Ion-exchange Beads Research Articles

Phosphotungstate anions supported on spherical beads of carbon as highly efficient catalysts for the dehydration of propan-2-ol to propene

There is a continuing need to develop new catalysts for the dehydration of propan-2-ol as opposed to di- iso-propyl ether. The reaction has been extensively studied using zeolites [1] or clays [2] with typical selectivities for propene in the region of 20–40%. The reaction has been used in order to monitor the catalytic behaviour of solids and activity correlates in many cases with the acidity of the solid [3–5]. Phosphotungstates supported on carbons, zirconias or silicas having been previously studied for the dehydration of propan-2-ol [6]. Uniform colloidal particles of salts of tungstophosphoric acids have been prepared by Matijevic and Perez-Maqueda [7], but in all cases, the product is present as a powder. In many cases, the above materials are extremely brittle and deteriorate rapidly when used as catalysts. As a contribution to the preparation of acidic/basic or redox catalysts in a bead form a versatile method has been developed for the support of porous oxides in pyrolysed ion-exchange beads. In previous cases, attempts have been made to remove the carbon matrix but this does not appear to be necessary. A conventional anion-exchange was used in this study, the strongly basic Amberlite IRA-93 (Cl) loaded with tungsten from an aqueous solution of sodium tungstate. After washing with water, converting to either tungstate or phosphotungstate and again washing with water the product, after pyrolysis of the metal-loaded ion-exchange resin, consists of discrete (typically 0.2–0.5 mm diameter) spherical particles of catalyst-loaded carbon, the spherical integrity of the unpyrolysed anion loaded resin being maintained. The tungstate was found to convert up to 80% propan-2-ol to propene and better than 95% conversion if phosphotungstate was used.

Preparation and characterization of cation‐exchange media based on flexible polyurethane foams

AbstractContinuous electrodeionization (CEDI) is used to deionize a solution to a level attained by mixed bed ion exchange without chemical regeneration. However, handling of ion‐exchange beads is laborious for a large‐scale CEDI system. In this study, a new type of ion‐exchange polyurethane foam containing sulfonic acid groups was synthesized by bulk condensation polymerization for use as a cation‐exchange medium. Polyurethanes are synthesized by the reaction between a diisocyanate and a polyol. Toluene diisocyanate 2,4‐80%, 2,6‐20% (http://www.tciamerica.com/) was reacted with poly(propylene glycol) to synthesize a polyurethane prepolymer and then N,N‐bis(2‐hydroxylethyl)‐2‐aminoethanesulfonic acid (BES) was added to give a foam containing sulfonic acid groups. The functional polyurethane prepolymers were characterized by proton nuclear magnetic resonance spectroscopy (1H NMR), Fourier transform infrared spectroscopy (FT‐IR), elemental analysis (EA), and gel permeation chromatography (GPC). The ion‐exchange capacity was measured as 2.5 meq/g, and the equiconductance point of the polyurethane foams were 14, 19, 29, and 33 μ/cm for BES molar ratios of 0.5, 0.7, 1.0, and 1.5, respectively. The porous plug model shows that the current flows dominantly through the solution phase of the polyurethane foam, which indicates the polyurethane foam is a suitable medium for use in a CEDI operation. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1773–1781, 2002

Electrodialysis: From an Idea to Realization

The historical validity demands some clarifications in the early history of electrodialysis. Early investigations with non-selective membranes are not to be recognized as electrodialysis-related although some of them have used the term “electrodialysis.” Only in 1950 the first synthetic ion-exchange membranes were produced (W. Juda and W.A. McRay). Preceding them membranes manufactured by T.R.E. Kressman were in fact a modified natural product—impregnated parchment membranes. Major electrodialysis" shortcomings such as membrane fouling and scaling were mostly overcome with electrodialysis reversal (EDR) commercial introduction by Ionics in 1967. Synthesis of novel sodium-selective membrane did not spark a burst in ED manufacturing. However higher demand for sodium-selective membranes can be expected in conjunction with nitrate-selective membranes for potable water denitrification—the essential step in water treatment in Europe. New break-through in ED progress was associated with development of apparatuses with ion-exchange resin beads filling desalting compartment. The analysis of energy consumption in ED process shows that the possibility of its reduction by traditional ways of membranes and spacers improvements is practically exhausted. However there is practically undeveloped option to lower the polarization resistance with help of electrodynamic instability (volumetric-charge development). Another possible improvement is creating membranes with special profile. As EDI targets higher levels of water purity the removal of weakly ionized ions becomes important issue.

