Fluorescent pH sensors with negligible sensitivity to ionic strength

Optical pH determination has the fundamental disadvantage of measuring a signal that depends on the ionic strength of the sample. The problem originates from the complex relationship between the proton activity and the concentration of the pH-sensitive dye. The effect of ionic strength on the signal depends on the charge of the indicator and its environment, e.g. the immobilisation matrix. We present novel lipophilic fluorescein esters carrying one negative charge. They are embedded in an uncharged, highly proton-permeable hydrogel to give optical pH sensors that show a negligible cross-sensitivity towards ionic strength. The fluorescent dyes differ in their substituents. This variation of substituents results in dissociation constants between 5.5 and 8.5. The indicators were made lipophilic by esterification of the carboxy group with a C(18) alkyl chain. Since their spectral properties are quite similar, two indicators may be used in one sensor. This results in an optical pH sensor with a dynamic range that extends from pH 4.5 to 8.     

Drying and pyrolysis of wood particles: experiments and simulation

The objective of this study is to develop a flexible and stable numerical method to predict the thermal decomposition of large wood particles due to drying and pyrolysis. At a later stage, this model is applied to each particle of a packed bed and thus, forms the entire packed bed process as a sum of individual particle processes. Therefore, this approach can deal with particles of different sizes, shapes and properties. A general formulation of the conservation equations allows the geometry of a fuel particle to be treated as a plate, cylinder or sphere. The various processes such as heat-up, drying and pyrolysis are described by a set of one-dimensional and transient conservation equations for mass and energy. This allows for simultaneous processes e.g. reactions in time and covers the entire range between transport-limited (shrinking core) and kinetically limited (reacting core) reaction regimes. The particles interact with a gas phase by heat and mass transfer taking into account the Stefan correction due to the gas outflow during conversion. Experiments carried out span a temperature range between T=300 and 900 °C for particle sizes varying between 8 and 17 mm. A comparison between measurements and predictions of drying models yielded satisfactory agreement only for the constant evaporation temperature model and thus, indicating, that the drying process is transport limited by heat transfer for large wood particles. Likewise, predicted results of pyrolysis for the above-mentioned range of temperatures and sizes agreed satisfactorily with measurements.     

Isocytosine as a hydrogen-bonding partner and as a ligand in metal complexes

Isocytosine (ICH; 1) exists in solution in an equilibrium of tautomers 1a and 1b with the N1 and N3 positions carrying the acidic proton, respectively. In the solid state, both tautomers coexist in a 1:1 ratio. As we show, the N3H tautomer 1b can selectively be crystallized in the presence of the model nucleobase 1-methylcytosine (1-MeC). The complex 1b x (1-MeC)2 x H2O (2) forms pairs through three hydrogen bonds between the components; hydrogen bonds between identical molecules are also formed, leading to an infinite tape structure. On the other hand, the N1H tautomer 1a co-crystallizes with protonated ICH to give [1a x ICH2]NO3 (3), again with three hydrogen bonds between the partners, yet the acidic proton is disordered over the two entities. With M(II)(dien) (M=Pt, Pd; dien=diethylenetriamine) preferential coordination of tautomer 1a through the N3 position is observed. DFT calculations, which were also extended to Pt(II)(tmeda) linkage isomers (tmeda=N,N,N',N'-tetramethylethylenediamine), suggest that intramolecular hydrogen bonding between the ICH tautomers and the co-ligands at M, while adding to the preference for N3 coordination, is not the major determining factor. Rather it is the inherently stronger Pt-N3 bond which favors complexation of 1a. With an excess of M(II)(dien), dinuclear species [M2(dien)2(IC-N1,N3)]3+ (M=Pd(II), 4 and Pt(II), 5) also form and were isolated as their ClO4(-) salts and structurally characterized. In strongly acidic medium 5 is converted to [Pt(dien)(ICH-N1)]2+ (6), that is, to the Pt(II) complex of tautomer 1b.     

