Microbiological treatment of industrial wastes containing toxic chromium involving successive use of bacteria,yeast and algae

Bacteria, yeasts, protozoa and algae, observed in industrial effluents from tanneries, were checked for their possible use in the detoxification of polluted water. Two bacterial strains were found to be highly resistant to Cr^VI. Several bacteria, yeasts and algae were observed to be capable of reducing Cr^VI to Cr^III.     

The time-related subcellular distribution of chromium in the rat liver cell after intravenous administration of Na2 51CrO4

After intravenous administration of Na₂ ⁵¹CrO₄ to rats the subcellular distribution of⁵¹Cr was determined at different time intervals after dosage. A time-related compartment shift from the cytosol into the mitochondrial and nuclear fractions was demonstrated. Dialysis studies indicated a firmer binding of⁵¹Cr to the mitochondrial and nuclear fractions than to the cytosol. Indirect evidence is presented that reduction from CrVI to CrIII takes place primarily inside the mitochondria. The hypothesis is put forward that reduction from Cr^VI to Cr^III may take place at any intracellular site where electron donors are available. Electron donors in the different intracellular organelles are discussed.     

Removal of chromium using Rhizobium leguminosarum

The chromium (Cr^III and Cr^VI) removal capability of Rhizobium leguminosarum was checked by estimating the amount of chromium in the medium before and after inoculation. To determine the efficiency of R. leguminosarum in removal of chromium, the influence of physical and chemical parameters such as temperature, pH and different concentrations (0.1–1.0 mM) of trivalent (Cr^III) and hexavalent (Cr^VI) chromium were studied. The chromium removal in aqueous solution by different size of active and inactivated biomass and immobilized cells of R. leguminosarum in a packed-bed column was also carried out. Results showed that in a medium containing up to 0.5 mM concentration of both Cr^III and Cr^VI, R. leguminosarum showed optimal growth. The maximum chromium removal was at pH 7.0 and 35°C. Active biomass removed 84.4 ± 3.6% of Cr^III and 77.3 ± 4.3% of Cr^VI in 24 h of incubation time. However, inactivated biomass removed maximum chromium after 36 h of incubation. Immobilized bacterial cells in a packed-bed column removed 86.4 ± 1.7% of Cr^III and 83.8 ± 2.2% of Cr^VI in 16 and 20 h of incubation time, respectively.     

Synthesis, structure and spectroscopic properties of chloranilate-bridged 4f-4f dinuclear complexes: a comparative study of the emission properties with Cr-Ln complexes

New 4f-4f chloranilate-bridged dinuclear Ln^III complexes, [(HBpz₃)₂Ln(μ-C₆Cl₂O₄)Ln(HBpz₃)₂] (Ln(CA)Ln: Ln=Eu, Tb, Yb), were synthesized and characterized by the X-ray analysis. Their structure and spectroscopic properties were compared with those of dinuclear 3d-4f assembled molecular systems [(acac)₂Cr^III(μ-ox)Ln^III(HBpz₃)₂] (Cr(ox)Ln: acac⁻=acetylacetonate, ox²⁻=oxalate, HBpz₃ ⁻=hydrotris(pyrazol-1-yl)borate) and [(acac)₂Cr(μ-bpypz)Ln(hfac)₃] (Cr(bpypz)Ln: bpypz⁻=3,5-di(2-pyridyl)pyrazolate, hfac=hexafluoroacetylacetonate). The complex Yb(CA)Yb shows strong 4f-4f emission due to the ligand to metal energy transfer from the triplet state of the CA²⁻ to the excited 4f state of Yb^III. On the other hand, the observation of only the 4f-4f emission in the Cr(bpypz)Yb complex is in contrast to the simultaneous observation of the low temperature 3d-3d and 4f-4f emissions associated with the energy transfer from Cr^III to Yb^III in the Cr(ox)Yb complexes. This indicates that the energy transfer from Cr^III to Yb^III is faster in the Cr(bpypz)Yb as compared to that in the Cr(ox)Yb even at low temperatures leading to the stronger 4f-4f luminescence in the former complex. No observation of either Tb^IIIor Eu^III emission in the Tb(CA)Tb or Eu(CA)Eu complexes suggests the energy transfer or back-transfer from the Tb or Eu ions to the CA²⁻ moiety. Conversely, the Cr(bpypz)Eu and Cr(bpypz)Tb complexes show 3d-3d emissions as similarly to the corresponding Cr(ox)Eu and Cr(ox)Tb complexes; indicating the energy transfer from the Eu or Tb to the Cr^III moiety.     

