Vladimir Simeon

Binding of nucleotides in water by phenanthridinium bis(intercaland) receptor molecules

The bis(phenanthridinium) receptor molecules 3–5 bind nucleotides in aqueous media by stacking interactions between cis-oriented receptor units and the intercalated nucleic base part of a nucleotide with stability constants in the range 105–106 dm3 mol–1, the highest measured to date for these type of receptors and substrates.

Ion association in aqueous solutions of strong electrolytes: a UV–Vis spectrometric and factor-analytical study

Ion association in aqueous solutions of varied concentrations of Co(NO3)2 and LiNO3 has been studied by means of UV–Vis absorption spectrometry assisted by principal-components and evolving-factor analyses. Formation of [Co(NO3)]+ and [LiNO3]0 associated ion pairs was detected at higher salt concentrations (>0.1 mol dm-3 for Co(NO3)2, >0.7 mol dm-3 for LiNO3). Absorption spectra of ion pairs were computed by means of EFA, as well as their equilibrium concentrations and formation constants in each solution. The fractions of ion pairs (relative to total salt concentration) amounted to ca. 0.6 ([Co(NO3)]+) or ca. 0.5 ([LiNO3]0), at the highest salt concentrations. Differences in the absorption spectra of ion pairs, suggesting different kinds of cation–anion interaction, were tentatively interpreted as a weak coordination in the case of [Co(NO3)]+ and as a classical electrostatic ion association (probably with intervening water molecule) in [LiNO3]0. No association was observed with Mg(NO3)2 solutions in the experimental concentration range (c⩽2 mol dm-3).

Raman spectra of aqueous solutions of strong electrolytes: evolving-factor analysis

Ion association in aqueous solutions of varied concentrations of LiNO3, Mg(NO3)2, Co(NO3)2, Li2SO4 and MgSO4 was studied by means of Raman spectrometry assisted by principal-components (PCA) and evolving-factor (EFA) analyses. Formation of one Raman-active associated species, {M···L(R)} (M=Li+, Mg2+, Co2+; L=NO3−, SO42−), was detected at higher salt concentrations (≳2 mol dm−3 for LiNO3, ≳1 mol dm−3 for Co(NO3)2, ≳0.2 mol dm−3 for Li2SO4, ≳0.5 mol dm−3 for MgSO4). Spectral profiles of L(aq) and {M···L(R)} species were computed by means of EFA, as well as their equilibrium concentrations in each solution. Maximum fractions of anions engaged in {M···L(R)} species amounted (at highest salt concentrations) to ≈0.6 (LiNO3), ≈0.4 (Co(NO3)2), ≈0.60 (Li2SO4) and ≈0.3 (MgSO4). Since, for a given salt concentration, the eqilibrium concentration of Raman-active species, [M···L(R)], was always lower than the concentration of analogous UV-active species, [M···L(UV)] (reported in a previous paper) it is suggested that {M···L(UV)} could be a precursor of {M···L(R)}. In all instances, apparent stability constants of {M···L(R)} species, falling into the range of (0.01≲K1/dm3 mol−1≲1), have a progressively upward trend with increasing salt concentration.

Existence of two forms of nitrate ion in dilute aqueous solutions. Thermodynamic parameters of interconversion

Two well-defined isosbestic points (at 265 and 313 nm) were found in the UV absorption spectra of dilute (0.06 mol dm−3) aqueous solutions of NaNO3, KNO3 and (NMe4)NO3, measured at 13 temperatures in the (10–70) °C range. This finding, together with the apparent cation-independence of the spectra (confirmed by several statistical tests) indicates the existence of a chemical equilibrium between two species differing in the structure of solvation shells: α-NO3− ⇌ β-NO3−. Assuming the constancy of standard conversion enthalpy, ΔrH°, in the experimental temperature range, a simple nonlinear programming algorithm, maximizing the linearity of the ln K° vs. 1/T relationship, was devised in order to compute optimized estimates of K° (T), ΔrH° and ΔrS°, as well as the individual spectra of α-NO3− and β-NO3− species. The following results were obtained: K° ranges from ≈0.08 (10 °C) to ≈0.25 (70 °C), ΔrH° = 15 kJ mol−1, ΔrS° = 31 J K−1 mol−1, λmax(α-NO3−) = 301 nm, emax(α-NO3−) = 7.8 cm2 mmol−1, λmax(β-NO3−) = 317 nm, emax(β-NO3−) = 5.7 cm2 mmol−1. Since the ΔrH° value falls inside the range of the energies of hydrogen-bond breaking and, in addition, ΔrS° > 0, it is hypothesized that the α → β conversion might include the rupture of one hydrogen bond (in the hydration shell) per one α-NO3− entity.

