Reactivity of an extremenly sterically crowded monofunctional Pt complex, [Pt(1-MeC-N3)3(H2O)]2+ (1-MeC=1-methylcytosine), toward model nucleobases and selectivity toward guanine in single- and double-stranded deoxyoligonucleotides

 Reactions of [Pt(1-MeC-N3)₃Cl]NO₃ (1-MeC-N3=1-methylcytosine, bound to Pt via N3) and the respective aqua species [Pt(1-MeC-N3)₃(H₂O)]²⁺ with the model nucleobases 9-ethylguanine (9-EtGH), 9-methyladenine (9-MeA), single-stranded 5′d(T₃GT₃), and double-stranded [5′d(GAGA₂GCT₂CTC)]₂ have been studied in solution by means of ¹H NMR spectroscopy, HPLC, and electrospray ionization mass spectrometry. Reactions are generally slow, in particular with the chloro species, and guanine is the only reactive base in the oligonucleotides. However, unlike (dien)Pt^II, which binds randomly to the guanines in the ds dodecamer, (1-MeC-N3)₃Pt^II binds selectively to the terminal guanine only, probably because base fraying takes place at the duplex ends. The X-ray crystal structures of [Pt(1-MeC-N3)₃(9-EtG-N7)]ClO₄·8H₂O (1b) and of [Pt(1-MeC-N3)₃(9-MeA-N7)](ClO₄)₂·0.5H₂O as well as NMR spectroscopic studies of [Pt(1-MeC-N3)₃(9-EtGH-N7)] (NO₃)₂·H₂O (1a) are reported. The tetrakis(nucleobase) complexes adopt a head-tail-head orientation of the three 1-MeC bases and an orientation of the fourth base (purine) that permits a maximum of intracomplex H bonds between exocyclic groups. As far as the guanine adduct (1a, 1b) is concerned, relative orientations of the four bases are identical in the model and in the oligonucleotide adduct. Received: 19 June 1998 / Accepted: 1 October 1998     

Coexistence of major and minor tautomers of 1-methylcytosine (1-MeC) in a single metal complex, trans-Pt(1-MeC-N3)(1-MeC-N4)X2 (X = Cl, I): metal migration N3 → N4 at acidic pH

Two complexes of composition trans-Pt(1-MeC-N3)(1-MeC-N4)I₂ · 2H₂O (4) and trans-Pt(1-MeC-N3)(1-MeC-N4)Cl₂ (5) are described and characterized by X-ray analysis, which simultaneously contain the preferred aminooxo tautomer I and the rare iminooxo tautomer II of 1-methylcytosine (1-MeC) bonded to the heavy metal, via N3 and N4, respectively. Formation of 4 originates from [Pt(1-MeC-N3)₃I]I (2), which likewise has been characterized by X-ray crystal structure analysis. A feasible way of formation of 4, which involves a metal migration process from N3 to N4 occurring at moderately acidic pH, is proposed. It appears to be yet another mechanism of metal migration, different from previously established cases which are redox-assisted and hydroxide-promoted, respectively.     

Metal-modified nucleobase pairs involving 7,9-dimethylguanine: trans-a2PtII analogues (a = NH3 or CH3NH2) of Watson-Crick GC and homo GG pairs

 The analogy between H-bonded nucleobase pairs and their metalated analogues is extended to the hemiprotonated pair of 7,9-dimethylguanine (7,9-DimeG) and the Watson-Crick and reversed Watson-Crick pair between 7,9-dimethylguaninium (7,9-DimeGH⁺) and 1-methylcytosine (1-MeC). The crystal structure analyses of two model compounds, trans–[Pt(CH₃NH₂)₂(7,9-DimeG-N1)₂](NO₃)₂ (1) and trans–[Pt(NH₃)₂(1-MeC-N3)(7, 9-DimeG-N1)](PF₆)₂· 2.5 H₂O (3a) are reported. Pt binding is through N1 of 7,9-DimeG and N3 of 1-MeC. In solution, 3a exists in a mixture with Watson-Crick and reversed Watson-Crick arrangements of the two bases, depending on solvent, concentration and anions. Received: 16 October 1996 / Accepted: 27 January 1997     

The role of intramolecular hydrogen bonding on nucleobase acidification following metal coordination: possible implications of an “indirect” role of metals in acid–base catalysis of nucleic acids

