Equilibrium Studies on Pd(II)-Amine Complexes with Bio-Relevant Ligands in Reference to Their Antitumor Activity

This review article presents an overview of the equilibrium studies on Pd-amine complexes with bio-relevant ligands in reference to their antitumor activity. Pd(II) complexes with amines of different functional groups, were synthesized and characterized in many studies. The complex formation equilibria of Pd(amine)²⁺ complexes with amino acids, peptides, dicarboxylic acids and DNA constituents, were extensively investigated. Such systems may be considered as one of the models for the possible reactions occurring with antitumor drugs in biological systems. The stability of the formed complexes depends on the structural parameters of the amines and the bio-relevant ligands. The evaluated speciation curves can help to provide a pictorial presentation of the reactions in solutions of different pH values. The stability data of complexes with sulfur donor ligands compared with those of DNA constituents, can reveal information regarding the deactivation caused by sulfur donors. The formation equilibria of binuclear complexes of Pd(II) with DNA constituents was investigated to support the biological significance of this class of complexes. Most of the Pd(amine)²⁺ complexes investigated were studied in a low dielectric constant medium, resembling that of a biological medium. Investigations of the thermodynamic parameters reveal that the formation of the Pd(amine)²⁺ complex species is exothermic.

Coordination Chemistry of Nucleotides and Antivirally Active Acyclic Nucleoside Phosphonates, including Mechanistic Considerations

Considering that practically all reactions that involve nucleotides also involve metal ions, it is evident that the coordination chemistry of nucleotides and their derivatives is an essential corner stone of biological inorganic chemistry. Nucleotides are either directly or indirectly involved in all processes occurring in Nature. It is therefore no surprise that the constituents of nucleotides have been chemically altered-that is, at the nucleobase residue, the sugar moiety, and also at the phosphate group, often with the aim of discovering medically useful compounds. Among such derivatives are acyclic nucleoside phosphonates (ANPs), where the sugar moiety has been replaced by an aliphatic chain (often also containing an ether oxygen atom) and the phosphate group has been replaced by a phosphonate carrying a carbon-phosphorus bond to make the compounds less hydrolysis-sensitive. Several of these ANPs show antiviral activity, and some of them are nowadays used as drugs. The antiviral activity results from the incorporation of the ANPs into the growing nucleic acid chain-i.e., polymerases accept the ANPs as substrates, leading to chain termination because of the missing 3'-hydroxyl group. We have tried in this review to describe the coordination chemistry (mainly) of the adenine nucleotides AMP and ATP and whenever possible to compare it with that of the dianion of 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA2- = adenine(N9)-CH2-CH2-O-CH2-PO32) [or its diphosphate (PMEApp4-)] as a representative of the ANPs. Why is PMEApp4- a better substrate for polymerases than ATP4-? There are three reasons: (i) PMEA2- with its anti-like conformation (like AMP2-) fits well into the active site of the enzyme. (ii) The phosphonate group has an enhanced metal ion affinity because of its increased basicity. (iii) The ether oxygen forms a 5-membered chelate with the neighboring phosphonate and favors thus coordination at the Pα group. Research on ANPs containing a purine residue revealed that the kind and position of the substituent at C2 or C6 has a significant influence on the biological activity. For example, the shift of the (C6)NH2 group in PMEA to the C2 position leads to 9-[2-(phosphonomethoxy)ethyl]-2-aminopurine (PME2AP), an isomer with only a moderate antiviral activity. Removal of (C6)NH2 favors N7 coordination, e.g., of Cu2+, whereas the ether O atom binding of Cu2+ in PMEA facilitates N3 coordination via adjacent 5- and 7-membered chelates, giving rise to a Cu(PMEA)cl/O/N3 isomer. If the metal ions (M2+) are M(α,β)-M(γ)-coordinated at a triphosphate chain, transphosphorylation occurs (kinases, etc.), whereas metal ion binding in a M(α)-M(β,γ)-type fashion is relevant for polymerases. It may be noted that with diphosphorylated PMEA, (PMEApp4-), the M(α)-M(β,γ) binding is favored because of the formation of the 5-membered chelate involving the ether O atom (see above). The self-association tendency of purines leads to the formation of dimeric [M2(ATP)]2(OH)- stacks, which occur in low concentration and where one half of the molecule undergoes the dephosphorylation reaction and the other half stabilizes the structure-i.e., acts as the "enzyme" by bridging the two ATPs. In accord herewith, one may enhance the reaction rate by adding AMP2- to the [Cu2(ATP)]2(OH)- solution, as this leads to the formation of mixed stacked Cu3(ATP)(AMP)(OH)- species, in which AMP2- takes over the structuring role, while the other "half" of the molecule undergoes dephosphorylation. It may be added that Cu3(ATP)(PMEA) or better Cu3(ATP)(PMEA)(OH)- is even a more reactive species than Cu3(ATP)(AMP)(OH)-. - The matrix-assisted self-association and its significance for cell organelles with high ATP concentrations is summarized and discussed, as is, e.g., the effect of tryptophanate (Trp-), which leads to the formation of intramolecular stacks in M(ATP)(Trp)3- complexes (formation degree about 75%). Furthermore, it is well-known that in the active-site cavities of enzymes the dielectric constant, compared with bulk water, is reduced; therefore, we have summarized and discussed the effect of a change in solvent polarity on the stability and structure of binary and ternary complexes: Opposite effects on charged O sites and neutral N sites are observed, and this leads to interesting insights.

