Aromatic-ring Stacking Interactions Research Articles

Synthesis, Structural Characterization, and Hirschfeld Surface Analysis of a New Cu(II) Complex and Its Role in Photocatalytic CO2 Reduction.

A new Cu(II) complex, [CuL1L2(CH3COO)2(H2O)]·H2O, was synthesized by the reaction of Cu(CH3COO)2·H2O, 6-phenylpyridine-2-carboxylic acid (HL1), and 4-[5-(pyridin-4-yl)-1,3,4-oxadiazol-2-yl]pyridine (L2) in ethanol-water (v:v = 1:1) solution. The Cu(II) complex was characterized using elemental analysis, IR, UV-vis, TG-DTA, and single-crystal X-ray analysis. The fluorescence properties of the copper complex were also evaluated. The structural analysis results show that the Cu(II) complex crystallizes in the triclinic system with space group P-1. The Cu(II) ion in the complex is five-coordinated with one O atom (O2) and one N atom (N1) from one 6-phenylpyridine-2-carboxylate ligand (L1), one N atom (N2) from 4-[5-(pyridin-4-yl)-1,3,4-oxadiazol-2-yl]pyridine ligand (L2), one O atom (O4) from acetate, and one O atom (O5) from a coordinated water molecule, and it adopts a distorted trigonal bipyramidal geometry. Cu(II) complex molecules form a two-dimensional layer structure through intramolecular and intermolecular O-H…O hydrogen bonding. The two-dimensional layer structures further form a three-dimensional network structure by π-π stacking interactions of aromatic rings. The analysis of the Hirschfeld surface of the Cu(II) complex shows that the H…H contacts made the most significant contribution (46.6%) to the Hirschfeld surface, followed by O…H/H…O, N…H/H…N and C…H/H…C contacts with contributions of 14.2%, 13.8%, and 10.2%, respectively. In addition, the photocatalytic CO2 reduction using Cu(II) complex as a catalyst is investigated under UV-vis light irradiation. The findings reveal that the main product is CO, with a yield of 10.34 μmol/g and a selectivity of 89.4% after three hours.

Supramolecular construction of a cyclobutane ring system with four different substituents in the solid state

Methods to form cyclobutane rings by an intermolecular [2 + 2] cross-photoreaction (CPR) with four different substituents are rare. These reactions are typically performed in the liquid phase, involve multiple steps, and generate product mixtures. Here, we report a CPR that generates a cyclobutane ring with four different aryl substituents. The CPR occurs quantitatively, without side products, and without a need for product purification. Generally, we demonstrate how face-to-face stacking interactions of aromatic rings can be exploited in the process of cocrystallization and the field of crystal engineering to stack and align unsymmetrical alkenes in CPRs to afford chiral cyclobutanes with up to four different aryl groups via binary cocrystals. Overall, we expect the process herein to be useful to generate chiral carbon scaffolds, which is important given the presence of four-membered carbocyclic rings as structural units in biological compounds and materials science.

Manifestation of intermolecular interactions in FTIR spectra of methylene blue molecules

Fourier-transform infrared (FTIR) spectra of cationic dye methylene blue (MB) were analyzed in its free form (gas-phase), hydrated crystalline state, and dried MB solution. A good agreement of experimental FTIR spectra with the results of theoretical calculations of MB vibration modes was observed. The calculations were performed by means of the density functional theory (DFT/B3LYP/6–31+G(d)), using Gaussian-03 software package. Vibrational bands of the N+(CH3)2 (1640–1650cm−1) and C-S-C (625-615, 1095cm−1) functional groups are characteristic of the presence of MB monomers in the studied samples. The participation of the N+(CH3)2 functional group in the formation of one bridging H-bond with OH groups of the solvent has been confirmed. The decrease in intensity of these bands with the increase of the fraction of dimers and H-aggregates of MB was observed. It was concluded that the configuration and primary localization of the charge in the heterocycle of ionic form MB+ are changed in the case of formation of dimers and H-aggregates. For samples with a dominant fraction of dimers and H-aggregates, the occurrence of intermolecular H-bonds of the type Nhet…Н-O⋯S+het was found. The occurrence of peaks at 1339–1356, 1481–1491, and 885cm−1 was observed. It was concluded that there is formation of dimers and H-aggregates due to interaction of sulfur and nitrogen atoms with OH groups of water molecules. Moreover, the manifestation of π-π stacking interactions of aromatic rings of the heterocycle of ionic form MB+ during self-association was determined on the basis of data on the behavior of the peaks of heterocyclic of СН (3050–3015cm−1), СС, СN (1600–1595cm−1), СС, and СN (1564–1546cm−1) vibrations.

