Silica, Alumina, and Clay-Catalyzed Alanine Peptide Bond Formation

The peptide bond formation of alanine (ala), ala + glycine (gly), ala + diglycine (gly₂), and ala + gly cyclic anhydride (cyc-gly₂) in drying/wetting cycles at 80°C was studied. Silica, alumina, and representative smectites—montmorillonite and hectorite—were used as catalysts, and the dependence of reaction yields on the available amount of water in the reaction systems was evaluated. Silica and alumina catalyze the formation of oligopeptide mainly in temperature fluctuation experiments, whereas higher amounts of water in the reaction system support clay-catalyzed reactions. Silica and alumina are much more efficient for amino acid dimerization than clays. Whereas only 0.1% of ala oligomerized on hectorite and no reaction proceeded on montmorillonite, about 0.9 and 3.8% alanine converted into its dimer and cyclic anhydride on silica and alumina, respectively. Clay minerals, on the other hand, seem to more efficiently catalyze peptide chain elongation than amino acid dimerization. The reaction yields of ala-gly-gly and gly-gly-ala from ala + gly₂ and ala + cyc-gly₂ reached about 0.3% on montmorillonite and 1.0% on hectorite. The possible mechanisms of these reactions and the relevance of the results for prebiotic chemistry are discussed. Received: 15 December 1996 / Accepted: 1 May 1997     

Thermoplastic starch–clay nanocomposites and their characteristics

In the quest for improved performance from polymers that offer biodegradation and therefore environmental acceptability, one approach is the addition of natural clays to produce nanocomposites. This study examines nanocomposites of glycerol-plasticized starch, with untreated montmorillonite and hectorite. Treated hectorite and kaolinite were added to produce conventional composites within the same clay volume fraction range for comparison. X-ray diffraction and transmission electron microscopy are used to confirm the type of composite. The ultrasonic pulse-echo technique was used to measure Young's and shear modulus. The nanocomposites presented greater increases in modulus for a given volume fraction of clay thus contributing to this new class of biodegradable and environmentally acceptable materials, although the results indicate that a plasticizer other than glycerol is preferable.     

Analysis of peptides synthesized in the presence of SAz-1 montmorillonite and Cu(2+) exchanged hectorite.

We have investigated the synthesis of oligopeptides containing glycine and tyrosine in the presence of the clay minerals montmorillonite (non-exchanged, SAz-1) and Cu(2+) exchanged hectorite. In both cases, homopolymers of the two amino acids are formed, as are mixed peptides. In the case of Cu(2+) hectorite, mixed oligopeptides up to trimers are detected in small amounts. For montmorillonite, heterogeneous oligopeptides up to hexamers are detected. Our experiments indicate montmorillonite is more effective in promoting oligopeptide formation than Cu(2+) hectorite. Analysis of the oligopeptide sequences formed on the montmorillonite surfaces indicates preferential synthesis of certain Gly-Tyr sequences over others.     

Adsorption of reovirus to clay minerals: effects of cation-exchange capacity, cation saturation, and surface area.

The adsorption of reovirus to clay minerals has been reported by several investigators, but the mechanisms defining this association have been studied only minimally. The purpose of this investigation was to elucidate the mechanisms involved with this interaction. More reovirus type 3 was adsorbed, in both distilled and synthetic estuarine water, by low concentrations of montmorillonite than by comparable concentrations of kaolinite containing a mixed complement of cations on the exchange complex. Adsorption to the clays was essentially immediate and was correlated with the cation-exchange capacity of the clays, indicating that adsorption was primarily to negatively charged sites on the clays. Adsorption was greater with low concentrations of clays in estuarine water than in distilled water, as the higher ionic strength of the estuarine water reduced the electrokinetic potential of both clay and virus particles. The addition of cations (as chloride salts) to distilled water enhanced adsorption, with divalent cations being more effective than monovalent cations and 10(-2) M resulting in more adsorption than 10(-3) M. Potassium ions suppressed reovirus adsorption to montmorillonite, probably by collapsing the clay lattices and preventing the expression of the interlayer-derived cation-exchange capacity. More virus was adsorbed by montmorillonite made homoionic to various mono-, di-, and trivalent cations (except by montmorillonite homoionic to potassium) than by comparable concentrations of kaolinite homoionic to the same cations. The sequence of the amount of adsorption to homoionic montmorillonite was Al greater than Ca greater than Mg greater than Na greater than K; the sequence of adsorption to kaolinite was Na greater than Al greater than Ca greater than Mg greater than K. The constant partition-type adsorption isotherms obtained when the clay concentration was maintained constant and the virus concentration was varied indicated that a fixed proportion of the added virus population was adsorbed, regardless of the concentration of infectious particles. A heterogeneity within the reovirus population was indicated.     