The interaction of D-galactonic acid with CrVI and CrIII. Structure,stability and physical properties of CrIII—aldonate complexes

The redox reaction between D-galactonic acid and potassium chromate yields ((lyxonateH₋₁)(galactonateH₋₁)Cr(OH₂))K·H₂O_n, with both aldonate molecules acting as bidentate ligands with the carboxylate and one alkoxo function as the donor sites. The shift of the CO₂^{poststaggered−} stretching vibration towards lower frequencies upon coordination and the high value of Δv indicate that the carboxylate acts as a monodentate donor site. Magnetic susceptibility data for the compound in the temperature range 3–300 K exhibit a drop in the effective magnetic moment with temperature below 70 K, which is indicative of antiferromagnetic interactions between the Cr^III centres. The molar magnetic susceptibility versus temperature plot could be fitted with the Fisher Hamiltonian for the case of infinite chains, equation-modified for the presence of monomeric species. The EPR and UV-Vis spectroscopic studies reveal that, in solution, the complex retains the distorted octahedral local coordination geometry. The ((lyxonateH₋₁)(galactonateH₋₁)Cr(OH₂))K_n dissociates slowly in aqueous solution but faster at high [H⁺], because of the rapid protonation of the alkoxo bridges linking the monomeric units. The potentiometric evaluation of the closely related binary system Cr^III-d-galactonate shows that the (Cr(galactonateH_−n)₂)^{1 − 2n} complexes are the major species in the 4–12 pH range, when a 1:2 Cr^III:ligand ratio is used. ¹³C NMR reveals that theCO₂^{poststaggered−} group is one of the coordination sites of the ligand.     

Electrospray identification of new polyoxochromate species

Electrospray ionization mass spectrometry (ESI MS) has been conducted on the ammonium and alkali metal (A=Li⁺, Na⁺ and K⁺) dichromate systems. A large number of previously unknown polyoxochromate species have been characterized. Major series that have been identified include [A_x+1H_xCr^VI_xO_4x]⁺ (Li⁺, x=1-5; Na⁺, x=1-7; K⁺, x=1-4) and [A_2x−1Cr^VI_xO_4x−1]⁺ (Li⁺, x=2, 3; Na⁺, x=2-4; K⁺, x=2, 3) in the alkali metal dichromate systems, and [HCr^VI_xO_3x+1]⁻ (x=1-5) in the ammonium dichromate system. Several series also contain mixed oxidation state species, ranging from Cr(V) to Cr(II) in conjunction with Cr(VI), which is consistent with the ease of reduction of Cr(VI). Negative ion ESI MS spectra clearly demonstrate the existence of [HCrO₄]⁻ as the most abundant ion at −20 V, suggesting that its existence in solution is not just hypothetical, as was previously thought. The polymerization units for the series observed include {AHCrO₄}, {A₂CrO₄} and {CrO₃}, with the latter prominent in the alkali metal systems. This presumably arises from the fragmentation of dichromate, A₂Cr₂O₇→{A₂CrO₄}+{CrO₃}. Moreover, the ESI MS of the dichromate compounds have illustrated that the preservation of tetrahedral stereochemistry is of paramount importance for these systems, which leads to only limited polymerization compared to the related molybdate and tungstate systems.     