Syntheses, Crystal Structures, and Magnetic Properties of Metal–Organic Hybrid Materials of Mn(II)/Co(II): Three-Fold Interpenetrated α-Polonium-like Network in One of Them

Three new 1,4-phenylenediacrylate bridged Mn(II) and Co(II) complexes of molecular formulas {[Mn2(ppda)(phen)4(H2O)2](ppda)2(H2O)} (1), {[Co(ppda)(dpyo)(H2O)3]·4(H2O)}n (2), and {[Co(ppda)(bpe)]·(0.5H2O)}n (3) [ppda = 1,4-phenylenediacrylate; phen = 1,10-phenanthroline; dpyo = 4,4′-dipyridyl N,N′-dioxide; bpe = 1,2-bis(4-pyridyl)ethane] have been synthesized and characterized by elemental analysis, IR spectra, single-crystal X-ray diffraction studies, and low-temperature magnetic measurements. The structural determination reveals that complex 1 is a discrete dinuclear species, 2 is a 1D polymeric chain, while 3 is a three-fold interpenetrated α-polonium network. Hydrogen-bonding interactions, formed by coordinated and/or lattice water molecules with ppda oxygen and π–π stacking interactions of aromatic rings, lead to a 3D supramolecular architecture in both complexes 1 and 2. Low-temperature magnetic study shows antiferromagnetic coupling in all the complexes. In addition, their electronic and fluorescent spectral properties have been investigated.

Left-Handed Dimer of EphA2 Transmembrane Domain: Helix Packing Diversity among Receptor Tyrosine Kinases

The Eph receptor tyrosine kinases and their membrane-bound ephrin ligands control a diverse array of cell-cell interactions in the developing and adult organisms. During signal transduction across plasma membrane, Eph receptors, like other receptor tyrosine kinases, are involved in lateral dimerization and subsequent oligomerization presumably with proper assembly of their single-span transmembrane domains. Spatial structure of dimeric transmembrane domain of EphA2 receptor embedded into lipid bicelle was obtained by solution NMR, showing a left-handed parallel packing of the transmembrane helices (535–559)2. The helices interact through the extended heptad repeat motif L535X3G539X2A542X3V546X2L549 assisted by intermolecular stacking interactions of aromatic rings of (FF557)2, whereas the characteristic tandem GG4-like motif A536X3G540X3G544 is not used, enabling another mode of helix-helix association. Importantly, a similar motif AX3GX3G as was found is responsible for right-handed dimerization of transmembrane domain of the EphA1 receptor. These findings serve as an instructive example of the diversity of transmembrane domain formation within the same family of protein kinases and seem to favor the assumption that the so-called rotation-coupled activation mechanism may take place during the Eph receptor signaling. A possible role of membrane lipid rafts in relation to Eph transmembrane domain oligomerization and Eph signal transduction was also discussed.

Associative π−π Interactions of Condensed Aromatic Compounds with Vanadyl or Nickel Porphyrin Complexes Are Not Observed in the Organic Phase

Association of vanadium and nickel petroporphyrin compounds with aromatic groups is commonly invoked as an important mechanism for aggregation of asphaltenes. In this study, the interaction of vanadium and nickel octaethylporphyrin with condensed aromatic compounds and bridged aromatic compounds was investigated. Neither UV−vis nor fluorescence spectroscopy detect any interactions between porphyrins and aromatic ring groups in either the ground or excited states. The lack of detectable binding of these model compounds suggests that the association of the majority of the vanadium and nickel petroporphyrins in crude oils with the asphaltene fraction may be due to other functionality appended to the porphyrin ring, rather than favorable π−π stacking interactions of aromatic rings with the porphyrin core itself.