The acidifying effect of Pt^II on nucleobase –NH and –NH₂ groups depends both on the site of metal coordination and on the efficiency of stabilization of the deprotonated nucleobase via intracomplex hydrogen bonding. Weakly acidic nucleobase protons with pK _a values between 9 and 17 can be acidified by a single Pt^II to have pK _a values which are well within the physiological pH range. This could open the possibility of an acid–base catalysis occurring at pH 7, with the metal–nucleobase entity functioning either as an acid or a base. Examples of Pt^II complexes studied here include, among others, mixed nucleobase systems of 1-methylcytosine and 1,9-dimethyladenine as well as a complex of the rare iminooxo tautomer of 1-methylcytosine having the metal bonded at N4.     

trans-(NH3)2PtII-modified deoxyoligonucleotides as potential antisense agents: cross-linking reactions between two 12-mers

 An approach is presented which probes the possible use of trans-[(NH₃)₂PtCl]⁺-modified deoxyoligonucleotides in the antisense strategy. It consists of (1) the selective platination of an oligonucleotide containing 11 pyrimidine (T, C) bases as well as a single guanine (G) as a Pt-anchoring group at the 5′-end to give trans-[(NH₃)₂Pt{5′-d(G^N7T₂C₂T₂C₂T₂C}Cl]^10– 1 ("antisense strand") and (2) subsequent hybridization with the purine 12-mer 5′-d(GA₂G₂A₂G₂A₂G)^11– ("sense strand"). According to HPLC, three major species 2–4 are formed during reaction (2), all of which are cross-linking adducts between 1 and the sense strand, as confirmed by ESI MS and melting temperature measurements. Only for the major product 3 can a structure be proposed on the basis of 1D and 2D NMR spectra. According to these, G₁ of the antisense strand is cross-linked with G₂₀ via trans-(NH₃)₂Pt^II. The complementary overhangs of the duplex represent "sticky ends" and are, in principle, capable of associating into multimers of the duplex. Received: 29 March 1999 / Accepted: 26 July 1999     

Phenoxyl-copper(II) complexes: models for the active site of galactose oxidase

 The reaction of the macrocycles 1,4,7-tris (3,5-di-tert-butyl-2-hydroxy-benzyl)-1,4,7-triazacyclononane, L¹H₃, or 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxy-benzyl)-1,4,7-triazacyclononane, L²H₃, with Cu(ClO₄)₂·6H₂O in methanol (in the presence of Et₃N) affords the green complexes [Cu^II(L¹H)] (1), [Cu^II(L²H)]·CH₃OH (2) and (in the presence of HClO₄) [Cu^II(L¹H₂)](ClO₄) (3) and [Cu^II(L²H₂)] (ClO₄) (4). The Cu^II ions in these complexes are five-coordinate (square-base pyramidal), and each contains a dangling, uncoordinated pendent arm (phenol). Complexes 1 and 2 contain two equatorially coordinated phenolato ligands, whereas in 3 and 4 one of these is protonated, affording a coordinated phenol. Electrochemically, these complexes can be oxidized by one electron, generating the phenoxyl-copper(II) species [Cu^II(L¹H)]^+ ·, [Cu(L²H)]^+ ·, [Cu^II(L¹H₂)]^2+ ·, and [Cu^II(L²H₂)]^2+ ·, all of which are EPR-silent. These species are excellent models for the active form of the enzyme galactose oxidase (GO). Their spectroscopic features (UV-VIS, resonance Raman) are very similar to those reported for GO and unambiguously show that the complexes are phenoxyl-copper(II) rather than phenolato-copper(III) species. Received: 10 February 1997 / Accepted: 7 April 1997     

Structural and reactivity studies on the ternary system guanine/methionine/trans-[PtCl2(NH3)L] (L=NH3, quinoline): implications for the mechanism of action of nonclassical trans-platinum antitumor complexes