Zinc-bound thiolate-disulfide exchange: a strategy for inhibiting metallo-beta-lactamases

The mononuclear zinc thiolate complexes [(Tp(PhMe))Zn(S-R)], where Tp(PhMe) is hydrotris((3-methyl-5-phenyl)pyrazolyl)borate and (S-R) is benzyl thiolate, 4-nitrophenylthiolate, 4-trifluoromethylphenylthiolate, 4-chlorophenylthiolate, phenylthiolate, 2-methylphenylthiolate, 4-methylphenylthiolate, 4-methoxyphenylthiolate, or 4-hydroxyphenylthiolate, were synthesized. Representative members of the class were also characterized structurally. The benzyl thiolate complex undergoes a thiolate-disulfide exchange reaction with a variety of diphenyl and dipyridyl disulfides. Kinetic studies revealed that the reaction shows saturation behavior in both complex and disulfide for most of the disulfides studied. Combined with studies of the lability of the coordinated thiolate, a mechanism is proposed where the reactive species is the zinc-coordinated thiolate. When the free benzyl thiol was allowed to react with the same disulfides, the reaction was slower by a factor of 20-200 than that for the zinc-thiolate complex, depending on the particular disulfide employed. Since most metallo-beta-lactamases contain one or more cysteine residues, the one in the active site being coordinated to zinc, the present study was extended to examine whether disulfides can be used as inhibitors of these enzymes by selective oxidation of the metal-bound cysteine. Several disulfides allowed to react with metallo-beta-lactamase CcrA from Bacteroides fragilis were moderate to potent irreversible inhibitors of the enzyme.     

Evidence for quasi-intramolecular acid-base reactions in solutions of transition metal ammine complexes

We have investigated the effect of temperature on the postulated quasi-intramolecular acid-base reactions in aqueous solutions of Cu(NH₃)₄(MnO₄)₂ and Ag(NH₃)₂ClO₄. Pure ammonium permanganate and perchlorate were obtained above 5 C and at room temperature, respectively, due to quasi-intramolecular acid-base reactions.     

Detection and measurement of noncoincidence between the principal axes of the g-matrix and zero-field splitting tensor using multifrequency powder EPR spectroscopy: application to cis-[(NH(3))(2)Pt(1-MeU)(2)Cu(H(2)O)(2)](SO(4)) x 4.5H(2)O (1-MeU = monoanion of 1-methyluracil)

Multifrequency continuous wave EPR spectra (4-34 GHz) on a powder of the title compound are consistent with a spin-triplet state. This arises from interaction between centrosymmetrically related pairs of copper(II) ions in the solid. The spectra at all frequencies have been simulated with a single set of spin-Hamiltonian parameters. The results show that there is noncoincidence between the principal axes of the g-matrices on each copper center and those of the zero-field splitting (D) tensor. This noncoincidence is a single rotation of 33 degrees +/- 2 degrees. The parameters from the powder spectra have been verified by a subsequent single-crystal EPR study which yielded the spin-Hamiltonian parameters g(XX) = 2.074, g(YY) = 2.093, g(ZZ) = 2.385, D(XX) = +/-0.0228 cm(-1), D(YY) = +/-0.0211 cm(-1), D(ZZ) = -/+0.0439 cm(-1) with Euler angles of alpha = 179 degrees, chi = 33.4 degrees, and gamma = 328 degrees. Analysis of the zero-field splitting tensor in terms of exchange indicates that the interaction between the pairs of copper(II) ions is almost entirely dipolar in origin. This study shows that multifrequency EPR spectroscopy on powders, coupled with spectrum simulation, can detect and measure noncoincidence between the principal axes of the g-matrix and zero-field splitting tensor, and does not necessarily require the presence of metal hyperfine interactions.     

Simplified ab initio calculations on small molecules with an extended balanced approximate MO formalism

Based on a test of different known simplified ab initio methods an approximate LCAO MO formalism is proposed which gives reasonable results in a minimal STO basis in comparison with ab initio calculations.     