Adsorption of reovirus to clay minerals: effects of cation-exchange capacity, cation saturation, and surface area

The adsorption of reovirus to clay minerals has been reported by several investigators, but the mechanisms defining this association have been studied only minimally. The purpose of this investigation was to elucidate the mechanisms involved with this interaction. More reovirus type 3 was adsorbed, in both distilled and synthetic estuarine water, by low concentrations of montmorillonite than by comparable concentrations of kaolinite containing a mixed complement of cations on the exchange complex. Adsorption to the clays was essentially immediate and was correlated with the cation-exchange capacity of the clays, indicating that adsorption was primarily to negatively charged sites on the clays. Adsorption was greater with low concentrations of clays in estuarine water than in distilled water, as the higher ionic strength of the estuarine water reduced the electrokinetic potential of both clay and virus particles. The addition of cations (as chloride salts) to distilled water enhanced adsorption, with divalent cations being more effective than monovalent cations and 10(-2) M resulting in more adsorption than 10(-3) M. Potassium ions suppressed reovirus adsorption to montmorillonite, probably by collapsing the clay lattices and preventing the expression of the interlayer-derived cation-exchange capacity. More virus was adsorbed by montmorillonite made homoionic to various mono-, di-, and trivalent cations (except by montmorillonite homoionic to potassium) than by comparable concentrations of kaolinite homoionic to the same cations. The sequence of the amount of adsorption to homoionic montmorillonite was Al greater than Ca greater than Mg greater than Na greater than K; the sequence of adsorption to kaolinite was Na greater than Al greater than Ca greater than Mg greater than K. The constant partition-type adsorption isotherms obtained when the clay concentration was maintained constant and the virus concentration was varied indicated that a fixed proportion of the added virus population was adsorbed, regardless of the concentration of infectious particles. A heterogeneity within the reovirus population was indicated.     

The adsorption of nucleotides and polynucleotides on montmorillonite clay

The binding of adenine derivatives to Na(+)-montmorillonite increases in the order 5'AMP, 3'-AMP, 5'ADP < adenosine < purine, adenine. With the exception of cytosine, cytosine derivatives bind less strongly than the corresponding adenine derivatives in the order 5'-CMP < cytidine < cytosine. There is little difference in the binding of uracil derivatives and these compounds bind less strongly than the corresponding adenine analogs. It is concluded that the adenine ring in adenine derivatives is protonated by the acidic montmorillonite surface and binding is a consequence of the electrostatic interaction between the protonated base and the negative charges on the surface of the montmorillonite. Different binding trends were observed with Cu(2+)-montmorillonite with AMP binding more strongly than adenosine and UMP binding more strongly than uridine. It is concluded that ligation to the Cu2+ is a major force in the binding of nucleotides to Cu(2+)-montmorillonite and are not readily washed from the clay. Factors contributing to the binding are discussed. Watson-Crick hydrogen bonding of 5'-AMP to poly(U) and 5'GMP to poly(C) was observed when the homopolymers are bound to the surface of the clay. No association of 5'-UMP to poly(U) bound to clay was detected. The possible role of montmorillonite clays in the prebiotic formation of RNA is discussed.     

Slow vapor‐phase desorption of toluene from several ion‐exchanged montmorillonites

The sorption and desorption of volatile compounds from soils and clays exhibit a wide range of kinetics. While much of the sorptive interaction is very rapid, a certain fraction of volatile compounds that enter soil and clays are only slowly desorbed. It is generally believed that the formation of this recalcitrant or slowly desorbing fraction of volatile organic compounds (VOCs) in soils is due to the diffusion of compounds to poorly accessible sorption sites. However, the exact nature of these sites is in doubt. In montmorillonite, there are two likely possibilities for formation of the recalcitrant fraction: sites between the clay lamella and sites within clay particle aggregates. Because montmorillonite may be an important fraction of many soils, we have explored the formation of slowly desorbing toluene on a montmorillonite clay that was ion exchanged with five different ions (K⁺, Na⁺, Ca²⁺, Mg²⁺, and Fe³⁺) to form mineralogically similar clays with varying interlamellar spacing. The recalcitrant fraction was quantified for varying sorption and desorption times. The type of ion exchanged into the clay appears to have an important influence on the formation of a recalcitrant fraction.     

