Species Distribution Curves Research Articles

Complex-Forming Properties of the Anti-Inflammatory Sialorphin Derivative Palmitic Acid-Lysine-Lysine-Glutamine-Histidine-Asparagine-Proline-Arginine with Cu(II) Ions in an Aqueous Solution.

The present work describes the complexation of the anti-inflammatory sialorphin derivative Pal-Lys-Lys-Gln-His-Asn-Pro-Arg (palmitic acid-lysine-lysine-glutamine-histidine-asparagine-proline-arginine) with Cu(II) ions in an aqueous solution, at a temperature of 25.0 ± 0.1 °C, over the whole pH range. The complexing properties were characterized by potentiometric and UV-Vis spectrophotometric methods. The potentiometric method was used to calculate the logarithms of the overall stability constants (log β) and the values of the stepwise dissociation constants (pKa) of the studied complexes. The percentage of each species formed in an aqueous solution was estimated from the species distribution curve as a function of pH. The absorbance (A) and molar absorption coefficient (ε) values for the Cu(II)-sialorphin derivative system were determined with UV-Vis spectroscopy. Our studies indicate that the sialorphin derivative forms stable complexes with Cu(II) ions, which may lead to future biological and therapeutic applications.

The protolytic properties of the sialorphin derivative Pal-Lys-Lys-Gln-His-Asn-Pro-Arg in an aqueous solution

The present work describes the protolytic properties of the sialorphin derivative (Pal-Lys- Lys-Gln-His-Asn-Pro-Arg) in an aqueous solution, at constant temperature 25.0 ± 0.1 °C and ionic strength 0.1 M (NaClO4), over the entire pH range. The overall protonation constants of the studied ligand were calculated by the potentiometric method. The logarithm values of the stepwise dissociation constants of the sialorphin derivative were found to be: [LH] = 11.88 ± 0.09, [LH2]+ = 9.96 ± 0.03, [LH3]2+ = 8.71 ± 0.03, [LH4]3+ = 5.49 ± 0.03, [LH5]4+ = 2.82 ± 0.03. The percentage of each species formed in an aqueous solution was estimated from the species distribution curves as a function of pH (82% [LH] at pH 11.0, 68% [LH2]+ at pH 9.3, 95% [LH3]2+ at pH 7.1, 90% [LH4]3+ at pH 4.2, 85% [LH5]4+ at pH  2.0). The five protonation constants indicate that the studied ligand can be very active in forming complexes with different metal ions. In addition, the sialorphin derivative can also be tested as a bioactive material, which warrants its further in vitro and in vivo studies.

COMPLEX-FORMING PROPERTIES OF SYNTHETIC PYRIMIDINONE NUCLEOSIDES WITH Ag(I) IONS IN AN AQUEOUS SOLUTION

The present work describes the complexation properties of two synthetic nucleosides (used as ligands), i.e. β-D-ribofuranoside-4-pyrimidinone (ribo) and β-D-deoxyribofuranoside-4-pyrimidinone (deoxy) with Ag(I) ions in aqueous solution. The stability constants of the studied coordination entities were calculated potentiometrically. The logarithm values of the overall stability constants of the coordination entities were found to be [Ag-deoxy]+ = 3.07, [Ag-(deoxy)2]+ = 6.90, [Ag-ribo]+ = 3.00 and [Ag-(ribo)2]+ = 6.63. Based on the species distribution curves as a function of CL/CM, the percentage of each coordination entity formed in aqueous solution was estimated (12 % [Ag-deoxy]+; 36 % [Ag-(deoxy)2]+; 16 % [Ag-ribo]+; 26 % [Ag-(ribo)2]+). It was found that the ligands form coordination entities with Ag(I) ions with stoichiometric ratios M : L = 1 : 1 and 1 : 2. Such coordination entities may show potential antiviral, antibacterial, and antitumor properties, which warrants further in vitro and in vivo studies. They may also be of great interest for their application in nanobiotechnology.