Ecophysiological responses of water hyacinth exposed to Cr3+ and Cr6+

Due to its wide industrial use, chromium (Cr) is considered a serious environmental pollutant of aquatic bodies. In order to investigate the ecophysiological responses of water hyacinth [Eichhornia crassipes (Mart.) Solms] to Cr treatment, plants were exposed to 1 and 10 mM Cr₂O₃ (Cr³⁺) and K₂Cr₂O₇ (Cr⁶⁺) concentrations for two or 4 days in a hydroponic system. Plants exposed to the higher concentration of Cr⁶⁺ for 4 days did not survive, whereas a 2 days treatment with 1 mM Cr³⁺ apparently stimulated growth. Analysis of Cr uptake indicated that most of the Cr accumulated in the roots, but some was also translocated and accumulated in the leaves. However, in plants exposed to Cr⁶⁺ (1 mM), a higher translocation of Cr from roots to shoots was observed. It is possible that the conversion from Cr⁶⁺ to Cr³⁺, which immobilizes Cr in roots, was not total due to the presence of Cr⁶⁺, causing deleterious effects on gas exchange, chlorophyll a fluorescence and photosynthetic pigment contents. Chlorophyll a was more sensitive to Cr than chlorophyll b. Cr³⁺ was shown to be less toxic than Cr⁶⁺ and, in some cases even increased photosynthesis and chlorophyll content. This result indicated that the F_v/F₀ ratio was more effective than the F_v/F_m ratio in monitoring the development of stress by Cr⁶⁺. There was a linear relationship between qP and F_v/F_m. No statistical differences were observed in NPQ and chlorophyll a/b ratio, but there was a tendency to decrease these values with Cr exposure. This suggests that there were alterations in thylakoid stacking, which might explain the data obtained for gas exchanges and other chlorophyll a fluorescence parameters.     

Cadmium uptake by roots: Contribution of apoplast and of high- and low-affinity membrane transport systems

The aim was to measure the respective contributions of apoplast and symplast to the Cd root uptake and to explain the linear component of the symplastic absorption. Two plants were used, maize (Zea mays L.) and two ecotypes of alpine pennycress (Noccaea caerulescens (J. Presl & C. Presl) F.K. Mey.), with contrasted abilities to accumulate Cd. Their roots were exposed to labelled Cd solutions of increasing concentrations. Root Cd was physico-chemically fractioned to obtain the exchanged apoplastic, non-exchanged apoplastic and symplastic pools. For both species, the proportion of Cd retained by the cell walls increased with Cd concentration in the exposure solution (ranging from 0.05 to 50 μmol L^?1), from approximately 30% to 90% of the total root Cd. This was modeled using Freundlich isotherms. The non-exchanged apoplastic Cd was negligible at the highest exposure concentrations, but reached almost 30% of the total root uptake at the lowest ones. The symplastic influx in roots of both species fitted a Michaelis-Menten function associated with a linear one. The linear component of the symplastic influx could reflect absorption through a low-affinity transport system (LATS). The strong adsorption of Cd on root apoplast might act as a driving force to extract the metal from the soil, compete with the symplastic absorption and contribute to the amount of element taken up by the plant, at least in its roots.     

Chromium uptake and transport in barley seedlings (Hordeum vulgare L.)

Summary Potassium chromate is more toxic to the growth of barley in solution culture than chromic chloride, though apparent uptake of the latter is much faster. Inhibitor studies indicate that CrO₄ ^2- uptake is active whereas Cr³⁺ uptake is passive, demonstrating that the two forms do not share a common uptake mechanism. Studies on the form of Cr inside root cells show that in plants fed CrO₄ ^2- the Cr remains largely unchanged whereas in plants fed Cr³⁺ a little CrO₄ ^2- (0.5 per cent) is produced. This conversion is dependent on the presence of living material and is probably enzymatic. Chromate uptake follows Michaelis-Menten kinetics at low concentration and is competitively inhibited by sulphate. Transport of chromium up the root is very slow, accounting for the low levels of Cr in the shoots. Chromate is transported better than Cr³⁺ though still to a very limited extent. These experiments provide a physiological basis for previous observations.     