The effect of smectite composition on the catalysis of peptide bond formation

Clay-catalyzed glycine and diglycine oligomerizations were performed as drying/wetting cycles at 80°C. Two trioctahedral smectites (hectorite and saponite), three pure montmorillonites, a ferruginous smectite, an Fe(II)-rich smectite, and three smectites containing goethite admixture were used as catalysts. Highest peptide bond formation was found with trioctahedral smectites. About 7% of glycine was converted to diglycine and diketopiperazine on hectorite after 7 days. In the case of dioctahedral smectites, highest yields were achieved using clays with a negative-layer charge localized in the octahedral sheets (up to 2% of converted glycine after 7 days). The presence of Fe(II) in clay is reflected in a higher efficiency in catalyzing amino acid dimerization (about 3.5% of converted glycine after 7 days). The possible significance of the results for prebiotic chemistry is discussed. Received: 20 February 1996 / Accepted: 26 April 1996     

Adsorption of Adenine, Cytosine, Thymine, and Uracil on Sulfide-Modified Montmorillonite: FT-IR, M?ssbauer and EPR Spectroscopy and X-Ray Diffractometry Studies

In the present work the interactions of nucleic acid bases with and adsorption on clays were studied at two pHs (2.00, 7.00) using different techniques. As shown by Mössbauer and EPR spectroscopies and X-ray diffractometry, the most important finding of this work is that nucleic acid bases penetrate into the interlayer of the clays and oxidize Fe²⁺ to Fe³⁺, thus, this interaction cannot be regarded as a simple physical adsorption. For the two pHs the order of the adsorption of nucleic acid bases on the clays was: adenine????cytosine?>?thymine?>?uracil. The adsorption of adenine and cytosine on clays increased with decreasing of the pH. For unaltered montmorillonite this result could be explained by electrostatic forces between adenine/cytosine positively charged and clay negatively charged. However for montmorillonite modified with Na₂S, probably van der Waals forces also play an important role since both adenine/cytosine and clay were positively charged. FT-IR spectra showed that the interaction between nucleic acid bases and clays was through NH⁺ or NH ₂ ⁺ groups. X-ray diffractograms showed that nucleic acid bases adsorbed on clays were distributed into the interlayer surface, edge sites and external surface functional groups (aluminol, silanol) EPR spectra showed that the intensity of the line g????2 increased probably because the oxidation of Fe²⁺ to Fe³⁺ by nucleic acid bases and intensity of the line g?=?4.1 increased due to the interaction of Fe³⁺ with nucleic acid bases. Mössbauer spectra showed a large decreased on the Fe²⁺ doublet area of the clays due to the reaction of nucleic acid bases with Fe²⁺.     

Clays as possible catalysts for peptide formation in the prebiotic era

From the point of view of prebiotic synthesis, clays might have performed functions of concentration, catalysis, and protection of molecules.The degrees of polymerization obtained, when amino acid adenylates are added to montmorillonite suspensions in water, are much higher than those obtained by polymerization in the absence of such a clay. In addition, they are of a discrete spectrum, usually multiplies of 6 or 7, and reach values of up to 40 mers. In the absence of clay a continuous spectrum of degrees of polymerization is obtained, and usually up to 4–6 mers only. Copolymerization in the absence of clays yields mostly random copolymers, in their presence mostly block copolymers are obtained.Optical density measurements show that after adsorption has taken place on the clay, stacking of its layers occurs. Polymerization starts only after these stacked layers have been formed. The distances between the layers — as measured by X-rays — increase during polymerization, probably because the resulting polymers settle in their interspace, while the adsorption site of the active monomers is at the edges of the clay.     

Cysteine, thiourea and thiocyanate interactions with clays: FT-IR, Mössbauer and EPR spectroscopy and X-ray diffractometry studies