Soil microarthropod distribution on the urban–rural gradient of Riga city: a study with robust sampling method application

AbstractTo address the new challenge of bringing more nature into the urban environment and developing adequate green infrastructure management methods, it is necessary to clarify the regularities of the distribution of the main ecosystem components—soil organism communities on the urban gradient. Microarthropods—collembolans and mites—are the most diverse soil animals and bioindicators of soil conditions. However, no suitable approaches exist so far to help reduce the high workload of soil zoological studies and make the data acquisition for soil assessment faster. To get closer to a solution to this problem, we propose a robust sampling approach using one pooled sample per site with surface area 58 cm2. This was tested in a microarthropod distribution study on the urban gradient of Riga city (Latvia) in six urban habitat types at 21 sites. The use of classical statistical methods for the processing of soil microarthropod data is limited because these data do not meet model requirements on which classical methods are based, first of all, conformity to the normal distribution. These problems are circumvented by bootstrapping methodology, which thanks to increasing computer performance now is implemented in the most modern program packages. We tested a set of such methods: one-way bootstrap-based analysis of variance, nonmetric multidimensional scaling (NMS), nonparametric multiplicative regression (NPMR), multi-response permutation procedure and Chao bootstrap-based rarefaction curves. NMS in combination with NPMR gave the best results providing statistically significant species distribution curves along the urban gradient which were broadly in line with species traits found by other studies.

Design, Synthesis and Application of Chemically Modified Amberlite XAD‐2 for Separation and Preconcentration of Pb(II)

AbstractA highly selective, efficient, green analytical method was developed to separate and preconcentrate trace amounts of lead from aqueous samples using gallic acid‐immobilized Amberlite XAD‐2. The method was designed by determining the stability constant of the Pb‐GA complex and plotting the chemical speciation diagram, which indicated strong complexation of Pb(II) with GA (log βPb‐GA=9.50). The species distribution curve for the Pb‐GA complex indicates almost 98 % complexation, forming Pb‐GA as the predominant species. Considering the strong interaction of GA toward the lead, GA was immobilized on the polymeric matrix of Amberlite XAD‐2, and the conditions for the uptake of lead from the aqueous solutions were optimized by both batch and column methods. The sorption capacity and concentration factor of modified Amberlite XAD‐2 for Pb(II) were found to be 4554 μg g−1 and100 respectively. The method was employed for the determination of lead in various wastewater samples. Molecular modeling studies were also performed on GA‐immobilized Amberlite XAD‐2 to determine the active structure of GA on the surface.

Influence of aprotic solvent on binary species of biomaterial with proteinogenic amino acid

The most unique, essential biomaterial in living beings is Calcium and their species are playing versatile roles in many biological functions. The proton containing and non proton forms of binary species of calcium ion with valine are ML+, MLH2+ and ML2H+ predicted at specific ion strength in disparate aprotic-aqua compositions. The stability constants of these species and their few influenced factors are examined. The differences in log β with respected to dielectric property of the aprotic solvent (1/D) and species formation equilibrium also discussed with reference to their percentages of species distribution curves.

The Effect of Deprotonation on the Reaction Kinetics of an Oxygen Scavenger Based on Gallic Acid

Oxygen scavengers based on gallic acid (GA) usually contain a base that establishes the alkaline conditions necessary for the humidity-induced scavenger reaction. Here we measured the effect of sodium carbonate (Na2CO3) and sodium hydroxide (NaOH) on the reaction kinetics of such scavengers. The time-dependent oxygen absorption of aqueous GA solutions (pH 2.9–13.8) was determined and the results were described using a second-order kinetic model. We calculated the degree of deprotonation (DoD) of GA in the solutions from the species distribution curves, and assessed the effect of the DoD on the reaction kinetics. This revealed that both the reaction rate coefficient k and the scavenger capacity n were significantly affected by the DoD. If the DoD fell below 0.25, there was no significant reaction. Although k increased with the DoD, n reached a maximum at DoD = 0.6–0.7. In principle, target DoD values can be achieved using any base, but a strong base is more efficient because lower quantities are required. In our experiments, the amount of Na2CO3 required to reach the maximum DoD was more than twice that of NaOH. Our results provide the basis for the functional design of active packaging systems incorporating optimized GA-based oxygen scavengers.

