilst the diet-related DNA adduct database can distinctly facilitate data interpretation.     

Factors influencing conformer equilibria in retro models of cisplatin-DNA adducts as revealed by moderately dynamic (N,N'-dimethyl-2,3-diaminobutane)PtG(2) retro models (G = a guanine derivative)

Typical cis-PtA(2)G(2) models of key DNA lesions formed by cis-type Pt anticancer drugs are very dynamic and difficult to characterize (A(2) = diamine or two amines; G = guanine derivative). Retro models have A(2) carrier ligands designed to decrease dynamic motion without eliminating any of three possible conformers with bases oriented head-to-tail (two: DeltaHT and LambdaHT) or head-to-head (one: HH). All three were found in NMR studies of eight Me(2)DABPtG(2) retro models (Me(2)DAB = N,N'-dimethyl-2,3-diaminobutane with S,R,R,S and R,S,S,R configurations at the chelate ring N, C, C, and N atoms, respectively; G = 5'-GMP, 3'-GMP, 5'-IMP, and 3'-IMP). The bases cant to the left (L) in (S,R,R,S)-Me(2)DABPtG(2) adducts and to the right (R) in (R,S,S,R)-Me(2)DABPtG(2) adducts. Relative to the case in which the bases are both not canted, canting will move the six-membered rings closer in to each other ("6-in" form) or farther out from each other ("6-out" form). Interligand interactions between ligand components near to Pt (first-first sphere communication = FFC) or far from Pt (second-sphere communication = SSC) influence stability. In typical cases at pH < 8, the "6-in" form is favored, although the larger six-membered rings of the bases are close. In minor "6-out" HT forms, the proximity of the smaller five-membered rings could be sterically favorable. Also, G O6 is closer to the sterically less demanding NH part of the Me(2)DAB ligand, possibly allowing G O6-NH hydrogen bonding. These favorable FFC effects do not fully compensate for possibly stronger FFC dipole effects in the "6-in" form. SSC, phosphate-N1H cis G interactions favor LambdaHT forms in 5'-GMP and 5'-IMP complexes and DeltaHT forms in 3'-GMP and 3'-IMP complexes. When SSC and FFC favor the same HT conformer, it is present at >90% abundance. In six adducts [four (S,R,R,S)-Me(2)DABPtG(2) and (R,S,S,R)-Me(2)DABPtG(2) (G = 3'-GMP and 3'-IMP)], the minor "6-out" HT form at pH approximately 7 becomes the major form at pH approximately 10, where G N1H is deprotonated, because the large distance between the negatively charged N1 atoms minimizes electrostatic repulsion and probably because the G O6-(NH)Me(2)DAB H-bond (FFC) is strengthened by N1H deprotonation. At pH approximately 10, phosphate-negative N1 repulsion is an unfavorable SSC term. This factor disfavors the LambdaHT R form of two (R,S,S,R)-Me(2)DABPtG(2) (G = 5'-GMP and 5'-IMP) adducts to such an extent that the "6-in" DeltaHT R form remains the dominant form even at pH approximately 10.     

Assembly of hybrid organic-organometallic materials through mechanochemical acid-base reactions