Interaction between ligands in ternary complexes of cadmium(II) with phenanthroline and phenylalkylmalonates in solution

The formation and stability of the ternary complexes of cadmium(II) with phenanthroline and malonate, phenylmalonate, benzylmalonate, 2-phenylethylmalonate, and 3-phenylpropylmalonate are studied by the potentiometric pH-titration technique in 40% (v/v) dioxane-water at 25°C and 0.1 mol/l ionic strength (NaNO3). In all systems, there are stable ternary complexes. The complexes possess enhanced stability relative to their statistical values. The possible reasons that lead to these results are discussed in terms of π-electron back donation from metal ions to ligands and the intramolecular ligand-ligand aromatic ring stacking interaction. 1H NMR measurements are used to demonstrate the aromatic ring-stacking interaction. The relative contribution of each interactions to the enhanced stability is also estimated. The results show that there are two weak interactions simultaneously observed in these ternary complexes. The interaction strength of aromatic ring stacking in the ternary complexes increases in the phenylalkylmalonate ligand order: L0 < L1 < L2 ≈ L3 (L0 = phenylmalonate, L1 = benzylmalonate, L2 = 2-phenylethylmalonate, L3 = 3-phenylpropylmalonate).

Di-mu-hydroxo-bis[aqua(1,10-phen-anthroline-kappa2N,N')copper(II)] trans-bis(2-([(E)-5-formyl-2-oxido-kappaO-benzylidene]amino-kappaN)ethanesulfonato(2-))copper(II) hexahydrate.

The title compound, [Cu(2)(OH)(2)(C(12)H(8)N(2))(2)(H(2)O)(2)][Cu(C(10)H(9)NO(5)S)(2)].6H(2)O, is comprised of a copper-centred complex cation and a copper-centred complex anion; the cation lies about an inversion centre and in the anion the Cu atom lies on an inversion centre. In the doubly charged bridged dicopper cation, each Cu centre has distorted square-pyramidal geometry. In the square-planar dianion, two sulfonate ligands are trans coordinated to the Cu atom via a deprotonated hydroxyl O atom and an imine N atom, forming two six-membered chelate rings. The structure is stabilized by an extensive hydrogen-bond system and aromatic-ring stacking interactions.

Mutational Analysis of Tyr-501 of HIV-1 Reverse Transcriptase: EFFECTS ON RIBONUCLEASE H ACTIVITY AND INHIBITION OF THIS ACTIVITY BY N-ACYLHYDRAZONES

N-(4-tert-Butylbenzoyl)-2-hydroxynaphthaldehyde hydrazone (BBNH) is a potent inhibitor of the ribonuclease H (RNase H) activity of human immunodeficiency virus (HIV)-1 reverse transcriptase (RT). Molecular modeling predicted that BBNH binds to the HIV-1 RT RNase H active site via two major interactions, coordination to the metal ion cofactor (Mg(2+) or Mn(2+)) in the enzyme active site and aromatic ring-stacking interaction between the naphthyl ring of BBNH and amino acid Tyr-501. The latter residue equivalent is conserved in virtually all RNases H, suggesting the need for an aromatic or pi-stacking interaction in this region. To assess the importance of Tyr-501 in the binding of BBNH for the inhibition of RT RNase H activity, we used site-specific mutagenesis to generate RT with a variety of substitutions at this position. Most substitutions resulted virtually in a complete loss of RNase H activity. However, three mutants, Y501F, Y501W, and Y501R, possessed RNase H activities comparable with wild-type enzyme. Whereas BBNH inhibited Y501F RT RNase H activity with potency equivalent to wild-type RT, the Y501W mutant showed a 6-fold resistance to inhibition by BBNH, and the Y501R mutant was completely resistant to inhibition by BBNH. The replication "fitness" of HIV molecular clones with the Y501W or Y510R mutation was significantly compromised compared with wild-type virus. Importantly, BBNH was an effective inhibitor of the DNA polymerase activity of all Y501X mutants tested. Our results highlight the importance of Tyr-501 in RT RNase H activity and in N-acylhydrazone inhibitor binding and suggest that drugs that target critical residues in HIV-1 proteins may be a useful approach in new antiviral development.