Effects of reduced sulfur on passive film properties of steam generator (SG) tubing: an overview

Purpose Corrosion is considered as one of the issues that threaten the safe operation of steam generator (SG) tubing. Some sulfur-related specie can cause corrosion degradation of SG tubing. Sulfur-induced corrosion of SG alloys in high temperature and high-pressure water is one of the most complicated processes. The purpose of this study is to study the effect of reduced sulphur on passive film properties of steam generator (SG) tubing. Design/methodology/approach In this paper, the effects of reduced sulfur on passive film properties of SG tubing were reviewed from the aspects of thermodynamic calculations and experimental. Findings Thermodynamic calculations are mainly presented by E-pH diagrams, volt equivalent diagrams and species distribution curves. The stability of sulphur species highly depends on temperature, solution pH, and electrochemical potential. Experimental data indicated that reduced sulfur species can interact with the passive film, which led to changes in film thickness, film structure, semiconductivity and pitting growth rate. Originality/value The state-of-the-art discussed in this paper gives basis for resolving engineering problems regarding with sulfur-induced corrosion.

Coordination, redox properties and SOD activity of Cu(II) complexes of multihistidine peptides

The results of electrochemical and SOD activity measurements of such copper(II) complexes of terminally protected multihistidine peptides that may mimic the active site of CuZnSOD enzyme are submitted and completed with solution equilibrium studies of some copper(II)-ligand systems. The equilibrium data confirm that the thermodynamic stabilities increase with the increasing number of histidyl residues in the amino acid sequence, the stability order, however, can be finely tuned by the number and quality of amino acids between histidine residues. Based on the cyclic voltammetric studies we concluded that the formal reduction potential values of imidazole nitrogen coordinated complexes decrease with the increasing number of imidazole donor atoms in the coordination sphere. However, the redox parameters of [CuH−1L]+ and [CuH−2L] complexes containing amide nitrogen coordination can be determined as well. All formal potential values of [CuL]2+, [CuH−1L]+ and [CuH−2L] complexes fall in the middle potential range of SOD activity. Finally, after the detailed analysis of species distribution curves based upon the equilibrium data SOD activity of copper(II) containing systems at two pH (pH=6.8 and 7.4) were determined. The imidazole coordinated [CuL]2+ complexes of the multihistidine peptide containing the HXH sequence exhibit the most significant activity, but the presence of amide nitrogen coordinated species with slightly distorted geometry could considerably contribute to the SOD activity.

CO2-Responsive wormlike micelles based on sodium oleate and hydrophobic tertiary amines

ABSTRACTWe reported two simple and novel CO2-responsive surfactant wormlike micellar systems consisting of commercial anionic surfactant sodium oleate (NaOA) and common hydrophobic tertiary amine N,N-dimethylcyclohexylamine (DMCHA), N,N-dimethylbenzylamine (DMBA). The conductivity, pH, and rheological measurements demonstrated the CO2-sensitive flowing behavior and property, which were attributed to the spherical-wormlike micelles transition, verified by cryogenic transmission electron microscopy (Cryo-TEM) and dynamic laser light scattering (DLS) measurements. Moreover, the transition can be easily cycled more than three times without deterioration of viscosity. Combined with the species distribution curve and 1H NMR spectra, a mechanism of the intermolecular electrostatic interaction and hydrophobic effects was proposed.

SOLUTION STATE AND THERMODYNAMIC STUDIES ON BINARY COMPLEXES OF THE TH(IV) WITH SOME HYDROXY ACIDS

A Solution state study on complexation behavior of Th(IV) with hydroxy acids (malic acid, lactic acid, salicylic acid and mandelic acid) has been carried out using the Irving–Rossotti titration technique in aqueous media and mixed media at different temperatures and ionic strengths. The dissociation constant and stability constants (logβ) of the resulting complexes have been calculated with the Fortran IV program BEST using the method of least squares and considering the BESTFIT model. Species distribution curves of complexes have been plotted as function of pH using the SPEPLOT Fortran IV program to visualize various species in equilibrium in the pH range 2–14. Thermodynamic properties were calculated and negative ∆G, ∆H and ∆S values indicated that complex formation is a spontaneous and exothermic process at these temperatures. To understand more about these equilibriums, the stability of these complexes was studied in presence of various systemic errors.