 The present model study explores the chemistry of methionine complexes and ternary methionine-guanine adducts formed by trans-[PtCl₂(NH₃)₂] (1) and antitumor trans-[PtCl₂(NH₃)quinoline] (2) using 1D (¹H, ¹⁹⁵Pt) and 2D NMR spectroscopy. Compound 2 was substitution inert in reactions with N-acetyl-lmethionine [AcMet(H)]. Reactions of trans-[PtCl(NO₃)(NH₃)quinoline] (5) ("monoactivated" 2) with AcMetH in water and acetone at various stoichiometries point to Pt(II)-S binding that requires prior activation of the Pt-Cl bond by labile oxygen donors. Trans-[PtCl{AcMet(H)-S}(NH₃)quinoline](NO₃) (6) and trans-[Pt{AcMet(H)-S}₂(NH₃)quinoline](NO₃)₂ (7) were isolated from these mixtures. At high [Cl^–], AcMet(H) is displaced from 7, giving 6. Frozen stereodynamics in 6 at the thioether-S and slow rotation about the Pt-N_quinoline bond result in four spectroscopically distinguishable diastereomers. ¹H NMR spectra of 7 show faster exchange dynamics due to mutual trans-labilization of the sulfur donors. Substitution of chloride in trans-[PtCl(9-EtGua)(NH₃)L]NO₃ (L=NH₃, 3; L=quinoline, 4; 9-EtGua=9-ethylguanine, which mimics the first DNA binding step of 1 and 2) by methionine-sulfur proceeded ca. 2.5 times slower for the quinoline compound. Both reactions, in turn, proved to be ca. 4 times faster than binding of a second nucleobase under analogous conditions. From the resulting mixtures the ternary adducts trans-[Pt(AcMet-S)(9-EtGua-N7)(NH₃)L](NO₃, Cl) (L=NH₃, 8; L=quinoline, 9) were isolated. A species analogous to 9 formed in a rapid reaction between 6 and 5′-guanosine monophosphate (5′-GMP). From NMR data an AMBER-based solution structure of the resulting adduct, trans-[Pt(AcMet-S)(5′-GMP-N7)(NH₃)quinoline] (10), was derived. The unusual reactivity along the N7-Pt-S axis in 8–10 resulted in partial release of both 9-EtGua and AcMet at high [Cl^–]. Possible consequences of the kinetic and structural effects (e.g., trans effect of sulfur, steric demand of quinoline) observed in these systems with respect to the (trans)formation of potential biological cross-links are discussed. Received: 25 May 1998 / Accepted: 6 August 1998     

The TpG chelate of cis(diammineplatinum) forms two head-to-head rotamers in H2O solution

 d(TpG)^– reacts with cis-[Pt(NH₃)₂(H₂O)₂]²⁺ in two steps to yield the platinum chelate cis-[Pt(NH₃)₂{d(TpG)-N3(1),N7(2)}]. In the latter, hindered rotation of the bases leads to an equilibrium between two rotamers interconverting slowly on the NMR time scale. The structure of the two rotameric chelates was studied by means of ¹H NMR and molecular modeling techniques. The major and minor rotamers could be assigned unambiguously to the two head-to-head conformational domains which are characterized by syn/anti and anti/anti sugar-base orientations, respectively. Molecular models derived for both rotamers show that the orientations of the bases are mutually quasi-enantiomeric. The interconversion between the two rotamers (k ≈ 1 s^–1 at 293 K) is approximately 10⁴ times faster than the analogous rotamer interconversion observed in cis-[Pt(NH₃)₂{r(CpG)-N3(1),N7(2)}]⁺ [Girault J-P, Chottard G, Lallemand J-Y, Huguenin F, Chottard J-C (1984) J Am Chem Soc 106 : 7227–7232], suggesting that the steric clash of the exocyclic amino group of the platinum-bound cytosine with the ligands in cis position is more severe than that of the two thymine oxo groups. Received: 23 June 1997 / Accepted: 30 September 1997     

Antitumor bifunctional dinuclear PtII complex BBR3535 forms interduplex DNA cross-links under molecular crowding conditions

When antitumor platinum drugs react with DNA they form various types of intrastrand and interstrand cross-links (CLs). One class of new antitumor platinum compounds comprises bifunctional Pt^II compounds based on the dinuclear or trinuclear geometry of leaving ligands. It has been shown that the DNA-binding modes of dinuclear or trinuclear bifunctional Pt^II agents are distinct from those of mononuclear cisplatin, forming markedly more intramolecular interstrand CLs. However, at least two types of DNA interstrand cross-linking by bifunctional Pt^II complexes can be envisaged, depending on whether the platinum complex coordinates to the bases in one DNA molecule (intramolecular interstrand CLs) or in two different DNA duplexes (interduplex CLs). We hypothesized that at least some antitumor bifunctional poly(di/tri)nuclear complexes could fulfill the requirements placed on interduplex DNA cross-linkers. To test this hypothesis we studied the interduplex cross-linking capability of a representative of antitumor polynuclear agents, namely, dinuclear Pt^II complex [{trans-PtCl(NH₃)₂}₂-μ-{trans-(H₂N(CH₂)₆NH₂(CH₂)₂NH₂(CH₂)₆NH₂)}]⁴⁺ (BBR3535). The investigations were conducted under molecular crowding conditions mimicking environmental conditions in the cellular nucleus, namely, in medium containing ethanol, which is a commonly used crowding agent. We found with the aid of native agarose gel electrophoresis that the DNA interduplex cross-linking efficiency of BBR3535 under molecular crowding conditions was remarkable: the frequency of these CLs was 54%. In contrast, the interduplex cross-linking efficiency of mononuclear cisplatin or transplatin was markedly lower (approximately 40-fold or 18-fold, respectively). We suggest that the production of interduplex CLs in addition to other DNA intramolecular adducts may provide polynuclear Pt^II compounds with a wider spectrum of cytotoxicity.     