Stacking interaction in metal complexes with compositions of DNA and heteroaromatic N-bases

The current development in the intramolecular aromatic-ring stacking interactions in the complexes with compositions of DNA and heteroaromatic N-bases has been reviewed to a great extent, especially the significant contributions in several important systems about ternary mixed-ligand complexes, including nucleotide-metal ion-polyaromatic amine, amino acid-metal ion-polyaromatic amine, nucleotide-metal ion-pyridine-like aromatic amine, nucleotide-metal ion-amino acid, nucleotide-metal ion-nucleic acid base, nucleic acid base-metal ion, and the important factors affecting the intramolecular aromatic-ring stacking interactions in the complexes. Based on the study of stacking interaction in the complexes, the mechanism of interaction between DNA molecules and complexes of heteroaromatic N-bases has been established, which is crucial for the design and synthesis of the complexes acting as molecular devices of DNA.

Ternary Complexes of Cadmium(II) with Phenylalanine and Phenylalkylmalonates in Dioxane–Water Mixtures

The formation and stability of ternary complexes of cadmium(II) with phenylalanine and malonate, phenylmalonate benzylmalonate, 2-phenylethylmalonate, and 3-phenylpropylmalonate, have been studied by the potentiometric pH titration technique in 40% (v/v) dioxane–water at 25°C and 0.10—ionic strength (NaNO3). All systems give stable ternary complexes, and the Δ log K and δK values demonstrated that the complexes possess enhanced stability. The possible reasons that lead to these results are discussed in terms of π–electron back donation from metal ions to ligands and intramolecular ligand–ligand aromatic ring-stacking interactions present in the complexes. The relative contribution of each interactions to the enhanced stability have also been evaluated.

Preparation, characterization, and the crystal structure of the inhibitor ZK-807834 (CI-1031) complexed with factor Xa.

Factor Xa plays a critical role in the formation of blood clots. This serine protease catalyzes the conversion of prothrombin to thrombin, the first joint step that links the intrinsic and extrinsic coagulation pathways. There is considerable interest in the development of factor Xa inhibitors for the intervention in thrombic diseases. This paper presents the structure of the inhibitor ZK-807834, also known as CI-1031, bound to factor Xa and provides the details of the protein purification and crystallization. Results from mass spectrometry indicate that the factor Xa underwent autolysis during crystallization and the first EGF-like domain was cleaved from the protein. The crystal structure of the complex shows that the amidine of ZK-807834 forms a salt bridge with Asp189 in the S1 pocket and the basic imidazoline fits snugly into the S4 site. The central pyridine ring provides a fairly rigid linker between these groups. This rigidity helps minimize entropic losses during binding. In addition, the structure reveals new interactions that were not found in the previous factor Xa/inhibitor complexes. ZK-807834 forms a strong hydrogen bond between an ionized 2-hydroxy group and Ser195 of factor Xa. There is also an aromatic ring-stacking interaction between the inhibitor and Trp215 in the S4 pocket. These interactions contribute to both the potency of this compound (K(I) = 0.11 nM) and the >2500-fold selectivity against homologous serine proteases such as trypsin.

Design, Synthesis, and Crystal Structure of a cis-Configuration N(2)S(2)-Coordinated Palladium(II) Complex: Role of the Intra- and Intermolecular Aromatic-Ring Stacking Interaction.

A cis-configuration bis(4,5-diazafluorene-9-one thiosemicarbazone) palladium trihydrate, which exhibits approximately five times SHG (second harmonic generation) efficiency of urea, has been designed, synthesized, and structurally characterized. The complex, C(24)H(16)N(10)PdS(2).3H(2)O, crystallizes in trigonal space group P3(1) with cell parameters a = 10.749(2) Å, c = 19.872(4) Å, V = 1988.3(6) Å(3), and Z = 3. The coordination geometry about the Pd(II) is surprisingly a cis-configuration square-planar formed by two imino nitrogen atoms N(3) and N(8) and two sulfur atoms S(1) and S(2) from two thiosemicarbazone ligands. Intramolecular contact analyses in the crystals and 1D and 2D (1)H NMR spectra in solution show that the cis-positioning of the two ligands was stabilized by the pi-pi interaction between the two delocalized diazafluorene moieties. Detailed crystal structure analyses demonstrate that both the intermolecular pi-pi stacking between the diazafluorene planes in different molecules and intermolecular hydrogen bonds between the water molecules and the deprotonated ligands might be the factors that influence the molecules to be packed in the acentric space group P3(1). The crystal structure of the free ligand was also reported here for comparison. The ligand, C(12)H(9)N(5)S, crystallizes in orthorhombic space group Pbca with cell parameters a = 8.432(2) Å, b = 15.427(3) Å, c = 17.387(3) Å, V = 2272.7(8) Å(3), and Z= 8. The thiosemaicarbazone moiety adopts an E configuration with N(1) cis to N(3).