Effect of imidazole on biomimetic cyclohexane oxidation by first-, second-, and third-generation manganese porphyrins using PhIO and PhI(OAc)2 as oxidants

In this work, spectrophotometric titrations of first- (MnIIITPPCl), second- (MnIIIAPTPPCl and MnIIIT4CMPPCl), and third- (MnIIIBr9APTPPCl and MnIIIBr8T4CMPPCl) generation manganese(III) porphyrins ([MnIIIP]+) were carried out in order to investigate the axial coordination equilibrium between imidazole (Im) and these metalloporphyrin complexes. Cyclohexane oxidation by PhIO or PhI(OAc)2 catalyzed by the aforementioned [MnIIIP]+, in the presence of various [MnIIIP]+/Im molar ratios, was investigated as a means to study the contribution that the penta-, [MnIIIP(Im)]+, or hexacoordinate, [MnIIIP(Im)2]+, species at equilibrium may exert into catalyst efficiency and oxidative stability. The computational program SQUAD was used to analyze the spectrophotometric data and calculate the equilibrium constants used by program HySS to generate the species distribution curves for the various [MnIIIP]+/Im systems. In general, higher catalytic efficiency in the PhIO systems was achieved with the use of imidazole ranging from 1:0.5 to 1:5 [MnIIIP]+/Im ratio, depending on the nature of [MnIIIP]+. The catalytic systems with PhI(OAc)2 as oxidant were more sensitive to Im addition, and optimum yields were achieved with lower [MnIIIP]+/Im ratio (up to 1:1). It is noteworthy that the presence of imidazole reduced the usual instability of the third-generation catalyst MnIIIBr8T4CMPPCl toward oxidative destruction by PhI(OAc)2, but did not exert such a protective effect in the PhIO oxidations.

Chemical Distribution and Structure of Quaternary Metal Chelates in Aqueous Solution Involving Asparagine and Uracil

Ternary and quaternary metal chelates of some biologically important bivalent transition metal ions viz. Co(II), Ni(II), Cu(II) and Zn(II) formed with biologically significant ligands, viz. Asparagine (A) and Uracil (B) have been investigated. The relevant stability constants and complexation equilibria of quaternary metal chelates in solution at 30±1 oC and at constant ionic strength (I=0.1M NaNO3) have been studied potentiometrically. Equilibrium concentrations of all species in multimetal-multiligand system, concentration of each metal and complexing agent have been calculated. Species distribution curves are obtained by plotting percent (%) concentration of the species obtained through SCOGS computer technique against pH. The distribution curves are finally sketched by running the computer program ORIGIN 4.0. The metal ligand formation constant of MA, MB, MAB and M1M2AB type of complexes follow Irving William order. The order of stability constants of quaternary systems has been observed and solution structures of metal complexes with said ligands have been discussed.

Ecological responses of plant species and communities to climate warming: upward shift or range filling processes?

The fate of alpine species in response to climate warming is still unclear. We analyze effects of climate warming on the composition of alpine plants communities and unravel the range filling of communities within a belt from long-term true upward shift processes. In the European Alps we re-sampled in 2003 the vegetation at sites studied in 1953 and analyzed the changes at intra- and inter-community level. Since 1953 all communities experienced a high species turnover, leading to an overall increase in species richness as new species exceeded species losses. The dominant species mainly declined allowing the potential expansion of competitors and/or of new species. The main recruitment sources are neighbor communities within the same elevation belt performing biotic exchanges with other plant communities in the same altitudinal belts. The changes of species distribution curves with elevation emphasized that more than half of the most widespread persisting species exhibited downward shifts instead of upward shifts. Upward shifts from lower elevation belts and of nonnative species were very limited. One third of the persisting species declined and could be used as a proxy to measure the extinction debt. Therefore the fate of plant communities will depend on the ability of the original species to persist and fill the available ecological gaps. Species persistence may be crucial in developing adaptation and environmental protection strategies.

A Solution State Study of the Complexation and Thermodynamic Parameters of Binary Complexes of the Inner Transition Metals with Piracetam in Aqueous and Mixed Solvents

An equilibrium study on complexation behavior of the inner transition metals, where M = {Y(III), La(III, Ce(III, Pr(III), Nd(III), Sm(III), Gd(III), Dy(III), Th(IV)} and L = piracetam, has been carried out using the Irving–Rossotti titration technique in aqueous media at different temperatures and ionic strengths. The protonation constant and stability constants (log10 β n ) of the resulting inner metal–ligand complexes have been calculated with the Fortran IV program BEST using the method of least squares and considering the BESTFIT model. Thermodynamic parameters were also evaluated, yielding negative ΔG°, ΔH° and positive ΔS° values that indicate complex formation is favorable at ordinary temperatures. Species distribution curves of complexes have been plotted as function of pH using the SPEPLOT FORTRAN IV program to visualize the presence of various species in equilibrium in the pH range 2–6. To understand more about these equilibria, the stability of these complexes was studied at different percentages of solvent (10–30 % v/v) in different aqua-organic solvent mixtures with methanol, ethanol, 1-butanol and dimethylformamide.