The effect of π-π stacking interaction of the indole ring with the coordinated phenoxyl radical in a nickel(ii)-salen type complex. Comparison with the corresponding Cu(ii) complex

In order to gain new insights into the effect of the π-π stacking interaction of the indole ring with the phenoxyl radical moiety as seen in the active form of galactose oxidase, we have prepared Ni(ii) complexes of a methoxy substituted salen-type ligand containing a pendent indole ring on the dinitrogen chelate backbone and characterized their one-electron oxidized forms. The X-ray crystal structure analysis and the other physicochemical experiments of the Ni(ii) complex revealed no significant intramolecular interaction of the indole ring with the coordination plane. On the other hand, the X-ray crystal structures of the oxidized Ni(ii) complex exhibited the π-π stacking interaction of the indole ring mainly with one of the two phenolate moieties. While the phenoxyl radical electron was delocalized on the two phenolate moieties in the Ni(ii)-salen coordination plane, the phenolate moiety in close contact with the indole moiety was considered to be the initial oxidation locus, indicating that the indole ring interacted with the phenoxyl radical by π-π stacking. The UV-vis-NIR spectrum of the oxidized Ni(ii) complex with the pendent indole ring was different from that of the complex without the side chain indole ring, but the differences were rather small in comparison with the oxidized Cu(ii)-salen complexes with the π-π stacking interaction of the indole ring. Such differences are due to the electronic structure difference, the localized radical electron on one of the phenolate moieties in the oxidized Cu(ii) complexes being more favorable for the π-π stacking interaction.

Structural and functional study for tRNA cleavage by Glycine o-phenanthroline CuII complex, [CuCl(phen)(gly)]∙4H2O

The o-phenanthroline gly Cu(II) complex, [CuCl(phen)(gly)]∙4H₂O 1, has been prepared and structurally characterized. The transfer RNA binding and degradation properties of complex 1 have been investigated by UV-vis spectroscopy, cyclic voltammetry (CV), capillary electrophoresis (CE) and atomic force microscopy (AFM) methods. The results showed that 1 can efficiently cleave tRNA in the physiological conditions (pH 7.0, and 37 °C), and has a digestion coefficient nearly up to 100% within 75 h. AFM image for 1/RNA exhibited arrayed tandem repetitions of tRNA segments. This study is targeting the destruction of the high-order structures of genetic biomacromolecules which paves an important way to novel materials for the decontamination of microorganisms (e.g., bacteria and viruses).