Intramolecular aromatic-ring stacking interaction of ternary Zn(UTP)L2- complexes

Intramolecular aromatic-ring stacking interaction of ternary Zn(UTP)L2- complexes

Thermodynamic stereoselectivity assisted by weak interactions in metal complexes. Copper(II) ternary complexes of cyclo-L-histidyl-L-histidine and L- or D-amino acids in aqueous solution

The thermodynamic stereoselectivity and spectroscopic characteristics of copper(II) ternary complexes with L- or D-amino acids (Xaa) and cyclo-L-histidyl-L-histidine, cyclo(-His-His-), have been investigated. Ternary complex formation with L-amino acids is more enthalpy and less entropy favoured than that of the analogous D-amino acids. For aromatic amino acids these differnces increase going from phenylalanine to tryptophan, suggesting involvement of aromatic ring-stacking interactions between one of the dipeptide imidazoles and the Xaa aromatic side chain in the mixed-ligand complexes. Enthalpy changes are the driving force for the thermodynamic stereoselectivity. A detailed analysis of the CD spectra of these ternary systems resulted in a correlation between Δ(Δε), the difference between the molar CD coefficients at the wavelength where it is maximized, and the ΔG° and ΔH° values. On the basis of EPR spectra, it is excluded that the differences between the mixed-ligand complexes with L- or D-amino acids are due to a different degree of solvation. It is proposed that hydrophobic and ring-stacking interactions, responsible for stereoselectivity, are favoured by a certain degree of tetrahedral distortion of the tetragonal copper(II) chromophore formed by the two imidazole nitrogen donors of cyclo(-His-His-) and the nitrogen and carboxylate oxygen donors of the L- or D-amino acids.

Ternary complexes in solution. 45. Intramolecular aromatic-ring stacking interactions in dependence on the ligand structure, geometry of the coordination sphere of the metal ion, and solvent composition

Ternary complexes in solution. 45. Intramolecular aromatic-ring stacking interactions in dependence on the ligand structure, geometry of the coordination sphere of the metal ion, and solvent composition

Ternary complexes in solution. 44. Dependence of an intramolecular aromatic-ring stacking interaction in mixed-ligand copper(II) complexes in aqueous solution on the distance of the stacking moiety from the coordinating group and the solid-state structure of the ternary copper(II)-1,10-phenanthroline-phenylpropionate complex

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTTernary complexes in solution. 44. Dependence of an intramolecular aromatic-ring stacking interaction in mixed-ligand copper(II) complexes in aqueous solution on the distance of the stacking moiety from the coordinating group and the solid-state structure of the ternary copper(II)-1,10-phenanthroline-phenylpropionate complexErich Dubler, Ulrich K. Haering, Kurt H. Scheller, Philippe Baltzer, and Helmut SigelCite this: Inorg. Chem. 1984, 23, 23, 3785–3792Publication Date (Print):November 1, 1984Publication History Published online1 May 2002Published inissue 1 November 1984https://doi.org/10.1021/ic00191a024RIGHTS & PERMISSIONSArticle Views288Altmetric-Citations79LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts

Intramolecular hydrophobic and aromatic-ring stacking interactions in ternary complexes in solution