Development of a Multi-Species Biotic Ligand Model Predicting the Toxicity of Trivalent Chromium to Barley Root Elongation in Solution Culture

Little knowledge is available about the influence of cation competition and metal speciation on trivalent chromium (Cr(III)) toxicity. In the present study, the effects of pH and selected cations on the toxicity of trivalent chromium (Cr(III)) to barley (Hordeum vulgare) root elongation were investigated to develop an appropriate biotic ligand model (BLM). Results showed that the toxicity of Cr(III) decreased with increasing activity of Ca²⁺ and Mg²⁺ but not with K⁺ and Na⁺. The effect of pH on Cr(III) toxicity to barley root elongation could be explained by H⁺ competition with Cr³⁺ bound to a biotic ligand (BL) as well as by the concomitant toxicity of CrOH²⁺ in solution culture. Stability constants were obtained for the binding of Cr³⁺, CrOH²⁺, Ca²⁺, Mg²⁺ and H⁺ with binding ligand: log K_CrBL 7.34, log K_CrOHBL 5.35, log K_CaBL 2.64, log K_MgBL 2.98, and log K_HBL 4.74. On the basis of those estimated parameters, a BLM was successfully developed to predict Cr(III) toxicity to barley root elongation as a function of solution characteristics.     

A nearly symmetric trinuclear chromium(III) oxo carboxylate assembly: preparation, molecular and crystal structure, and magnetic properties of [Cr3O(O2CPh)6(MeOH)3](NO3) · 2MeOH

Complex [Cr₃O(O₂CPh)₆(MeOH)₃](NO₃) · 2MeOH (1 · 2MeOH) has been synthesized from the one-pot reaction between Cr(NO₃)₃ · 9H₂O and NaO₂CPh in MeOH. The structure of the complex has been solved by single-crystal X-ray crystallography. It crystallizes in the monoclinic space group P2₁/n with a=14.716(6) Å, b=22.569(8) Å, c=15.755(6) Å, β=95.02(1)°, V=5212.5(4) ų and Z=4. Although the cation does not possess any crystallographically imposed symmetry element, its {Cr₃(μ₃-O)} core is nearly symmetric. Each Cr^III…Cr^III vector is further bridged by two η¹:η¹:μ₂ benzoates, with a terminal MeOH molecule completing octahedral coordination at each metal ion. The crystal structure consists of layers that are parallel to (0 1 0) crystallographic plane and are formed through π-π stacking interactions and hydrogen bonds. Variable-temperature magnetic susceptibility and solid-state ¹H NMR studies indicate that the total spin value of the ground state is 1/2. EPR experiments reveal the existence of a distribution of trimers with axial anisotropy in the g tensor.     

Reactivity of a low-valent chromium dinitrogen complex

(Nn^iPrCr)₂(μ₂-κ²:κ²-N₂) (1, Nn^iPr = bis(2,6-diisopropylphenyl)pentane-2,4-ketiminato), formally a chromium(I) complex, was exposed to a variety of small molecules possessing hetero- or homo-atomic single, double, or triple bonds. Reaction of 1 with adamantyl azide yielded complexes in various higher oxidation states, such as (Nn^iPrCr^II)₂(μ₂-NAd) (2), Nn^iPrCr^III(κ²-N₄Ad₂) (3), or Nn^iPrCr^V(NAd)₂ (4). Compound 1 was found to reductively couple 3-pentanone to form Nn^iPrCr(κ²-O₂C₂Et₄) (5) and reductively couple benzylidene aniline to form both Nn^iPrCr^III(cis-κ²-C₂₈H₂₂N₂) (6a) an Nn^iPrCr^III(trans-κ²-C₂₈H₂₂N₂) (6b). Reaction with a stochiometric amount of benzylidene aniline yielded the imine complex Nn^iPrCr(η²-NPhCHPh) (7). Exposure of 1 to a bulky isocyanide formed the octahedral Nn^iPrCr^I[CN(C₆H₄(Me)₂]₄ (9). Complex 1 was also found to break the O-O, NO, and S-S bonds in various small molecules to form Nn^iPrCr^II(OCMe₃) (11), Nn^iPrCr^V(O)(NPh) (8), and [Nn^iPrCr^II(μ-SPh)]₂ (10).     