The present study examined the adsorption of cysteine, thiourea and thiocyanate on bentonite and montmorillonite at two different pHs (3.00, 8.00). The conditions used here are closer to those of prebiotic earth. As shown by FT-IR, Mössbauer and EPR spectroscopy and X-ray diffractometry, the most important finding of this work is that cysteine and thiourea penetrate into the interlayer of the clays and reduce Fe³⁺ to Fe²⁺, and as consequence, cystine and c,c′-dithiodiformamidinium ion are formed. This mechanism resembles that which occurs with aconitase. This is a very important result for prebiotic chemistry; we should think about clays not just sink of molecules, but as primitive vessels of production of biomolecules. At pH 8.00, an increasing expansion was observed in the following order for both minerals: thiourea > thiocyanate > cysteine. At pH 3.00, the same order was not observed and thiourea had an opposite behavior, being the compound producing the lowest expansion. Mössbauer spectroscopy showed that at pH 8.00, the proportion of Fe²⁺ ions in bentonite increased, doubling for thiourea, or more than doubling for cysteine, in both clays. However, at pH 3.00, cysteine and thiourea did not change significantly the relative amount of Fe²⁺ and Fe³⁺ ions, when compared to clays without adsorption. For thiocyanate, the amount of Fe²⁺ produced was independent of the pH or clay used, probably because the interlayers of clays are very acidic and HSCN formed does not reduce Fe³⁺ to Fe²⁺. For the interaction of thiocyanate with the clays, it was not possible to identify any potential compound formed. For the samples of bentonite and montmorillonite at pH 8.00 with cysteine, EPR spectroscopy showed that intensity of the lines due to Fe³⁺ decreased because the reaction of Fe³⁺/cysteine. Intensity of EPR lines did not change when the samples of bentonite at pH 3.00 with and without cysteine were compared. These results are in accordance with those obtained using Mössbauer and FT-IR spectroscopy.     

INFLUENCE OF THE CLAY TYPES ILLITE AND MONTMORILLONITE ON THE UPTAKE OF65 Zn BY SCENEDESMUS OBLIQUUS.

SUMMARY

The stimulatory effect of the clays illite and montmorillonite on zinc uptake by Scenedesmus obliquus is discussed. The competitive effects of hydrogen, phosphate, magnesium and copper ions on zinc uptake are illustrated. This competition is reduced in the presence of illite and montmorillonite and zinc uptake is stimulated. Ferrous ions have no effect on zinc uptake, while calcium ions antagonize zinc uptake at high concentrations. The latter effect was remarkable in the presence of montmorillonite.     

Clays in prebiological chemistry

Summary In this review an attempt is made to highlight the structures and properties of clay that may contribute to a better understanding of the role of clays in chemical evolution. The adsorption of organic molecules on clays has been demonstrated, as has the synthesis of bioorganic monomers in the presence of clays. For instance, amino acids (glycine, aspartic acid, threonine, alanine and others) as well as purines and pyrimidines, have been obtained from CO and NH₃ in the presence of clays at relatively high temperatures (250-325°C). Carbohydrates are also easily derived from formaldehyde at relatively low temperatures (80°C). The oligomerization of biochemical monomers, mediated by clays has also been shown to result in the formation of polymer molecules basic to life. For instance the condensation of amino acyl adenylates at room temperature in the presence of montmorillonite is known to yield polypeptides in discrete ranges of molecular weights with degrees of polymerization up to 56. Clays have also been found to affect the condensation of mononucleotides to oligonucleotides. Although the role of clays in the origin of metabolic pathways has not been demonstrated, it is possible that clays may have played a cooperative role with catalytic peptides in an intermediate stage of prebiological chemistry preceding the emergence of life on this planet.     

The effect of clays on the oligomerization of HCN

Summary The reaction of 0.1 M HCN and dilute solutions of diaminomaleonitrile (DAMN) at pH 8–9 and 25°C in the presence of suspensions of montmorillonite (bentonite) clays were investigated. Montmorillonite clays inhibit the oligomerization of aqueous solutions of HCN. Yields of colored oligomers, urea, and DAMN, are all diminished by clays, but the rate of loss of cyanide is not significantly decreased. The inhibition of oligomer formation is due to the clay-catalyzed decomposition of DAMN. The absence of strong binding of DAMN to clays was suggested by our failure to detect DAMN when a clay that had been incubated with DAMN was washed with spermidine (6 × 10^–3 g/{ie317-1}). It was established that DAMN does not simply bind to the clays by the observation that the bulk of the radioactivity was recovered from the supernatant in the reaction of¹⁴C-DAMN with montmorillonite. The clay-catalyzed decomposition of DAMN was observed when montmorillonite from two different sources was used and with a variety of homoionic montmorillonites and bentonites. A modification of the established procedure for using the cyanide electrode for cyanide analyses was used to follow the release of HCN from DAMN. This new method can be used in both the acidic and basic pH range and it does not result in the destruction of DAMN by the reagents used for the analysis. Quantitative analyses of the reaction solution from the clay-catalyzed decomposition of DAMN revealed the formation of 1–2 equivalents of HCN per mole of DAMN. The possible significance of these clay-catalyzed reactions in chemical evolution is discussed. Chemical Evolution 35: For the previous papers in this series see Ferris, J P. and Joshi, P.C., (1978), Science 201, 361–362; Ferris, J.P., Narang, R.S., Newton, T.A. and Rao, V.R., (1979), J. Org. Chem. 44, 1273–1278; Ferris, J.P. (1979), Science 203, 1135–1136; Ferris, J.P. and Joshi, P.C., (1979), J. Org. Chem. 44, 2133–2137     