One possibility of influencing the stability and structure of mixed ligand complexes [1] is through the formation of intramolecular ligand-ligand bonds. Covalent bond formation is well-known, e.g., coordinated pyruvate and glycinate form a Schiff base within the coordination sphere of a metal ion [2]. Ionic bonds may be formed between oppositely charged side chains: several amino acids are predestinated for such electrostatic interactions [3, 4]. Hydrogen bond formation also seems possible [5] ▪ and is known to exist in solid mixed ligand complexes [6]. Aromatic-ring stacking is another very subtle possibility for an influence and equilibrium (1) may even be shifted to its right side, i.e. for the logarithm of the corresponding equilibrium constant, Δ log K M ▪ (eqn. 2) [1], positive values are observed. For example, for the Mg 2+ of Ca 2+ /1,10-phenanthroline (Phen)/adenosine 5′-triphosphate (AT 4− system Δ log K M ⋍ 0.5 due to the intramolecular stacking between the purine moiety of ATP 4− and the aromatic-ring system of Phen (studied by potentiometric pH-titrations, UV-difference spectrophotometry and 1H-NMR shift measurements) [7]. This leads to an intramolecular equilibrium (3) between, an ‘open’ and ‘closed’ isomer of the ternary complex ▪ M(A)(B). About 90% of Ca(Phen)(ATP) 2− exist in the ‘closed’ form [4], while for M(tryptophanate)(ATP) 3− where M 2+ = Mn 2+, Cu 2+ or Zn 2+ the corresponding percentages are 55, 41 and 76 [4]. Hydrophobic interactions are observed between the isopropyl residue of leucinate (Leu) and Phen; e.g. of Zn(Phen)(Leu) + exist about 30% in the ‘closed’ form [8]. Estimation for M(phenylalaninate)(norvalinate) complexes, where M 2+ = Co 2+, Ni 2+ or Cu 2+, give 21, 5 and 11%, respectively, for the isomer with an intramolecular hydrophobic interaction [4, 8]. Research supported by the Swiss National Science Foundation.

Binary and ternary complexes of metal ions, nucleoside 5′-monophosphates, and amino acids

The aromatic-ring stacking interactions between the indole moiety of L -tryptophan and the purine or pyrimidine moieties of AMP 2− or CMP 2+, respectively, were studied by 1H NMR spectroscopy. Under the influence of increasing concentrations of L -trytophan the resonance of H-2, H-8 and H-1′ of AMP 2− of a H-5 and H-6 of CMP 2− are shifted upfield. Computer curve fitting of the shift data gave the stability constants K (AMP)(H·Trp) (AMP) = 6.83 ± 0.81 M −1 and K (CMP)(H·Trp) CMP = 0.77 ± 0.35 M −1 (in D 2O; 27°C; I = 0.1 M) which show that the purine moiety forms the more stable stacks. Repetition of the experiments under conditions where the amino acid exists as the anion, i.e. as tryptophanate, gave K (AMP)(Trp) (AMP) = 2.24 ± 0.29 M −1 and K (CMP)(Trp) (CMP) = 0.14 ± 0.05 M −1 (in D 2O; 27°C; I = 0.1–0.15 M). Hence, it is evident that the formation of an ionic bridge between the negatively charged phosphate moiety of the nucleotides and the positively charged ammonium group of L -tryptophan favors the formation of the stack between the nucleic bases and the indole residue. In the case with AMP 2− the stability constant of this ion pair formation could be estimated, i.e. K IP ≅ 0.6 M −1 (in D 2O; 27°C; I = 0.1–0.15 M), a value which compares well with K IP = 1.35 ± 0.76 M −1 for the (CH 3) 4N +/Trp − interaction. Intramolecular equilibrium constants for the ion pair interaction within the stacked complex and for the stacking interaction within the ion pair complex are also estimated and discussed. Stability constant of metal ion complexes could only be determined with Ni 2+ as a precipitate formed with Cu 2+ and Zn 2+. The stability constants of the binary systems, Ni 2+/AMP 2−, Ni 2+/CMP 2−, Ni 2+/Trp −, and Ni 2+/Ala −, were determined by potentiometric pH-titrations. The somewhat enhanced stability of Ni(Trp) 2 compared with Ni(Ala) 2, and of Ni(AMP) 2 2− compared with Ni(CMP) 2 2− (for which no evidence of formation was observed) is explained by self-stacking within these binary 1:2 complexes. Based on the data of the binary systems, the following ternary systems were studied and the corresponding stability constants determined: Ni 2+/AMP 2−/Trp −, Ni 2+/AMP 2−/Ala −, Ni 2+/CMP 2−/Trp −, and Ni 2+/CMP 2−/Ala −. A most likely intramolecular purine/indole stack in the ternary Ni(AMP)(Trp) − complex is not significantly reflected in the measured stability constants. This observation and the implications of stacking with regard to the stability of ternary complexes, as well as the general importance of “bridged” stacking adducts for biological systems are briefly discussed.