The synthesis, characterization, and alkylation of nacnac chromium triflate derivatives

A Cr(III) triflate coordinated by the bulky β-diketiminate ^MeL^iPr (^MeL^iPr = 2,4-pentane N,N′-bis(2,6-diisopropylphenyl)diketiminate) was synthesized from the corresponding bridging iodide complex [^MeL^iPrCr(μ-I)]₂ by ligand substitution and subsequent oxidation with silver triflate (AgOTf). ^MeL^iPr Cr^III(OTf)₂ exhibits rare trigonal bipyramidal geometry about Cr(III). Attempts to alkylate this triflate synthon with 1,4-dilithiobutane (Li(CH₂)₄Li) led to reduction, while reaction with dimethylzinc (ZnMe₂) led to a mono-alkylated product; only the reaction with methyl lithium (MeLi) was successful in generating a dialkyl.     

Syntheses, structures and antimicrobial activities of various metal complexes of hinokitiol

Metal-oxygen bonding complexes (M = Mg^II, Mn^II, Ni^II, Mo^VI, W^VI, Pd^II, Sb^III, Bi^III, Fe^III, Ti^IV, K^I, Ba^II, Zr^IV and Hf^IV) with a hinokitiol (Hhino; 2-hydroxy-4-isopropylcyclohepta-2,4,6-trienone or β-thujaplicin) ligand, which has two unequivalent oxygen donor atoms, were synthesized and characterized by elemental analysis, TG/DTA, FT-IR and solution (¹H and ¹³C) NMR spectroscopy. Single-crystal X-ray structure analysis revealed various molecular structures for the complexes, which were classified into several families of family, i.e. type A [M^II(hino)₂(L)]₂ (M = Mg^II, Mn^II, Ni^II; L = EtOH or MeOH), with a dimeric structure consisting of one bridging hino⁻ anion, one chelating hino⁻ anion and one alcohol or water molecule, type B, with the octahedral, cis-dioxo, bis-chelate complexes cis-[M^VIO₂(hino)₂] (M = Mo^VI, W^VI), type C, with square planar complex [M^II(hino)₂] (M = Pd^II), type D, with tris-chelate, 7-coordinate complexes with one inert electron pair [M^III(hino)₃] (M = Sb^III, Bi^III), type D′, with the bis-chelate, pseudo-6-coordinate complexes with one inert electron pair [M^III(hino)₂X] (M = Sb^III, X = Br), type E, with tris-chelate, 6-coordinate complexes with Δ and Λ isomers [M^III(hino)₃] (M = Fe^III), type E′ of bis-chelate, 6-coordinate complex [M^IV(hino)₂X₂] (M = Ti^IV, X = Cl), type F, with water-soluble alkali metal salts [M^I(hino)] (M = K^I), and type H, with tetrakis-chelate, 8-coordinate complexes [M^IV(hino)₄](M = Zr^IV, Hf^IV). These structural features were compared with those of metal complexes with a related ligand, tropolone (Htrop). The antimicrobial activities of these complexes, evaluated in terms of minimum inhibitory concentration (MIC; μg mL⁻¹) in two systems, were compared to elucidate the relationship between structure and antimicrobial activity.     

Effects of Cr on proton extrusion, potassium uptake and transmembrane electric potential in maize root segments

