Sorption of copper and zinc to the plasma membrane of wheat root.

Sorption of Cu(2+) and Zn(2+) to the plasma membrane (PM) of wheat root (Triticum aestivum L cv. Scout 66) vesicles was measured at different pH values and in the presence of organic acids and other metals. The results were analyzed using a Gouy-Chapman-Stem model for competitive sorption (binding and electrostatic attraction) to a negative binding site. The binding constants for the two investigated cations as evaluated from the sorption experiments were 5 M(-1) for Zn(2+) and 400 M(-1) for Cu(2+). Thus, the sorption affinity of Cu(2+) to the PM is considerably larger than that of Ca(2+), Mg(2+) or Zn(2+). The greater binding affinity of Cu(2+) was confirmed by experiments in which competition with La(3+) for sorption sites was followed. The amount of sorbed Cu(2+) decreased with increasing K(+), Ca(2+), or La(3+) concentrations, suggesting that all these cations competed with Cu(2+) for sorption at the PM binding sites, albeit with considerable differences among these cations in effectiveness as competitors with Cu(2+). The sorption of Cu(2+) and Zn(2+) to the PM decreased in the presence of citric acid or malic acid. Citric acid (as well as pH) affected the sorption of Cu(2+) or Zn(2+) to PM more strongly then did malic acid.

Characteristics of the quenching of 9-aminoacridine fluorescence by liposomes made from plant lipids.

Several laboratories have determined the surface charge density of membranes utilizing methods based on vesicle-induced quenching of the fluorescence of 9-aminoacridine and its relief by other cations. However, the computational methods by which surface charge density were calculated have not been verified in a model system. In this study, the quenching of 9-aminoacridine fluorescence by liposomes made from varying amounts of digalactosyldiacylglyceride and phosphatidic acid and relief of quenching by salts was examined. Quenching of 9-aminoacridine fluorescence increased with increasing amounts of phosphatidic acid added, independent of the composition of the added liposomes. In certain instances, the computational methods did not yield the surface charge density of the liposomes expected from their composition. However, when the effects of background ionic strength on surface potential were considered, there was a positive correlation between expected and calculated values. Therefore, the data support the contention that changes in the fluorescence of 9-aminoacridine can be used to calculate surface charge density of membranes.

Adsorption and Interactions of Methyl Green with Montmorillonite and Sepiolite.

The divalent organic cation, methyl green (MG), undergoes a slow transformation (6 h) to a monovalent cation, carbinol (MGOH(+)) upon dilution of its solution (10 mM), or in a buffer at neutral pH. Adsorption isotherms of MG on montmorillonite were determined by two procedures, both of which yield a final pH of suspensions between 7 to 7.4. When the amounts of MG in suspension were lower than the cation-exchange capacity (CEC) of the clay (0.8 mol(c)/kg clay), no measurable amount of MG remained in solution. The maximal amounts of MGOH(+) adsorbed were larger than those of MG(2+), being 1.15 and 0.75 mol MG/kg clay, respectively, corresponding to 140% of the CEC in the first case. On a charge basis the adsorption of added MG(2+) amounts to 185% of the CEC, which raises the possibility that a certain fraction of MG(2+) transformed into the monovalent form during the incubation period, since other divalent organic cations previously studied only adsorbed up to the CEC (paraquat), or slightly above it (diquat). Adsorption of MG on sepiolite (CEC=0.15 mol(c)/kg) further emphasizes the two patterns of its adsorption. The maximal adsorbed amounts of MG(2+) and MGOH(+) were 0.09 and 0.30 mol/kg clay, respectively. X-ray diffraction measurements gave lower values for the basal spacings for montmorillonite-MG(+) than for MGOH(+), suggesting that MG(2+) binds two clay platelets together, as in the case of other divalent cations. A competition for adsorption between MG and the monovalent organic cation, acriflavin (AF), gave lower adsorbed amounts of AF when competing with MG(+), which is interpreted to be due to the smaller basal spacing in this case, which partially inhibits the entry of AF molecules into the interlammelar space. Spectra of montmorillonite-MG particles in the visible range exhibited significant differences between clay-MG and clay-carbinol. Copyright 2000 Academic Press.

Interactions of Monovalent Organic Cations with Pillared Clays.