Structure and electronic spectra of purine-methyl viologen charge transfer complexes

The structure and properties of the electron donor-acceptor complexes formed between methyl viologen and purine nucleosides and nucleotides in water and the solid state have been investigated using a combination of experimental and theoretical methods. Solution studies were performed using UV-vis and (1)H NMR spectroscopy. Theoretical calculations were performed within the framework of density functional theory (DFT). Energy decomposition analysis indicates that dispersion and induction (charge-transfer) interactions dominate the total binding energy, whereas electrostatic interactions are largely repulsive. The appearance of charge transfer bands in the absorption spectra of the complexes are well-described by time-dependent DFT and are further explained in terms of the redox properties of purine monomers and solvation effects. Crystal structures are reported for complexes of methyl viologen with the purines 2'-deoxyguanosine 3'-monophosphate (DAD'DAD' type) and 7-deazaguanosine (DAD'ADAD' type). Comparison of the structures determined in the solid state and by theoretical methods in solution provides valuable insights into the nature of charge-transfer interactions involving purine bases as electron donors.

Equilibrium Studies of Dibutyltin(IV)-Zwitterionic Buffer Complexation

Equilibrium studies in aqueous solution are reported for dibutyltin(IV) (DBT) complexes of the zwitterionic buffers “Good’s buffers” Mes and Mops. Stoichiometric and formation constants of the complexes formed were determined at different temperatures and ionic strength 0.1 mol·L⁻¹ NaNO₃. The results show that the best fit of the titration curves were obtained when the complexes ML, MLH₋₁, MLH₋₂ and MLH₋₃ were considered beside the hydrolysis product of the dibutyltin(IV) cation. The thermodynamic parameters ΔH^o, ΔS^o and ΔG^o calculated from the temperature dependence of the formation constant of the dibutyltin(IV) complexes with 2-(N-morpholino)ethanesulfonic acid (Mes) and 3-(N-mor-pholino)-propanesulfonic acid (Mops) were investigated. The effect of dioxane as a solvent on the formation constants of DBT–Mes and DBT–Mops complexes decrease linearly with the increase of dioxane proportion in the medium. The concentration distribution of the various complexes species was evaluated as a function of pH.

Stability and structure of mixed-ligand metal ion complexes that contain Ni2+, Cu2+, or Zn2+, and Histamine, as well as adenosine 5'-triphosphate (ATP4-) or uridine 5'-triphosphate (UTP(4-): an intricate network of equilibria

With a view on protein-nucleic acid interactions in the presence of metal ions we studied the "simple" mixed-ligand model systems containing histamine (Ha), the metal ions Ni(2+), Cu(2+), or Zn(2+) (M(2+)), and the nucleotides adenosine 5'-triphosphate (ATP(4-)) or uridine 5'-triphosphate (UTP(4-)), which will both be referred to as nucleoside 5'-triphosphate (NTP(4-)). The stability constants of the ternary M(NTP)(Ha)(2-) complexes were determined in aqueous solution by potentiometric pH titrations. We show for both ternary-complex types, M(ATP)(Ha)(2-) and M(UTP)(Ha)(2-), that intramolecular stacking between the nucleobase and the imidazole residue occurs and that the stacking intensity is approximately the same for a given M(2+) in both types of complexes: The formation degree of the intramolecular stacks is estimated to be 20 to 50%. Consequently, in protein-nucleic acid interactions imidazole-nucleobase stacks may well be of relevance. Furthermore, the well-known formation of macrochelates in binary M(2+) complexes of purine nucleotides, that is, the phosphate-coordinated M(2+) interacts with N7, is confirmed for the M(ATP)(2-) complexes. It is concluded that upon formation of the mixed-ligand complexes the M(2+)-N7 bond is broken and the energy needed for this process corresponds to the stability differences determined for the M(UTP)(Ha)(2-) and M(ATP)(Ha)(2-) complexes. It is, therefore, possible to calculate from these stability differences of the ternary complexes the formation degrees of the binary macrochelates: The closed forms amount to (65±10)%, (75±8)%, and (31±14) % for Ni(ATP)(2-), Cu(ATP)(2-), and Zn(ATP)(2-), respectively, and these percentages agree excellently with previous results obtained by different methods, confirming thus the internal validity of the data and the arguments used in the evaluation processes. Based on the overall results it is suggested that M(ATP)(2-) species, when bound to an enzyme, may exist in a closed macrochelated form only, if no enzyme groups coordinate directly to the metal ion.