Abstract Proton extrusion of maize root Zea mays segments, was inhibited by the presence of Cr (o.n. + 6; present in solution as CrO₄^2-, Cr₂O₇^2-) in the incubation medium: the minimum inhibiting concentration was 2 × 10^?3 mol m^?3 and the inhibition progressively increased with Cr concentration. Cr inhibited proton extrusion. Also, when this activity was stimulated by the presence of K⁺ or fusicoccin (FC) in the incubation medium, the K⁺ and FC stimulating effect was still present when proton extrusion was inhibited by Cr. In addition, Cr inhibited K⁺ uptake. This inhibition was higher (50%) at K⁺ concentrations up to 1 mol m^?3 lower (15%) at higher K⁺ concentrations. This result indicates that the system responsible for K⁺ uptake operating at low K⁺ concentrations is more sensitive to Cr inhibition. Cr had no effect on transmembrane electric potential (PD). The depolarizing and hyper-polarizing effect of K+ and FC, respectively, were not affected by Cr; but Cr enhances the depolarizing effect of the uncoupler carbonylcyanide m-chlorophenylhydrazone (CCP). These results indicate that Cr inhibited the proton translocating mechanism coupled with K⁺ uptake, but did not change the net transport of charges through the plasmalemma. The Cr effect is discussed, taking into account the possibility of a direct effect of Cr at the membrane level or, alternatively, of an effect on some metabolic processes controlling membrane function.     

The effect of Cr(III) upon the thermal denaturation of DNA

Thermal denaturation studies of Cr-DNA solutions at pH 6.0 were carried out by monitoring the uv absorbance at 260 nm. The melting curves of solutions of calf thymus and Escherichia coli DNA with added Cr(ClO₄)₃ were broadened and shifted to higher temperatures. As the ratio of Cr: DNA increased, the melting temperature increased until it reached a maximum at Cr: DNA ratios of 0.7 (E. coli) and 0.9 (calf thymus). At higher concentrations of Cr³⁺ the melting temperature decreased and then leveled off, but it never fell as low as that of the pure DNA.     

Distinct and effective biotransformation of hexavalent chromium by a novel isolate under aerobic growth followed by facultative anaerobic incubation

A bacterial isolate (G161) with high Cr(VI)-reducing capacity was isolated from Cr(VI)-contaminated soil and identified as Leucobacter sp. on the basis of 16S rRNA gene sequence analysis. The isolate was a Gram-positive, aerobic rod. The hexavalent chromate-reducing capability of the isolate was investigated under three conditions of oxygen stress. The isolate was found to reduce Cr(VI) under all conditions but performed most effectively during aerobic growth followed by facultative anaerobic incubation. Under these conditions, the isolate tolerated K₂Cr₂O₇ concentrations up to 1,000 mg/l and completely reduced 400 mg/l K₂Cr₂O₇ within 96 h. The strain reduced Cr(VI) over a wide range of pH (6.0–11.0) and temperatures (15–45 °C) with optimum performance at pH?8.0 and 35 °C. The presence of other metals, such as Ca²⁺, Co²⁺, Cu²⁺, Mn²⁺, Ni²⁺, and Zn²⁺, induced no effect or else played a stimulatory role on Cr(VI)-reduction activity of the strain. The strain was tested for Cr(VI) removal in wastewaters and proved capable of completely reducing the contained Cr(VI). This is the novel report of a bacterial growth and Cr(VI)-reduction process under sequential aerobic growth and facultative anaerobic conditions. The study suggested that the isolate possesses a distinct capability for Cr(VI) reduction which could be harnessed for the detoxification of chromate-contaminated wastewaters.     

Kinetics,biochemical and factorial analysis of chromium uptake in a multi-ion system by Tradescantia pallida (Rose) D. R. Hunt

Discharge of wastewater from electroplating and leather industries is a major concern for the environment due to the presence of toxic Cr⁶⁺ and other ions, such as sulfate, nitrate, phosphate, etc. This study evaluated the potential of Tradescantia pallida, a plant species known for its Cr bioaccumulation, for the simultaneous removal of Cr⁶⁺, SO₄^2?, NO₃^?, and PO₄^3?. The effect of different co-ions on Cr⁶⁺ removal by T. pallida was examined following the Plackett-Burman design of experiments carried out under batch hydroponics conditions. The results revealed a maximum removal of 84% Cr⁶⁺, 87% SO₄^2?, 94% NO₃^? and 100% PO₄^3? without any phytotoxic effect on the plant for an initial Cr⁶⁺ concentration in the range 5–20 mg L^?1. SO₄^2? and NO₃^? enhanced Cr uptake at a high initial Cr concentration (20 mg L^?1), whereas PO₄^3? did not affect Cr uptake both at high and low initial Cr concentrations. The Cr⁶⁺ removal kinetics in the presence of different ions was well described by the pseudo-second-order kinetic model which revealed that both biosorption and bioaccumulation of the metal played an important role in Cr⁶⁺ removal. Increase in the total carbohydrate and protein content of the plant following Cr⁶⁺ and co-ions exposure indicated a good tolerance of the plant toward Cr⁶⁺ toxicity. Furthermore, enhancement in the lipid peroxidation and catalase activity in T. pallida upon Cr⁶⁺ exposure revealed a maximum stress-induced condition in the plant. Overall, this study demonstrated a very good potential of the plant T. pallida for Cr⁶⁺ removal from wastewater even in the presence of co-ions.     

