Complexes of Ag(I), Hg(I) and Hg(II) with multidentate pyrazolyl-pyridine ligands: From mononuclear complexes to coordination polymers via helicates, a mesocate, a cage and a catenate

The coordination chemistry of a series of di- and tri-nucleating ligands with Ag(I), Hg(I) and Hg(II) has been investigated. Most of the ligands contain two or three N,N'-bidentate chelating pyrazolyl-pyridine units pendant from a central aromatic spacer; one contains three binding sites (2 + 3 + 2-dentate) in a linear sequence. A series of thirteen complexes has been structurally characterised displaying a wide range of structural types. Bis-bidentate bridging ligands react with Ag(I) to give complexes in which Ag(I) is four-coordinate from two bidentate donors, but the complexes can take the form of one-dimensional coordination polymers, or dinuclear complexes (mesocate or helicate). A tris-bidentate triangular ligand forms a complicated two-dimensional coordination network with Ag(I) in which Ag...Ag contacts, as well as metal-ligand coordination bonds, play a significant role. Three dinuclear Hg(I) complexes were isolated which contain an {Hg2}2+ metal-metal bonded core bound to a single bis-bidentate ligand which can span both metal ions. Also characterised were a series of Hg(II) complexes comprising a simple mononuclear four-coordinate Hg(II) complex, a tetrahedral Hg(II)4 cage which incorporates a counter-ion in its central cavity, a trinuclear double helicate, and a trinuclear catenated structure in which two long ligands have spontaneously formed interlocked metallomacrocyclic rings thanks to cyclometallation of two of the Hg(II) centres.     

Predictive theory for the combination kinetics of two alkyl radicals

An ab initio transition state theory based procedure for accurately predicting the combination kinetics of two alkyl radicals is described. This procedure employs direct evaluations of the orientation dependent interaction energies at the CASPT2/cc-pvdz level within variable reaction coordinate transition state theory (VRC-TST). One-dimensional corrections to these energies are obtained from CAS+1+2/aug-cc-pvtz calculations for CH3 + CH3 along its combination reaction path. Direct CAS+1+2/aug-cc-pvtz calculations demonstrate that, at least for the purpose of predicting the kinetics, the corrected CASPT2/cc-pvdz potential energy surface is an accurate approximation to the CAS+1+2/aug-cc-pvtz surface. Furthermore, direct trajectory simulations, performed at the B3LYP/6-31G* level, indicate that there is little local recrossing of the optimal VRC transition state dividing surface. The corrected CASPT2/cc-pvdz potential is employed in obtaining direct VRC-TST kinetic predictions for the self and cross combinations of methyl, ethyl, iso-propyl, and tert-butyl radicals. Comparisons with experiment suggest that the present dynamically corrected VRC-TST approach provides quantitatively accurate predictions for the capture rate. Each additional methyl substituent adjacent to a radical site is found to reduce the rate coefficient by about a factor of two. In each instance, the rate coefficients are predicted to decrease quite substantially with increasing temperature, with the more sterically hindered reactants having a more rapid decrease. The simple geometric mean rule, relating the capture rate for the cross reaction to those for the self-reactions, is in remarkably good agreement with the more detailed predictions. With suitable generalizations the present approach should be applicable to a wide array of radical-radical combination reactions.     

Total synthesis of the marine natural product (-)-clavosolide A

[Structure: see text] The total synthesis of the marine metabolite clavosolide A is reported which confirms the structure and absolute configuration of the natural product as the symmetrical diolide glycosylated by permethylated D-xylose moieties, 2.     

Applications of differential scanning calorimetry in developing cryopreservation strategies for Parkia speciosa, a tropical tree producing recalcitrant seeds