Carbonyl trichlorostannate complexes of platinum: chemistry and 119Sn, 195Pt,and 13C NMR spectroscopy

The reactions of the carbonyl anion [PtCl₃(CO)]^- with SnCl₂ in the presence of CO in both methylene chloride and acetone are reported. In the former solvent, only Pt^II-SnCl₃ species are formed. These have been identified by ¹³C, ¹¹⁹Sn and ¹⁹⁵Pt NMR measurements as cis-[PtCl₂(SnCl₃)(CO)]^-, (I), trans- [PtCl(SnCl₃)₂(CO)]^-, (II), and [Pt(SnCl₃)₄(CO)]^2-, (III). Salts of these complexes have been isolated. In contrast, when acetone is the solvent, reduction of the platinum occurs to give two new complexes. On the basis of NMR measurements, we assign one of these as the Pt^I dimer [Pt₂(SnCl₃)₄(CO)₂]^2-, (IV), and the other as a platinum triangle (VI) containing terminal CO ligands and two types of Sn ligand. The Pt^II compound (IV) can also be generated by treating a CH₂Cl₂ solution of trans-[PtCl(SnCl₃)_2- (CO)]^-, (II), with dihydrogen. NMR spectroscopic data, including those from measurements on samples of the complexes containing ¹³C-enriched CO, are reported and discussed.     

Reaction of PtOH complexes with CO2: synthesis and X-ray structure of (PBz3)4Pt2(Ph)2(η1,η1,μ-CO3)·(toluene)

The reaction of (diphoe)Pt(CH₂CN)(OH) and trans-(PBz₃)₂Pt(Ph)(OH) with CO₂ is reported as producing dimeric, bridged carbonato complexes of the type: P₄Pt₂(R)₂(CO₃). The crystal and molecular structure of (PBz₃)₄Pt₂(Ph)₂(μ-CO₃)·(toluene) is also reported, showing η¹, η¹ bonding. The reaction is recognized to proceed in a stepwise fashion through bicarbonato intermediates.     

A coordination polymer formed by cis-[(PMePh2)2Pt(NO3)2] and 2,5-bis(4-pyridyl)-1,3,4-thiadiazole

The 1:1 adduct formed from cis-[(PMePh₂)₂Pt(NO₃)₂] and 2,5-bis(4-pyridyl)-1,3,4-thiadiazole (bpytdz) in chloroform crystallizes out as a 1D coordination polymer built up of cis-(PMePh₂)₂Pt units connected by the bpytdz ligand, an unusual self-assembly for a complex formed from a (phosphine)₂Pt^II entity and a N,N linker.     

Thermodynamic stability and energetics of DNA duplexes containing major intrastrand cross-links of second-generation antitumor dinuclear PtII complexes