On the interaction of compounds of chromium(VI) with hydrogen peroxide. A study of chromium(VI) and (V) peroxides in the acid-basic pH range

A computational study of chromium(VI) and (V) peroxides, which exhibit important genotoxic and mutagenic activity, is reported. Energies and equilibrium geometries for [Cr^VI(O)(O₂)₂(OH)]⁻, [Cr^VI(O)(O₂)₂(OH₂)], [Cr^VI(O)(O₂)₂(py)], [Cr^VI(OH)(O₂)₂(OH₂)]⁺, [Cr^V(O)(O₂)₂(OH₂)]⁻ and species were calculated using molecular mechanics calculations (MMFF94 and MM⁺), quantum calculations with semi-empirical methods (RHF and UHF/PM3) and density functional theory (pBP86/DN* or pBP/DN* and B3LYP/6-31G(d). Equilibrium geometries for the compounds [Cr^V(O₂)₃(OH)]²⁻ and [Cr^V(O₂)₄]³⁻ were determined by molecular mechanics. Vibrational frequencies, standard thermodynamic quantities and electronic spectra were calculated using B3LYP/6-31G(d). The structural relationship between all these species and an explanation of the formation of peroxo species in the acid-basic pH range are given. An experimental study of peroxo species in basic medium was also performed (synthesis, X-ray powder diffraction patterns and infrared spectra of the peroxo complexes isolated) but did not confirm the existence of a tri-peroxo complex in the solid phase.     

Operationally defined apoplastic and symplastic aluminum fractions in root tips of aluminum-intoxicated wheat

Knowledge of the mechanistic basis of differential aluminum (Al) tolerance depends, in part, on an improved ability to quantify Al located in the apoplastic and symplastic compartments of the root apex. Using root tips excised from seedlings of an Al-tolerant wheat cultivar (Triticum aestivum L. cv Yecora Rojo) grown in Al solutions for 2 d, we established an operationally defined apoplastic Al fraction determined with six sequential 30-min washes using 5 mm CaCl₂ (pH 4.3). Soluble symplastic Al was eluted by freezing root tips to rupture cell membranes and performing four additional 30-min CaCl₂ washes, and a residual fraction was determined via digestion of root tips with HNO₃. The three fractions were then determined in Yecora Rojo and a sensitive wheat cultivar (Tyler) grown at 18, 55, or 140 μm total solution Al (Al_T). When grown at equal Al_T, Tyler contained more Al than Yecora Rojo in all fractions, but both total Al and fractional distribution were similar in the two cultivars grown at Al_T levels effecting a 50% reduction in root growth. Residual Al was consistently 50 to 70% of the total, and its location was elucidated by staining root tips with the fluorophore morin and examining them using fluorescence and confocal laser scanning microscopy. Wall-associated Al was only observed in tips prior to any washing, and the residual fraction was manifested as distinct staining of the cytoplasm and nucleus but not of the apoplastic space. Accordingly, the residual fraction was allocated to the symplastic compartment for both cultivars, and recalculated apoplastic Al was consistently approximately 30 to 40% of the total. Distributions of Al in the two cultivars did not support a symplastic detoxification hypothesis, but the role of cytoplasmic exclusion remains unsettled.