Protonation Equilibria of Carminic Acid and Stability Constants of Its Complexes with Some Divalent Metal Ions in Aqueous Solution

The dissociation constants of carminic acid (7-D-glucopyronosyl-3,5,6,8-tetra- hydroxy-1-methyl-9,10-dioxo-anthracene-2-carboxylic acid) (CA), together with the stability constants of its Cu(II), Zn(II), Ni(II), Co(II) and Hg(II) complexes, were studied potentiometrically in aqueous medium at 25.0 (1) °C, and at the ionic background of 0.1 mol⋅dm−3 of NaCl, and determined with the SUPERQUAD computer program. It has been observed that carminic acid has five dissociation constants, and for H5L their values are 3.39 (7), 5.78 (7), 8.35 (7), 10.27 (7), and 11.51 (7). This ligand behaves as a bi-dentate ligand, and the carboxyl and the ortho hydroxy groups of the ligand coordinate to the metal ions. Various metal complexes were produced in solution under the experimental conditions, for each metal ion used, including hydrolyzed species. The species distribution curves of the complexes formed in the solution were calculated and reviewed. The stability of the complexes was found to follow the order: Cu(II) > Zn(II) > Ni(II) > Co(II) > Hg(II).

A Solution Study of the Interaction of the Cu(II) Ions with HisGlyGlyTrp Tetrapeptide and Its Evaluation as Superoxide Dismutase Mimetic Complex

The superoxide anion radical is a highly reactive toxic species produced during the metabolic processes. A number of copper (II) complexes with amino acids and peptides are known to show superoxide dismutase (SOD) like activity. The design and application of synthetic low molecular weight metal complexes as SOD mimics have received considerable attention during the last decade. A variety of di- and tri-peptides containing histidyl residue in different positions have been employed to bind Cu(II) and to show the activity. But reports on Cu(II) complex with tetra-peptide having histidine amino acid in this regard are limited. As the HGGGW peptide having His at its N-terminal is reported to be a potential moiety for Cu(2+) binding, in the present work the synthesis of HisGlyGlyTrp peptide and its complexation with copper (II) ions has been reported. The interaction of synthesized peptide with Cu(II) was studied by electron spray ionization-mass spectrometer (ESI-MS) and UV-Vis spectroscopic methods. The species distribution was studied by combined spectrophotometric and potentiometric methods. The studies were performed at 25 ± 0.1 °C with constant ionic strength (µ = 0.1 M NaNO(3)) in aqueous solution using Bjerrum-Calvin's pH-titration technique as adopted by Irving and Rossotti for binary systems. The solution studies suggested that the pH of the medium play important role in the different species formation of the copper complexes. Species distribution curves indicate that Cu complexation takes place at all physiological pH values from 3-11. The resultant copper (II) peptide complex at physiological pH was tested for superoxide dismutase activity using standard NBT method. The complex has SOD activity with the IC(50) value of 1.32 µM.

ChemInform Abstract: Phosphate Anion Binding by Macrocyclic Dinucleating Ligands and Their Metal Complexes