Interactions between an acid-activated pillared clay and several organic cations including dyes (methylene blue, MB; crystal violet, CV; acriflavin, AF) and benzyl derivatives (benzyltrimethylammonium, BTMA; benzyltriethylammonium, BTEA) were studied by adsorption measurements and X-ray diffraction. When the dyes were adsorbed from low ionic strength solutions, adsorption was irreversible but saturated at levels below the cation exchange capacity (CEC) of the clay (0.6 meq/g). The difference with CEC value was largest for CV. This mode of adsorption was interpreted in terms of interlayer adsorption with steric hindrance in the pillared galleries. On the other hand, when the dyes were adsorbed from high ionic strength solutions, adsorption levels well beyond the CEC of the clay could be reached, in particular for MB and CV. This was interpreted in terms of a second adsorption mode, involving formation of molecular aggregates on the outer surface of the clay, as evidenced by X-ray diffraction. The behavior of the cationic benzyl derivatives was markedly different, with an adsorption level always below the CEC and a decrease of adsorption as the ionic strength was increased, as expected for non-complex-forming cations. Copyright 1999 Academic Press.

Computation of surface electrical potentials of plant cell membranes . Correspondence To published zeta potentials from diverse plant sources

A Gouy-Chapman-Stern model has been developed for the computation of surface electrical potential (psi0) of plant cell membranes in response to ionic solutes. The present model is a modification of an earlier version developed to compute the sorption of ions by wheat (Triticum aestivum L. cv Scout 66) root plasma membranes. A single set of model parameters generates values for psi0 that correlate highly with published zeta potentials of protoplasts and plasma membrane vesicles from diverse plant sources. The model assumes ion binding to a negatively charged site (R- = 0.3074 &mgr;mol m-2) and to a neutral site (P0 = 2.4 &mgr;mol m-2) according to the reactions R- + IZ &rlharr; RIZ-1 and P0 + IZ &rlharr; PIZ, where IZ represents an ion of charge Z. Binding constants for the negative site are 21, 500 M-1 for H+, 20,000 M-1 for Al3+, 2,200 M-1 for La3+, 30 M-1 for Ca2+ and Mg2+, and 1 M-1 for Na+ and K+. Binding constants for the neutral site are 1/180 the value for binding to the negative site. Ion activities at the membrane surface, computed on the basis of psi0, appear to determine many aspects of plant-mineral interactions, including mineral nutrition and the induction and alleviation of mineral toxicities, according to previous and ongoing studies. A computer program with instructions for the computation of psi0, ion binding, ion concentrations, and ion activities at membrane surfaces may be requested from the authors.

Binding and electrostatic attraction of lanthanum (La3+) and aluminum (Al3+) to wheat root plasma membranes.

A general model for the sorption of trivalent cations to wheat-root (Triticum aestivum L cv. Scout 66) plasma membranes (PM) has been developed and includes the first published coefficients for La3+ and Al3+ binding to a biological membrane. Both ions are rhizotoxic, and the latter ion is the principal contributor to the toxicity of acidic soils around the world. The model takes into account both the electrostatic attraction and the binding of cations to the negatively charged PM surface. Ion binding is modeled as the reaction P- + IZ <==> 'PIZ-1 in which P- represents a negatively charged PM ligand, located in an estimated area of 540 A2, and IZ represents an ion of charge Z. Binding constants for the reaction were assigned for K+ (1 M-1) and Ca2+ (30 M-1) and evaluated experimentally for La3+ (2200 M-1) and H+ (21,500 M-1). Al sorption is complicated by Al3+ hydrolysis that yields hydroxoaluminum species that are also sorbed. Binding constants of 30 and 1 M-1 were assigned for AlOH2+ and Al(OH)+2, respectively, then a constant for Al3+ (20,000 m-1) was evaluated experimentally using the previously obtained values for K+, Ca2+ and H+ binding. Electrostatic attraction was modeled according to Gouy-Chapman theory. Evaluation of parameters was based upon the sorption of ions to PM vesicles suspended in solutions containing variable concentrations of H+, Ca2+ and La3+ or Al3+. Use of small volumes, and improved assay techniques, allowed the measurement of concentration depletions caused by sorption to vesicles. Some independent confirmation of our model is provided by substantial agreement between our computations and two published reports of La3+ effects upon zeta potentials of plant protoplasts. The single published report concerning the electrostatic effects of Al on cell membranes is in essential agreement with the model.