Shoot-tips of Parkia speciosa, a recalcitrant seed producing tropical leguminous tree withstood cryopreservation using encapsulation-vitrification in combination with trehalose preculture. Differential scanning calorimetry (DSC) revealed that trehalose moderated the thermal characteristics of the shoot-tips. A 30 min PVS2 treatment had the lowest glass transition temperature (Tg) (-50.2 +/- 1.1 degree C) when applied in combination with 5% (w/v) trehalose. The Tg increased to -40.2 +/- 1.0 degree C as the sugar concentration was decreased to 2.5 percent (w/v). Tg heat capacity for shoot-tips treated with 2.5 percent and 5 percent (w/v) trehalose and exposed to PVS2 for 30 min increased from 0.17 +/ 0.05 to 0.23 +/- 0.01 J per gram, respectively. Enthalpies of the melt-endotherm varied in proportion to trehalose concentration, for the 30 min PVS2 treatment, whereas the melt enthalpy for control shoots was greater than 150 J per gram and decreased to ca. 60 J per gram with 2.5 percent (w/v) trehalose. For 5 percent and 10 percent (w/v) trehalose treatments, enthalpy declined to ca. 24 and 12 J per gram respectively and freezing points were depressed to -75 degree C and -85 degree C with 2.5 percent and 5 percent trehalose (w/v), respectively. DSC elucidated the critical points at which vitrification occurred in germplasm exposed to trehalose and PVS2. A 60 min PVS2 treatment supporting ca. 70 percent survival was found optimal for stable glass formation during cooling and on rewarming.     

Deployment of the encapsulation-dehydration protocol to cryopreserve polar microalgae held at the Czech Republic Academy of Sciences Institute of Botany

Polar isolates of four chlorococcal microalgae originating from the Arctic and Antarctica withstand cryopreservation using encapsulation-dehydration. Viability assessments, which initially used chloroplhyll fluorescence (Kautsky) induction kinetics, revealed that all strains suffered photosynthetic impairment during early post-cryopreservation recovery. This cryoinjury was reversible, as indicated by cell regrowth in three of the four strains. Lack of growth in the fourth isolate was due to contaminating bacteria rather than cryogenic factors.     

Deployment of the encapsulation-dehydration protocol to cryopreserve microalgae held at the Sammlung von Algenkulturen, Universitat Gottingen, Germany

Encapsulation-dehydration was applied to cryopreserve 14 diverse algal strains, representing eukaryotic terrestrial microalgae; of these 12 survived to form cell colonies after recovery from cryostorage. Surviving algae had varying degrees of tolerance to osmotic dehydration and desiccation in this vitrification-based cryoprotective strategy. The extent of algal regrowth was affected by the mode of desiccation (silica gel or air-flow), the duration of evaporative desiccation and exposure to light during early recovery phase. This paper: (i) demonstrates the versatility of the encapsulation/dehydration method to cryopreserve diverse microalgae; (ii) confirms the successful transfer of this cryostorage technology to the Culture Collection of Algae at Gottingen University (SAG); and (iii) recommends encapsulation/dehydration as a feasible alternative to controlled rate cooling for preserving algae held in international culture collections.     

Unconventional methyl galactan synthesized via the thexyldimethylsilyl intermediate: preparation, characterization, and properties

Reaction of a beta-(1 --> 4) linked galactan with TDMS chloride followed by methylation and desilylation yields methyl galactans with unconventional functionalization patterns. The products were characterized via FTIR and NMR of the intact polymer and by CE after controlled depolymerization. A TDMS-derivatized methyl galactan contains differently methylated secondary hydroxyl groups. SEC and analytical ultracentrifugation showed a consistent decrease in the molecular weight after the consecutive reaction steps. Biological studies revealed that the methyl galactans are less active in complement fixation assays as compared with a 3-O-methyl galactan-enriched polysaccharide fraction isolated from Acanthus ebracteatus.     

Microkinetic simulations of the oxidation of CO on Pd based nanocatalysis: a model including co-dependent support interactions

The catalysed oxidation of CO using mass-selected Pd(13) clusters supported on thin MgO films was modelled using a microkinetic simulation of the reaction. The model of the system includes reverse spill-over calculations which were intrinsically incorporated into the formulation of the kinetics. The spill-over model is based on a capture-zone approach including a co-dependence on the variables of the kinetic equations. The experimental values were determined using dual pulsed-molecular beam measurements and recorded at a range of temperatures. The experiment allowed the turn-over frequency and reaction probability to be determined as a function of mole fraction. Comparison of the kinetic model with the experimental data gives excellent agreement and strongly highlights the importance of substrate effects. In particular, the origin of the low temperature catalysis of the Pd clusters is elucidated. The model allows the mole fraction and temperature dependent values such as the sticking coefficients for these clusters to be predicted.