Complex formation reactions of palladium(II)-1,3-diaminopropane with various biologically relevant ligands. Kinetics of hydrolysis of glycine methyl ester through complex formation

The interaction of [Pd(DAP)(H2O)2]2+ (DAP = 1,3-diaminopropane) with some selected bio-relevant ligands, containing different functional groups, were investigated. The ligands used are dicarboxylic acids, amino acids, peptides and DNA constituents. Stoichiometry and stability constants of the complexes formed are reported at 25°C and 0.1 M ionic strength. The results show the formation of 1:1 complexes with amino acids and dicarboxylic acids. The effect of chelate ring size of the dicarboxylic acid complexes on their stability constants is examined. Peptides form both 1:1 complexes and the corresponding deprotonated amide species. DNA constituents form 1:1 and 1:2 complexes. The effect of dioxane on the acid dissociation constants of CBDCA and the formation constant of its complex with Pd(DAP)2+ was reported. The kinetics of hydrolysis of glycine methyl ester bound to [Pd(DAP)(H2O)2]2+ was studied at 25°C and 0.1M ionic strength.

Xanthosine 5'-monophosphate (XMP). Acid-base and metal ion-binding properties of a chameleon-like nucleotide

The four acidity constants of threefold protonated xanthosine 5'-monophosphate, H(3)(XMP)(+), reveal that in the physiological pH range around 7.5 (X - H x MP)(3-) strongly dominates and not XMP(2-) as commonly given in textbooks and often applied in research papers. Therefore, this nucleotide, which participates in many metabolic processes, should be addressed as xanthosinate 5'-monophosphate as is stated in this critical review. Micro acidity constant schemes allow quantification of intrinsic site basicities. In 9-methylxanthine nucleobase deprotonation occurs to more than 99% at (N3)H, whereas for xanthosine it is estimated that about 30% are (N1)H deprotonated and for (X - H x MP)(3-) it is suggested that (N1)H deprotonation is further favored, especially in macrochelates where the phosphate-coordinated M(2+) interacts with N7. The formation degree of these macrochelates in the (X - H x MP x M)(-) species of Co(2+), Ni(2+), Cu(2+), Zn(2+) or Cd(2+) amounts to 90% or more. In the monoprotonated (M x X - H x MP x H)(+/-) complexes, M(2+) is located at the N7/[(C6)O] unit as the primary binding site and it forms macrochelates with the P(O)(2)(OH)(-) group to about 65% for nearly all metal ions considered (i.e., including Ba(2+), Sr(2+), Ca(2+), Mg(2+)); this indicates outer-sphere binding to P(O)(2)(OH)(-). Finally, a new method quantifying the chelate effect is applied to the M(X - H x MP)(-) species, stabilities and structures of mixed-ligand complexes are considered, and the stability constants for several M(X - H x DP)(2-) and M(X - H x TP)(3-) complexes are estimated (112 references).

Evidence for intramolecular aromatic-ring stacking in the physiological pH range of the monodeprotonated xanthine residue in mixed-ligand complexes containing xanthosinate 5′-monophosphate (XMP)

The stability constants of the mixed-ligand complexes formed between Cu(Arm)2+ [Arm = 2,2'-bipyridine (Bpy) or 1,10-phenanthroline (Phen)], and the di- or trianion of xanthosine 5'-monophosphoric acid [= XMP(2-) or (XMP - H)(3-)] were determined by potentiometric pH titration in aqueous solution (25 degrees C; I = 0.1 M, NaNO3). Those for the monoanion, i.e., the Cu(Arm)(H;XMP)+ complexes, could only be estimated; for these species it is concluded that the metal ion is overwhelmingly bound at N7 and the proton resides at the phosphate group. Similarly, in the Cu(Arm)(XMP)+/- [= Cu(Arm)(X - H.MP.H)+/-] complexes Cu(Arm)2+ is also at N7 but the xanthine residue has lost a proton whereas the phosphate group still carries one, i.e., stacking plays, if at all, only a very minor role, yet, the N7-bound Cu(Arm)2+ appears to form an outer-sphere macrochelate with P(O)2(OH)-, its formation degree being about 60%. All this is different in the Cu(Arm)(XMP - H)- complexes, which are formed by the (XMP - H)(3-) species, that occur at the physiological pH of 7.5 and for which previously evidence has been provided that in a tautomeric equilibrium the xanthine moiety loses a proton either from (N1)H or (N3)H. In Cu(Arm)(XMP - H)- the phosphate group is the primary binding site for Cu(Arm)2+ and the observed increased complex stability is mainly due to intramolecular stack (st) formation between the aromatic-ring systems of Phen or Bpy and the monodeprotonated xanthine residue of (XMP - H)(3-); e.g., the stacked Cu(Phen)(XMP - H) isomer occurs with approximately 76%. Regarding biological systems the most important result is that at physiological pH the xanthine moiety has lost a proton from the (N1)H/(N3)H sites forming (XMP - H)(3-) and that its anionic xanthinate residue is able to undergo aromatic-ring stacking.