Possible Mechanisms of Intercalation

Recent knowledge of the kinetics and intercalation mechanisms are summarized and accompanied by examples of intercalation reactions of water and ethanol into anhydrous vanadyl phosphate and redox intercalation of alkali metal cations into vanadyl phosphate dihydrate. Three possible mechanisms of intercalation are presented which are based on: (i) a concept of exfoliation of layers; (ii) the formation of stages and randomly stacked layers; (iii) co-existence of intercalated and non-intercalated parts of crystals of the host separated by an advancing phase boundary. The corresponding kinetic curves are ascribed to mechanisms (ii) and (iii).     

Head-to-head (HH) and head-to-tail (HT) conformers of cis-bis guanine ligands bound to the [Re(CO)3]+ core

We have prepared four complexes of the type [Re(guanine)(2)(X)(CO)(3)] (guanine = 9-methylguanine or 7-methylguanine, X = H(2)O or Br) in order to understand the factors determining the orientation of coordinated purine ligands around the [Re(CO)(3)](+) core. The 9-methylguanine ligand (9-MeG) was chosen as the simplest N(9) derivatized guanine, and 7-methylguanine (7-MeG) was chosen because metal binding to N(9) does not impose steric hindrance. Two types of structures have been elucidated by X-ray crystallography, an HH (head-to-head) and HT (head-to-tail) conformer for each of the guanines. All complexes crystallize in monoclinic space groups: [Re(9-MeG)(2)(H(2)O)(CO)(3)]ClO(4) (2) in P2(1)/n with a = 12.3307(10) A, b = 16.2620(14) A, c = 13.7171(11) A, and beta = 105.525(9) degrees, V = 2650.2(4) A(3), with the two bases in HT orientation and its conformer [Re(9-MeG)(2)(H(2)O)(CO)(3)]Br (3) in P2(1)/n with a = 15.626(13) A, b = 9.5269(5) A, c = 15.4078(13) A, and beta = 76.951(1) degrees, V = 2234.5(3) A(3), and the two bases in an HH orientation. Similarly, [Re(7-MeG)(2)(H(2)O)(CO)(3)]ClO(4) (4) crystallizes in P2(1)/c with a = 13.0708(9) A, b = 15.4082(7) A, c = 14.316(9) A, and beta = 117.236(7) degrees, V = 2563.5(3) A(3), and exhibits an HT orientation and [ReBr(7-MeG)(2)(CO)(3)] (5) in P2/c with a = 17.5117(9) A, b = 9.8842(7) A, c = 15.3539(1) A, and beta = 100.824(7) degrees, V = 2610.3(3) A(3), and shows an HH orientation. When crystals of any of these complex pairs are dissolved in D(2)O, the (1)H NMR spectrum shows a single peak for the H(8) resonance of the respective coordinated purine indicating a rapid equilibrium between HH and HT conformations in solution. DFT calculations simulating the rotation of one ligand around its Re-N bond showed energetic barriers of less than 8.7 kcal/mol. We find no hypochromic effect in the Raman spectrum of 3, which showed base stacking in the solid state. Neither steric interactions nor hydrogen bonding are important in determining the orientation of the ligands in the coordination sphere.     

Supramolecular approaches to generate libraries of chelating bidentate ligands for homogeneous catalysis

The process of catalyst discovery and development relying on combinatorial methods has suffered so far from the difficult access to structurally diverse and large libraries of ligands, in particular the structurally more complex class of bidentate ligands. A completely new approach to streamline the difficult ligand synthesis process is to use structurally less complex monodentate ligands that self-assemble in the coordination sphere of a metal center through noncovalent attractive ligand-ligand interactions to generate bidentate, chelating ligands. When complementary attractive ligand-ligand interactions are employed, it is even possible to generate libraries of defined chelate-ligand catalysts by simply mixing two different monomeric ligands. This Minireview summarizes the first approaches and results in this new field of combinatorial homogeneous catalysis.