Rheology of starch–clay nanocomposites

The effects of incorporating various montmorillonite nanoclays into wheat, potato, corn, and waxy corn starch samples were examined by rheology and X-ray diffraction. The nanoclays included the hydrophilic Cloisite Na+ clay as well as the more hydrophobic Cloisite 30B, 10A, and 15A clays. Frequency sweep and creep results for wheat starch–nanoclay samples at room temperature indicated that the Cloisite Na+ samples formed more gel-like materials than the other nanoclay samples. X-ray diffraction results showed no intercalation of Cloisite Na+ clays at room temperature, suggesting that starch granules interacted only with the clay surface and not the interlayer. When the various wheat starch–nanoclay samples were heated to 95 °C, the Cloisite Na+ samples exhibited a large increase in modulus. In contrast, the more hydrophobic nanoclay samples had comparable modulus values to the neat starch sample. These results suggested that during gelatinization, the leached amylose interacted with the Cloisite Na+ interlayer, producing better reinforcement and higher modulus values. X-ray diffraction results supported this interpretation since the data showed greater intercalation of Cloisite Na+ clay in the gelatinized samples. The samples containing wheat and corn starch showed comparable elastic modulus values during gelatinization. However, the potato and waxy corn samples had modulus values that rapidly decreased at higher temperatures.     

Effects of Water and pH on the Oxidative Oligomerization of Chloro an Methoxyphenol by a Montmorillonite Clay

This study focussed on the capacity of a montmorillonite clay to oxidize organic contaminants having activating (methoxyphenol) and deactivating (chlorophenol) substituent groups when pH and water conditions are changing. The amount and strength of Lewis and Br?nsted acidity of the clay was measured using organic indicator and titration methods. Water plays two distinct roles in the oxidation of such contaminants by clays: (1) it neutralizes the clay's Lewis acidity, thereby preventing chlorophenol from getting oxidized in significant yields; (2) it does not successfully compete with methoxyphenol for Lewis acid sites because high dimer yields are observed. The high capacity of Na⁺, Ca²⁺, and Fe³⁺ clays to oxidize phenolic compounds at high pH appears to be caused by phenolates being more reactive than the protonated form. The Lewis and Br?nsted acidity measurement of the various homoionic clays tested help explain the high capacity of the clays to oxidize phenolic compounds at low and high pH and their low capacity at near neutral pH. Finally, the results also clarify the effects of exchangeable cations on the capacity of clays to oxidize organic contaminants.     

Red‐ and green‐emitting nano‐clay materials doped with Eu3+ and/or Tb3+

Trivalent europium (Eu³⁺) and terbium (Tb³⁺) ions are important activator centers used in different host lattices to produce red and green emitting materials. The current work shows the design of new clay minerals to act as host lattices for rare earth (RE) ions. Based on the hectorite structure, nano‐chlorohectorites and nano‐fluorohectorites were developed by replacing the OH^? present in the hectorite structure with Cl^? or F^?, thus avoiding the luminescence quenching expected due to the OH^? groups. The produced matrices were characterized through X‐ray powder diffraction (XPD), transmission electron microscopy (TEM), FT‐IR, ²⁹Si MAS (magic angle spinning) NMR, nitrogen sorption, thermogravimetry‐differential scanning calorimetry (TGA‐DSC) and luminescence measurements, indicating all good features expected from a host lattice for RE ions. The nano‐clay materials were successfully doped with Eu³⁺ and/or Tb³⁺ to yield materials preserving the hectorite crystal structure and showing the related luminescence emissions. Thus, the present work shows that efficient RE³⁺ luminescence can be obtained from clays without the use of organic ‘antenna’ molecules.     

Clays as possible catalysts for peptide formation in the prebiotic era.