Visibility of peripheral journals through the science citation index

Abstract The effect of the inclusion of a journal into the SCI source journals selection on the journals “visibility” was studied by analyzing the number of independent citations of the articles published in two journals ( Croatica Chemica Acta (CCA) and Roczniki Chemii (RC)) in two periods. The “echo factor,” i.e. the citation count normalized by the size of CCA and RC publication source pools and of SCI citing pool, did not show any increase upon the inclusion of either of the two journals into SCI selection: for CCA it even showed a decrease and for RC remained essentially constant. A statistically significant increase in the SCI citing pool, that took place around 1976, was also noticed.

Principal components and Procrustean analyses of stripping voltammograms

Abstract Principal components analysis (PCA), applied to differential-pulse anodic stripping voltammograms of a series of solutions containing various amounts of Pb 2+ and Cd 2+ , revealed three principal components, accounted for by the presence of the two metal ions and of an appreciably curved baseline. The analysis of numerically simulated voltammograms showed that serious errors can arise from the use of incomplete target vectors unless a “tangent fit” procedure is used. By using so-modified voltammetric profiles of blank, lead and cadmium solutions as target vectors in the Procrustean analysis [target factor analysis (TFA)], it was possible to calculate accurately the concentrations of both metals in all solutions examined. Compared with the existing methods of evaluating voltammograms, the proposed PCA-TFA approach has the advantages of being global and, at the same time, simple (linear) and straightforward.

Voltammetric investigations of cadmium(II) complexes with glycine, di- and triglycine

Abstract Aqueous solutions containing Cd 2+ and glycine, diglycine or triglycine and KNO 3 (0.1 ml/1) supporting electrolyte were investigated by means of five voltammetric techniques; dc polarography, pulse and differential pulse polarography, differential-pulse anodic stripping voltammetry and square-wave voltammetry. Since cadmium reduction and oxidation were found to be reversible (with the techniques employed) it was concluded that these systems contained labile cadmium complexes. Thus, it was possible to adopt the well-known DeFord-Hume approach and to determine stability constants by using two (linear and non-linear) weighted regression treatments. Since a rather wide range of free-ligand concentrations was scanned, the presence of Cd(ligand) 3 complexes, which had not been observed so far, was inferred from the data for all the three ligands studied. Being the dominant species at and above physiological pH (>/ 7.4), these tris-ligand complexes might have some biological significance.

Transition metal complexes of N,N-dimethylthreonine: Stability and UV/Vis spectra

SummaryAcidity (dehydronation) constants of N,N-dimethylthreonine (DMT) and stability constants of its complexes with Cu+2, Ni+2, and Co+2 were determined in aqueous solution by means of potentiometric titration. UV/Vis spectra were also taken during the titration. It is suggested thatDMT acts as a bidentate ligand toward copper(II) by engaging either (a) amino and carboxyl groups (in [Cu(DMT)] and [Cu(DMT)2]), or, (b) upon dehydronation, amino and hydroxyl groups (in [Cu(DMT)H−1], [Cu(DMT)2H−1], and [Cu(DMT)2H−2]). It is suggested that the coordination in threoninato andallo-threoninato complexes is similar to that described under (a).ZusammenfassungDehydronierungskonstanten des N,N-Dimethylthreonins(DMT)sowie die Stabilitätskonstanten seiner Komplexe mit Co+2, Ni+2 und Cu+2 in wässriger Lössung wurden mittels potentiometrischer Titration bestimmt. Auch UV/Vis Spektren wurden während der Titration aufgenommen. Die Daten zeigen, daß in den [Cu(DMT)]- und [Cu(DMT)2]-Komplexen die Amino- und Carboxylgruppen koordiniert sind, während in den dehydronierten Komplexen ([Cu(DMT)H−1],[Cu(DMT)2H−1] und [Cu(DMT)2H−2]) die Donorgruppen-NH2 und -OH sind.

COORDINATION OF COPPER(II) TO CYCLIC PEPTIDES WITH A CYSTEINIC DISULFIDE BRIDGE: COMPLEX STABILITY AND VISIBLE ABSORPTION SPECTRA

Abstract Solutions containing copper(II) and an [sbnd]S[sbnd]S[sbnd] bridged cyclic oligopeptide (general formula CG n C, 0 ≤ n ≤ 4) in aqueous KCl (0.1 mol dm−3) were studied at 25°C by potentiometric titration methods and absorption spectrometry. The ligands studied were found to give rise to a number of complexes; besides several (3 to 5) monoligand complexes, the formation of at least one (up to 5) bis-ligand species was detected for each ligand. Stability constants of all detected complex species (4 to 11 per ligand) were determined. From the acidity constants of individual complexes (computed from stability constants) the hapticities of the ligands were inferred. Visible absorption spectra of all examined solutions contain one peak whose position shifts hypsochromically (∼740 nm … 540 nm) with increasing deprotonation and hapticity, indicating changes in the coordination geometry of the copper(II) ion, probably from tetragonal towards distorted tetrahedral. With CG2C, and additional absorption peak ...