Abstract Four closely related dinuclear macrocyclic ligands are described: 1,13-dioxa-4,7,10,16,19,11-hexaazacyclotetracosane (OBISDIEN), 3,6,9,17,20,23-hexaazatricyclo[23.3.1.1 11,15 ]triconta-(1(29),11(30),12,14,25,27-hexaaza (BMXD), 3,6,9,16,19,22-hexaaza-27-28-dioxatricyclo-[22.2.1.1 11,14 ]-octacos-1,(26),11,13,24-tetraene (BFBD), and 3,6,9,17,20,23,29,30-octaazatri-cyclo[23.3.1.1 11,15 ]triconta-1(29),11,13,15,25,27-hexaene (BPBD). These macrocycles may be considered hexaaza ligands with two diethylenetriamine moieties bound together by bridging groups, except for BPBD, for which the bridging groups are pyridine rings, making this an octaaza ligand; however the remaining six nitrogens are analogous to those in OBISDIEN, BFBD, and BMXD. The syntheses of these ligands are outlined and compared. Each ligand forms six protonated species, which bind anions through hydrogen bonding and coulombic forces, the strength of binding increases with the number of protons, corresponding to an increase in the number of hydrogen bonds and to an increase in the coulombic attractive forces. The binding of these macrocyclic ligands to the nucleotides ATP, ADP, and AMP and their inorganic analogs, triphosphate, pyrophosphate, and phosphate, are described in detail, with equilibrium constants given for each species formed. The selectivities of the macrocyclic ligands for the nucleotides are described. With the aid of species distribution curves in competitive systems, the use of macrocycles for the separation of ATP, ADP, and AMP is suggested. It is shown that these aza macrocycles form both mono- and dinuclear metal complexes. The stabilities of the mononuclear ternary Cu(II) complexes are increased by hydrogen bonds between the coordinated phosphate and the protonated amino groups, with a maximum attained with three hydrogen bonds. For dinuclear ternary complexes protonation and deprotonation reduce the stability since the former reduces the number of amino donors coordinating the Cu(II), while the latter increases the number of hydroxide ions coordinated directly with the metal ion. The dinuclear Cu(II) complex shows a preference for the binding of ATP over ADP, especially in moderately alkaline solution.

Novel mono- and dinucleating ligands-containing artificial di- and tetrahistidine and their zinc(II) complexes as a structural phosphotriesterase models for the hydrolysis of p-nitrophenyl diphenylphosphate (p-NPDPP)

Two new artificial peptides with histidine side chains, namely N-methyl N,N′-bis(Im-bzl-l-histidylmethylestermethyl)amine L1 and N,N′,N″,N‴-tetrakis(Im-bzl-l-histidylmethyl-estermethyl)ethylene diamine L2 have been synthesized and were shown to form stable zinc complexes [L1Zn(H2O)2](ClO4)2 1 and [L2Zn2(H2O)3](ClO4)4 2, respectively. Solution studies (pH–1NMR titrations) of the ligand L2 in the presence of zinc ions were also reported. The catalytic activity of zinc complex species 1 and 2 as structural phosphotriestrase models were tested on the hydrolysis/detoxification of p-nitrohenl diphenylphosphate (p-NPDPP). From the correlation between the pH-rate profiles and the species distribution curves, the catalytically active species could be identified. On the basis of R-dependence as well as the rate acceleration of each complex, the possibility of cooperative action of zincs in dinuclear zinc complex is debated.

Dramatic Increase of Selectivity for Heavy Lanthanide(III) Cations by Tuning the Flexibility of Polydentate Chelators

Two novel octadentate ligands have been synthesized by attaching two terminal iminodiacetic groups to either 1,4-diazepane (BCAED) or piperazine (BCAEP) as central scaffold. The introduction of the seven- or six-membered ring into the ligand backbone is expected to modify their overall flexibility and then to affect the stability of the corresponding lanthanide(III) complexes. In this work, thermodynamic stability data are determined for the formation of the complexes of BCAED and BCAEP with La(3+), Nd(3+), Eu(3+), Gd(3+), Ho(3+), and Lu(3+). The ligand BCAED shows a strong binding affinity for Lu(3+) (logK = 20.99), moderate for Gd(3+) (logK = 17.15) and rather weak for La(3+) (logK = 12.77). Thus, the variation of logK across the Ln series assumes the remarkable value of 8.22, the largest so far reported. This points to a predominant role of a suitable size match between the metal ion and the ligand cavity, determined by its structure and flexibility. The ligand BCAEP forms less stable complexes with lanthanide(III) cations although it retains a good selectivity (DeltalogK(La-Lu) = 5.66). The Gd(III) complexes have been investigated in aqueous solution by measuring their relaxivity as a function of pH, at 20 MHz and 25 degrees C. The results can be interpreted very well in terms of the species distribution curves calculated from the thermodynamic data and indicate that in these complexes Gd(3+) is octacoordinated, without any bound water molecule. This coordination geometry is maintained in the solid state as shown by the X-ray crystal structure of [Na(H(2)O)(2)][Gd(BCAED)] where the metal ion is at the center of a bicapped-trigonal prism. Finally, the (13)C NMR spectra (9.4 T, 25 degrees C) of the diamagnetic La(3+), Y(3+), and Lu(3+) complexes show that a pronounced stereochemical rigidity is associated with the thermodynamically more stable complexes.