The effects of major DNA intrastrand cross-links of antitumor dinuclear Pt^II complexes [{trans-PtCl(NH₃)₂}₂-μ-{trans-(H₂N(CH₂)₆NH₂(CH₂)₂NH₂(CH₂)₆NH₂)}]⁴⁺ (1) and [{PtCl(DACH)}₂-μ-{H₂N(CH₂)₆NH₂(CH₂)₂NH₂(CH₂)₆NH₂)}]⁴⁺ (2) (DACH is 1,2-diaminocyclohexane) on DNA stability were studied with emphasis on thermodynamic origins of that stability. Oligodeoxyribonucleotide duplexes containing the single 1,2, 1,3, or 1,5 intrastrand cross-links at guanine residues in the central TGGT, TGTGT, or TGTTTGT sequences, respectively, were prepared and analyzed by differential scanning calorimetry. The unfolding of the platinated duplexes was accompanied by unfavorable free energy terms. The efficiency of the cross-links to thermodynamically destabilize the duplex depended on the number of base pairs separating the platinated bases. The trend was 1,5→1,2→1,3 cross-link of 1 and 1,5→1,3→1,2 cross-link of 2. Interestingly, the results showed that the capability of the cross-links to reduce the thermodynamic stability of DNA (ΔG ₂₉₈⁰) correlated with the extent of conformational distortions induced in DNA by various types of intrastrand cross-links of 1 or 2 determined by chemical probes of DNA conformation. We also examined the efficiency of the mammalian nucleotide excision repair systems to remove from DNA the intrastrand cross-links of 1 or 2. The efficiency of the excinucleases to remove the cross-links from DNA depended on the length of the cross-link; the trend was identical to that observed for the efficiency of the intrastrand cross-links to thermodynamically destabilize the duplex. Thus, the results are consistent with the thesis that an important factor that determines the susceptibility of the intrastrand cross-links of dinuclear platinum complexes 1 and 2 to be removed from DNA by nucleotide excision repair is the efficiency of these lesions to thermodynamically destabilize DNA.     

Kinetics of formation and stability of {Pt(dien)}2+ complexes with octamer and 14-mer DNA oligonucleotides containing a GG sequence

 Reaction of [Pt(dien)Cl]⁺ (1) with the 14-mer oligonucleotide 5′-d(ATACATGGTACATA) (I) gave rise to two major species which corresponded to the 5′-G and 3′-G platinated monofunctional adducts, and a minor amount of the bis-platinated adduct formed during the later stages of the reaction. The reaction of (1) with the related octamer 5′-d(ATACATGG) (II) was also investigated. Kinetic data obtained by HPLC showed that the 5′-G and 3′-G bases of the 14-mer oligonucleotide were platinated at similar rates: the second-order rate constant is 53×10^–2 M^–1 s^–1 at 298 K in 0.1 M NaClO₄. However, the platination rate of 5′-G of the octamer (II) (k=69×10^–2 M^–1 s^–1) was enhanced by a factor of three compared to the rate of platination at 3′-G (k=22×10^–2 M^–1 s^–1). All the adducts were separated by HPLC and characterized by NMR spectroscopy, enzymatic digestion and MALDI-TOF mass spectrometry. ¹H and ¹⁵N NMR shifts suggest that there are distinct conformational differences between 14-mer duplexes platinated at 5′-G (I5′ _ds) and 3′–G (I3′ _ds). Molecular mechanics modelling indicates that rotation around the Pt-N7 bond is more restricted in the case of the 5′-G adduct than in that of the 3′-G adduct. The binding of {Pt(dien)}²⁺ to 5′-GN7 and 3′-GN7 in the monofunctional adducts of (I) was shown to be reversible upon the addition of high concentrations of chloride ions. Received: 3 July 1998 / Accepted: 10 November 1998     

Photoactive trans ammine/amine diazido platinum(IV) complexes

The synthesis and characterisation of eight new octahedral Pt^IV complexes of the type trans,trans,trans-[Pt(N₃)₂(OH)₂(NH₃)(Am)] where Am = methylamine (2), ethylamine (4), thiazole (6), 2-picoline (8), 3-picoline (10), 4-picoline (12), cyclohexylamine (14), and quinoline (16) are reported, including the X-ray crystal structures of complexes 2, 8, and 14 as well as that of two of the precursor Pt^II complexes (trans-[Pt(N₃)₂(NH₃)(methylamine)] (1) and trans-[Pt(N₃)₂(NH₃)(cyclohexylamine)] (13)). Irradiation with UVA light rapidly induces loss in intensity of the azide-to-Pt^IV charge-transfer bands and gives rise to photoreduction of platinum. These complexes have potential for use as photoactivated anticancer agents.     