From the point of view of prebiotic synthesis, clays might have performed functions of concentration, catalysis, and protection of molecules. The degree of polymerization obtained when amino acid adenylates are added to montmorillonite suspensions in water, are much iigher than those obtained by polymerization in the absence of such a clay. In addition, they are of a discrete spectrum, usually multiplies of 6 or 7, and reach values of up to 40 mers. In the absence of clay a continuous spectrum of degrees of polymerization is obtained, and usually up to 4-6 mers only. Copolymerization in the absence of clays yields mostly random copolymers in their presence mostly block copolymers are obtained. Optical density measurements show that after adsorption has taken place on the clay, stacking of its layers occurs. Polymerization starts only after these stacked layers have been formed. The distance between the layers - as measured by X-rays - increase during polymerization, probably because the resulting polymers settle in their interspace, while the adsorption site of the active monomers is at the edges of the clay.     

Chemical evolution 40. clay-mediated oxidation of diaminomaleonitrile

Summary The previously reported inhibition of the oligomerization of HCN by montmorillonite clays was investigated. The inhibition is due to the oxidation of diaminomaleonitrile (DAMN) by the Fe³⁺ in the clay lattice. Fe²⁺ and oxalic acid were shown to be the reaction products. From these reaction products and the previous report that two equivalents of HCN are formed per equivalent ofDAMN, it was established that diiminosuccinonitrile (DISN) is the initial reaction product, which is rapidly hydrolyzed to oxalic acid and HCN. The same oxidative transformations are effected by Fe³⁺ bound to Dowex 50, Fe³⁺ in solution and Ni(NH₃)₆ ²⁺. The rate of reaction of DAMN decreased in the order Fe³⁺ > Fe³⁺-Dowex > montmorillonite, indicating no catalytic role for the clay in the oxidation of DAMN. Little reaction of DAMN was observed with montmorillonite in which the bulk of the iron was in the Fe²⁺ oxidation state. The possible significance of these redox reactions to chemical evolution is discussed.For the previous papers in this series see Ferris JP, Alwis KW, Edelson EH, Mount N, Hagan Jr J (1980) Origin of Life Wolman Y (ed) Reidel, Dordrecht, p 125–128 Ferris JP, Edelson EH, Auyeung JM, Joshi PC (1981) J Mol Evol 17:69-77     

Heavy metals alter the electrokinetic properties of bacteria, yeasts, and clay minerals.

The electrokinetic patterns of four bacterial species (Bacillus subtilis, Bacillus megaterium, Pseudomonas aeruginosa, and Agrobacterium radiobacter), two yeasts (Saccharomyces cerevisiae and Candida albicans), and two clay minerals (montmorillonite and kaolinite) in the presence of the chloride salts of the heavy metals, Cd, Cr, Cu, Hg, Ni, Pb, and Zn, and of Na and Mg were determined by microelectrophoresis. The cells and kaolinite were net negatively charged at pH values above their isoelectric points (pI) in the presence of Na, Mg, Hg, and Pb at an ionic strength (mu) of 3 x 10(-4); montmorillonite has no pI and was net negatively charged at all pH values in the presence of these metals. However, the charge of some bacteria, S. cerevisiae, and kaolinite changed to a net positive charge (charge reversal) in the presence of Cd, Cr, Cu, Ni, and Zn at pH values above 5.0 (the pH at which charge reversal occurred differed with the metal) and then, at higher pH values, again became negative. The charge of the bacteria and S. cerevisiae also reversed in solutions of Cu and Ni with a mu of greater than 3 x 10(-4), whereas there was no reversal in solutions with a mu of less than 3 x 10(-4). The clays became net positively charged when the mu of Cu was greater than 3 x 10(-4) and that of Ni was greater than 1.5 x 10(-4). The charge of the cells and clays also reversed in solutions containing both Mg and Ni or both Cu and Ni (except montmorillonite) but not in solutions containing both Mg and Cu (except kaolinite) (mu = 3 x 10(-4)). The pIs of the cells in the presence of the heavy metals were at either higher or lower pH values than in the presence of Na and Mg. Exposure of the cells to the various metals at pH values from 2 to 9 for the short times (ca. 10 min) required to measure the electrophoretic mobility did not affect their viability. The specific adsorption on the cells and clays of the hydrolyzed species of some of the heavy metals that formed at higher pH values was probably responsible for the charge reversal. These results suggest that the toxicity of some heavy metals to microorganisms varies with pH because the hydrolyzed speciation forms of these metals, which occur at higher pH values, bind on the cell surface and alter the net charge of the cell.(ABSTRACT TRUNCATED AT 400 WORDS)