Acid-base and metal-ion-binding properties of 9-[2-(2-phosphonoethoxy)ethyl]adenine (PEEA), a relative of the antiviral nucleotide analogue 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA). An exercise on the quantification of isomeric complex equilibria in solution

The acidity constants of 3-fold protonated 9-[2-(2-phosphonoethoxy)ethyl]adenine, H3(PEEA)+, and of 2-fold protonated (2-phosphonoethoxy)ethane, H2(PEE), and the stability constants of the M(H;PEEA)+, M(PEEA), and M(PEE) complexes with M2+ = Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, or Cd2+ have been determined (potentiometric pH titrations; aqueous solution; 25 degrees C; I = 0.1 M, NaNO3). It is concluded that in the M(H;PEEA)+ species, the proton is at the phosphonate group and the metal ion at the adenine residue. The application of previously determined straight-line plots of log K(M(R-PO3))M versus pK(H(R-PO3))H for simple phosph(on)ate ligands, R-PO3(2-), where R represents a residue that does not affect metal-ion binding, proves that the M(PEEA) complexes of Co2+, Ni2+, Cu2+, Zn2+, and Cd2+ as well as the M(PEE) complexes of Co2+, Cu2+, and Zn2+ have larger stabilities than is expected for a sole phosphonate coordination of M2+. For the M2+ complexes without an enhanced stability (e.g., Mg2+ or Mn2+), it is concluded that M2+ binds in a monodentate fashion to the phosphonate group of the two ligands. Combination of all of the results allows the following conclusions: (i) The increased stability of the Co(PEE), Cu(PEE), Zn(PEE), and Co(PEEA) complexes is due to the formation of six-membered chelates involving the ether-oxygen atom of the aliphatic residue (-CH2-O-CH2CH2-PO3(2-)) of the ligands with formation degrees of about 15-30%. (ii) Cd(PEEA) forms a macrochelate with N7 of the adenine residue (formation degree about 30%); Ni(PEEA) has similar properties. (iii) With Zn(PEEA), both mentioned types of chelates are observed, that is, Zn(PEEA)(cl/O) and Zn(PEEA)(cl/N7), with formation degrees of about 13 and 41%, respectively; the remaining 46% is due to the "open" isomer Zn(PEEA)(op) in which the metal ion binds only to the PO3(2-) group. (iv) Most remarkable is Cu(PEEA) because a fourth isomer, Cu(PEEA)(cl/O/N3), is formed that contains a six-membered ring involving the ether oxygen next to the phosphonate group and also a seven-membered ring involving N3 of the adenine residue with a very significant formation degree of about 50%. Hence, PEEA(2-) is a truly ambivalent ligand, its properties being strongly dependent on the kind of metal ion involved. Comparisons with M2+ complexes formed by the dianions of 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA) and related ligands reveal that five-membered chelates involving an ether-oxygen atom are considerably more stable than the corresponding six-membered ones. This observation offers an explanation of why PMEA is a nucleotide analogue with excellent antiviral properties and PEEA is not.

Nucleoside 5'-triphosphates: self-association, acid-base, and metal ion-binding properties in solution

Adenosine 5'-triphosphate (ATP(4-)) and related nucleoside 5'-triphosphates (NTP(4-)) serve as substrates in the form of metal ion complexes in enzymic reactions taking part thus in central metabolic processes. With this in mind, the coordination chemistry of NTPs is critically reviewed and the conditions are defined for studies aiming to describe the properties of monomeric complexes because at higher concentrations (>1 mM) self-stacking may take place. The metal ion (M(2+)) complexes of purine-NTPs are more stable than those of pyrimidine-NTPs; this stability enhancement is attributed, in accord with NMR studies, to macrochelate formation of the phosphate-coordinated M(2+) with N7 of the purine residue and the formation degrees of the resulting isomeric complexes are listed. Furthermore, the formation of mixed-ligand complexes (including also those with buffer molecules), the effect of a reduced solvent polarity on complex stability and structure (giving rise to selectivity), the use of nucleotide analogues as antiviral agents, and the effect of metal ions on group transfer reactions are summarized.