Effects of geometric isomerism in dinuclear antitumor platinum complexes on their interactions with N-acetyl-L-methionine

In this study, the reactions of N-acetyl-L-methionine (AcMet) with [{trans-PtCl(NH₃)₂}₂-μ-H₂N(CH₂)₆NH₂](NO₃)₂ (BBR3005: 1,1/t,t 1) and its cis analog [{cis-PtCl(NH₃)₂}₂-μ-{H₂N(CH₂)₆NH₂}]Cl₂ (1,1/c,c 2) were analyzed to determine the rate and reaction profile of chloride substitution by methionine sulfur. The reactions were studied in PBS buffer at 37°C by a combination of multinuclear (¹⁹⁵Pt, {¹H-¹⁵N} HSQC) magnetic resonance (NMR) spectroscopy and electrospray ionization time of flight mass spectrometry (ESITOFMS). The diamine linker of the 1,1/t,t trans complex was released as a result of the trans influence of the coordinated sulfur atom, producing trans-[PtCl(AcMet)(NH₃)₂]⁺ (III) and trans-[Pt(AcMet)₂(NH₃)₂]²⁺ (IV). In contrast the cis geometry of the dinuclear compound maintained the diamine bridge intact and a number of novel dinuclear platinum compounds obtained by stepwise substitution of sulfur on both platinum centers were identified. These include (charges omitted for clarity): [{cis-PtCl(NH₃)₂}-μ-NH₂(CH₂)₆NH₂-{cis-Pt(AcMet)(NH₃)₂}] (V); [{cis-Pt(AcMet)(NH₃)₂}₂-μ-NH₂(CH₂)₆NH₂] (VI); [{cis-PtCl(NH₃)₂}-μ-NH₂(CH₂)₆NH₂-{PtCl(AcMet)NH₃] (VII); [{PtCl(AcMet)(NH₃)}₂-μ-NH₂(CH₂)₆NH₂] (VIII); [{trans-Pt(AcMet)₂(NH₃)}-μ-NH₂(CH₂)₆NH₂-{PtCl(AcMet)(NH₃)] (IX) and the fully substituted [{trans-Pt(AcMet)₂(NH₃)}₂-μ-{NH₂(CH₂)₆NH₂] (X). For both compounds the reactions with methionine were slower than those with glutathione (Inorg Chem 2003, 42:5498–5506). Further, the 1,1/c,c geometry resulted in slower reaction than the trans isomer, because of steric hindrance of the bridge, as observed previously in reactions with DNA and model nucleotides.     

A “directed” approach toward a cationic molecular square containing four isonicotinamidate ligands and (4 + 2) (en)Pt metal entities

The use of 4-cyanopyridine (4-CNpy) and 3-cyanopyridine (3-CNpy) as ditopic ligands with 180° and 120° directionalities, respectively, for the construction of molecular architectures with the 90° metal fragments (en)Pt^II and (en)Pd^II in water is hampered by the ease with which these ligands undergo hydrolysis to isonicotinamide (4-C(O)NH₂py) and nicotinamide (3-C(O)NH₂py). As described in this article, out of six X-ray structurally characterized complexes (1-6), only a single one (1) reveal coordination of the unchanged ligand (4-CNpy) to (en)Pt^II. Nevertheless also the hydrolysis products are of interest in the context of obtaining discrete metallacyclic compounds: Thus, (en)Pt^II and 4-C(O)NH₂py form a hexanuclear complex, [PF₆⊂{(en)Pt}₆(4-C(O)NHpy)₄](NO₃)₇·10H₂O (2), in which the anionic isonicotinamidate ligands function as tridentate, bridging ligands to produce a hybrid between a metallasquare and a 2-floor open box. The resulting cation with a +8 charge accommodates a single hexafluorophosphate anion in its interior. Adjacent cations of 2 pack in such a way as to develop Pt₄ chains as typically seen in “platinum blues” and their [Pt^II]₄ precursors.     

Macrocyclic zinc(II) complexes for selective recognition of nucleobases in single- and double-stranded polynucleotides