Manual grinding of the organometallic complex [Fe(eta(5)-C(5)H(4)COOH)(2)] with a number of solid bases, namely 1,4-diazabicyclo[2.2.2]octane, C(6)H(12)N(2), 1,4-phenylenediamine, p-(NH(2))(2)C(6)H(4), piperazine, HN(C(2)H(4))(2)NH, trans-1,4-cyclohexanediamine, p-(NH(2))(2)C(6)H(10), and guanidinium carbonate [(NH(2))(3)C](2)[CO(3)], generates quantitatively the corresponding adducts, [HC(6)H(12)N(2)][Fe(eta(5)-C(5)H(4)COOH)(eta(5)-C(5)H(4)COO)] (1), [HC(6)H(8)N(2)][Fe(eta(5)-C(5)H(4)COOH)(eta(5)-C(5)H(4)COO)] (2), [H(2)C(4)H(10)N(2)][Fe(eta(5)-C(5)H(4)COO)(2)] (3), [H(2)C(6)H(14)N(2)][Fe(eta(5)-C(5)H(4)COO)(2)].2 H(2)O, (4.2 H(2)O), and [C(NH(2))(3)](2)[Fe(eta(5)-C(5)H(4)COO)(2)].2 H(2)O, (5.2 H(2)O), respectively. Crystallization from methanol in the presence of seeds of the ground sample allows the growth of single crystals of these adducts; therefore we were able to determine the structures of the adducts by single-crystal X-ray diffraction. This information was used in turn to identify and characterize the polycrystalline materials obtained by the grinding process. In the case of [HC(6)N(2)H(12)][Fe(eta(5)-C(5)H(4)COOH)(eta(5)-C(5)H(4)COO)] (1), the base can be removed by mild treatment regenerating the starting dicarboxylic acid, while in all other cases decomposition is observed. The solid-solid processes described herein imply molecular diffusion through the lattice, breaking and reassembling of hydrogen-bonded networks, and proton transfer from acid to base.     

Kinetics and mechanism for reversible chloride transfer between mercury(II) and square-planar platinum(II) chloro ammine, aqua, and sulfoxide complexes. Stabilities, spectra, and reactivities of transient metal-metal bonded platinum-mercury adducts

The Hg2+aq- and HgCl+aq-assisted aquations of [PtCl4]2- (1), [PtCl3(H2O)]- (2), cis-[PtCl2(H2O)2] (3), trans-[PtCl2(H2O)2] (4), [PtCl(H2O)3]+ (5), [PtCl3Me2SO]- (6), trans-[PtCl2(H2O)Me2SO] (7), cis-[PtCl(H2O)2Me2SO]+ (8), trans-[PtCl(H2O)2M32SO]+ (9), trans-[PtCl2(NH3)2] (10), and cis-[PtCl2(NH3)2] (11) have been studied at 25.0 degrees C in a 1.00 M HClO4 medium buffered with chloride, using stopped-flow and conventional spectrophotometry. Saturation kinetics and instantaneous, large UV/vis spectral changes on mixing solutions of platinum complex and mercury are ascribed to formation of transient adducts between Hg2+ and several of the platinum complexes. Depending on the limiting rate constants, these adducts are observed for a few milliseconds to a few minutes. Thermodynamic and kinetics data together with the UV/vis spectral changes and DFT calculations indicate that their structures are characterized by axial coordination of Hg to Pt with remarkably short metal-metal bonds. Stability constants for the Hg2+ adducts with complexes 1-6, 10, and 11 are (2.1 +/- 0.4) x 10(4), (8 +/- 1) x 10(2), 94 +/- 6, 13 +/- 2, 5 +/- 2, 60 +/- 6, 387 +/- 2, and 190 +/- 3 M-1, respectively, whereas adduct formation with the sulfoxide complexes 7-9 is too weak to be observed. For analogous platinum(II) complexes, the stabilities of the Pt-Hg adducts increase in the order sulfoxide < aqua < ammine complex, reflecting a sensitivity to the pi-acid strength of the Pt ligands. Rate constants for chloride transfer from HgCl+ and HgCl2 to complexes 1-11 have been determined. Second-order rate constants for activation by Hg2+ are practically the same as those for activation by HgCl+ for each of the platinum complexes studied, yet resolved contributions for Hg2+ and HgCl+ reveal that the latter does not form dinuclear adducts of any significant stability. The overall experimental evidence is consistent with a mechanism in which the accumulated Pt(II)-Hg2+ adducts are not reactive intermediates along the reaction coordinate. The aquation process occurs via weaker Pt-Cl-Hg or Pt-Cl-HgCl bridged complexes.