Equilibrium, kinetic and solvent effect studies on the reactions of [RuIII(Hedta)(H2O)] with thiols

The interaction of [Ru(III)(edta)(H(2)O)](-) with a series of selected thiols having extra functional groups was investigated potentiometrically and kinetically. The pK(a) values of the uncoordinated carboxylic acid group and coordinated water molecule are 3.12 and 7.41, respectively, in aqueous solution at 25 degrees C and 0.1 M ionic strength. The formation constants of the complexes were determined in the pH range 3-9, and the concentration distribution of the various complex species was evaluated as a function of pH. The effect of dioxane on the pK(a) values of [Ru(III)(Hedta)(H(2)O)] and the formation constants of the corresponding thiol complexes is presented. The study also provides mechanistic information on the reaction of [Ru(III)(edta)(H(2)O)](-) with the thiols. The low values of DeltaH(not equal) and negative values of DeltaS(not equal) and DeltaV(not equal) for the substitution reactions of [Ru(III)(edta)(H(2)O)](-) clearly support the associative character of the substitution process.

Intramolecular stacking interactions in ternary copper(II) complexes formed by a heteroaromatic amine and 9-[2-(2-phosphonoethoxy)ethyl]adenine, a relative of the antiviral nucleotide analogue 9-[2-(phosphonomethoxy)ethyl]adenine☆

The stability constants of the mixed-ligand complexes formed between Cu(Arm)2+, where Arm=2,2'-bipyridine (Bpy) or 1,10-phenanthroline (Phen), and the dianions of 9-[2-(2-phosphonoethoxy)ethyl]adenine (PEEA2-) and (2-phosphonoethoxy)ethane (PEE2-), also known as [2-(2-ethoxy)ethyl]phosphonate, were determined by potentiometric pH titrations in aqueous solution (25 degrees C; I=0.1 M, NaNO3). The ternary Cu(Arm)(PEEA) complexes are considerably more stable than the corresponding Cu(Arm)(R-PO3) species, where R-PO3(2-) represents a phosph(on)ate ligand with a group R that is unable to participate in any kind of interaction within the complexes. The increased stability is attributed to intramolecular stack formation in the Cu(Arm)(PEEA) complexes and also, to a smaller extent, to the formation of 6-membered chelates involving the ether oxygen atom present in the -CH2-O-CH2-CH2-PO3(2-) residue of PEEA2-. This latter interaction is separately quantified by studying the ternary Cu(Arm)(PEE) complexes which can form the 6-membered chelates but where no intramolecular ligand-ligand stacking is possible. Application of these results allows a quantitative analysis of the intramolecular equilibria involving three structurally different Cu(Arm)(PEEA) species; e.g., of the Cu(Bpy)(PEEA) system about 11% exist with the metal ion solely coordinated to the phosphonate group, 4% as a 6-membered chelate involving the ether oxygen atom of the -CH2-O-CH2CH2-PO3(2-) residue, and 85% with an intramolecular stack between the adenine moiety of PEEA2- and the aromatic rings of Bpy. In addition, the Cu(Arm)(PEEA) complexes may be protonated, leading to Cu(Arm)(H;PEEA)+ species for which it is concluded that the proton is located at the phosphonate group and that the complexes are mainly formed (50 and 70%) by a stacking adduct between Cu(Arm)2+ and the adenine residue of H(PEEA)-. Finally, the stacking properties of adenosine 5'-monophosphate (AMP2-), of the dianion of 9-[2-(phophonomethoxy)ethyl]adenine (PMEA2-) and of several of its analogues (=PA2-) are compared in their ternary Cu(Arm)(AMP) and Cu(Arm)(PA) systems. Conclusions regarding the antiviral properties of several acyclic nucleoside phosphonates are shortly discussed.