 As an extension of our earlier discoveries that Zn^II-cyclen complex (1) (cyclen=1,4,7,10-tetraazacyclododecane) and Zn^II-acridine-pendant cyclen complex Zn^II-N-(9-acridin)ylmethyl-cyclen (3) are the first compounds to selectively recognize thymidine and uridine nucleosides in aqueous solution at physiological pH, the interaction of these and a relevant complex, bis(Zn^II-cyclen) (7), has been investigated with a series of polynucleotides, single-stranded poly(U) and poly(G), and double-stranded poly(A)·poly(U), poly(dA)·poly(dT) and poly(dG)·poly(dC). These Zn^II-cyclen complexes interact with the imide-containing nucleobases in the single-stranded poly(U), unperturbed by the presence of the anionic phosphodiester backbone. The affinity constant of 1 for each N(3)-deprotonated uracil base in poly(U) is determined to be log K= 5.1 by a kinetic measurement, which is almost the same as log K=5.2 for the interaction of 1 with uridine. Thus, they disrupt the A-U (or A-T) hydrogen bonds to unzip the duplex of poly(A)·poly(U) or poly(dA)·poly(dT), as demonstrated by lowering of the melting temperatures (T _m) of poly(A)·poly(U) and poly(dA)·poly(dT) in 5 mM Tris-HCl buffer (pH 7.6, 10 mM NaCl) with increase in their concentrations. The order of the denaturing efficiency is well correlated with that of the 1 : 1 affinity constants for each complex with uracil or thymine;7>3>1. The comparison of circular dichroism (CD) spectra for poly(A)·poly(U), poly(A), and poly(U) in the presence of 3 has revealed a structural change from poly(A)·poly(U) to two single strands, poly(A) and poly(U), caused by 3 binding exclusively to uracils in poly(U). On the other hand, the acridine-pendant cyclen complex 3, which earlier was found to associate with guanine by the Zn^II coordinating with guanine N(7), in addition to the π-π stacking, interacts with guanine in the double helix of poly(dG)·poly(dC) from outside and stabilized the double-stranded structure, as indicated by higher T _m. Received: 31 December 1997 / Accepted: 23 February 1998     

Synthesis and X-ray crystal structure analysis of 1:1 and 1:2 complexes of cisplatin with the model nucleobase 9-methyladenine in its protonated form and a unique HNO3 adduct of cis-[(NH3)2Pt(9-MeAH-N7)2]

The 1:1 and 1:2 complexes of cis-(NH₃)₂Pt^II with 9-methyladeninium cations, 9-MeAH⁺, have been prepared and characterized by X-ray crystallography: cis-[(NH₃)₂Pt(9-MeAH-N7)Cl](NO₃)₂ (1) and cis-[(NH₃)₂Pt(9-MeAH-N7)₂](NO₃)₄ · 2HNO₃ · 2H₂O (2). The pK_a values for 9-MeAH⁺ in H₂O are 1.7 in 1 as well as 0.4 (pK_a1) and 1.3 (pK_a2) for 2, as determined by pD dependent ¹H NMR spectroscopy. Compound 2 is special in that it crystallizes with two equivalents of HNO₃ per Pt entity. The HNO₃ molecules are stacked in rectangular channels provided by cis-(NH₃)₂Pt^II units, 9-methyladeninium ligands and nitrate anions, which form a porous network of hydrogen bonds.     

Syntheses and characterization of vitamin B<Subscript>12</Subscript>–Pt(II) conjugates and their adenosylation in an enzymatic assay

Aiming at the use of vitamin B₁₂ as a drug delivery carrier for cytotoxic agents, we have reacted vitamin B₁₂ with trans-[PtCl(NH₃)₂(H₂O)]⁺, [PtCl₃(NH₃)]⁻ and [PtCl₄]²⁻. These Pt(II) precursors coordinated directly to the Co(III)-bound cyanide, giving the conjugates [{Co}–CN–{trans-PtCl(NH₃)₂}]⁺ (5), [{Co}–CN–{trans-PtCl₂(NH₃)}] (6), [{Co}–CN–{cis-PtCl₂(NH₃)}] (7) and [{Co}–CN–{PtCl₃}]⁻ (8) in good yields. Spectroscopic analyses for all compounds and X-ray structure elucidation for 5 and 7 confirmed their authenticity and the presence of the central “Co–CN–Pt” motif. Applicability of these heterodinuclear conjugates depends primarily on serum stability. Whereas 6 and 8 transmetallated rapidly to bovine serum albumin proteins, compounds 5 and 7 were reasonably stable. Around 20% of cyanocobalamin could be detected after 48 h, while the remaining 80% was still the respective vitamin B₁₂ conjugates. Release of the platinum complexes from vitamin B₁₂ is driven by intracellular reduction of Co(III) to Co(II) to Co(I) and subsequent adenosylation by the adenosyltransferase CobA. Despite bearing a rather large metal complex on the β-axial position, the cobamides in 5 and 7 are recognized by the corrinoid adenosyltransferase enzyme that catalyzes the formation of the organometallic C–Co bond present in adenosylcobalamin after release of the Pt(II) complexes. Thus, vitamin B₁₂ can potentially be used for delivering metal-containing compounds into cells. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.