Coupling sequential injection on-line preconcentration by means of a renewable microcolumn with ion-exchange beads with detection by electrothermal atomic absorption spectrometry: Comparing the performance of eluting the loaded beads with transporting them directly into the graphite tube, as demonstrated for the determination of nickel in environmental and biological samples

The design of a flow injection/sequential injection (FIA/SIA) on-line preconcentration system incorporating a renewable microcolumn with ion-exchange beads and interfaced with an electrothermal atomic absorption spectrometry (ETAAS) detector is described, and its practical applicability is demonstrated for the determination of trace level concentrations of nickel. The column is loaded with a defined volume of SP Sephadex C-25 cation-exchange resin beads and subsequently exposed to a metered volume of sample solution. The analyte loaded beads are afterwards eluted with 30 μl of diluted nitric acid, and the eluate transported via air segmentation into the graphite tube. The used beads are then discarded. The ETAAS determination is performed in parallel with the preconcentration process of the ensuing sample. An improved precision of R.S.D. 1.5% is obtained as compared to the 3.4% for the procedure in which the loaded beads are transported directly to the graphite furnace for pyrolysis and atomization, and even improved in comparison to the traditional unidirectional and bidirectional repetitive elution procedures which under comparable conditions yield R.S.D.-values of 5.8 and 4.9%, respectively. The tolerance limits for cations such as Pb(II), Zn(II), Co(II) and Mn(II) were improved up to 10–50-folds, and the linear calibration range extended to comprise 0.02–1.20 μg l −1. Because of lower operating temperature, the life time of the graphite tube is extended. An enrichment factor of 71.1 and a detection limit of 10.2 ng l −1 along with a sampling frequency of 12 h −1 were obtained, which are at the same levels as those for the previously described procedure without elution. The present approach was validated by determination of the nickel contents in two certified reference materials, an industrial waste water sample and a human urine sample.

Design of high efficiency polymeric cation exchange fibers1

AbstractIn this study, we have been exploring a new ion exchange material in the form of a fiber that could yield a number of important advantages over conventional ion exchange beads. In this approach, ion exchange fibers are prepared by 1) coating low‐cost glass fiber substrates with a polystyrene/divinylbenzene oligomer, 2) curing, and 3) sulfonating. The sulfonation process and effects of varying degrees of crosslinking were characterized through diffuse reflectance infrared spectroscopy and acid‐base neutralization titrations. Capacities of 4.7 meq/g (on a per resin basis) were easily obtained. Kinetics experiments showed the contact efficiencies of the new systems were greatly improved over the traditional beads due to greater surface associated with the film morphology and shorter diffusion lengths. This translated into an order of magnitude increase in ion exchange rate. As a result of the thin coatings, the use of solvents prior to functionalization, and preswelling of the finished product prior to end‐use were eliminated. Scanning electron microscopy was used to image the fibers and track­their mechanical integrity through various stages of­the process. Finally, repetitive regenerations proved­the long‐term stability of the spent fibers. Copyright­© 2001 John Wiley & Sons, Ltd.

Intraparticle Diffusion during Selective Sorption of Trace Contaminants: The Effect of Gel versus Macroporous Morphology

Ion exchanger beads are commercially available in two different physical morphologies, namely, gel and macroporous. While gel-type (or microporous) exchanger particles are essentially homogeneous solid phases with closely spaced functional groups, a single macroporous ion exchanger bead may be viewed as an ensemble of tiny microgels with an interconnected network of pores. Macroporous exchangers are gaining acceptance in environmental applications due to their durability, resistance to oxidation, and less susceptibility to fouling by natural organic matters. Many environmentally significant synthetic aromatic compounds, namely, chlorophenols, naphthalene and benzene sulfonic acids, nitrophenols, and quaternary benzylammonium compounds exist as ions in the aqueous phase over a wide range of pH and are very selectively removed by polymeric exchangers with hydrophobic matrixes. Anionic pentachlorophenol (PCP-), a widely used pesticide and wood preservative, was used in the study as a representative aromatic anion. This paper presents significant differences in intraparticle diffusion behaviors during sorption of trace PCP- onto gel and macroporous anion exchangers. While an increase in competing chloride ion concentration had no appreciable effect on the effective intraparticle diffusivity for gel exchangers, the same resulted in a significant enhancement of intraparticle diffusivity of macroporous exchangers. For exchangers with gel morphology, solid-phase diffusion is the primary intraparticle solute transport mechanism. For macroporous exchangers, on the contrary, the slower pore diffusion between the microgels of a single spherical bead is the rate-limiting step. The experimental observations were substantiated using an intraparticle parallel solute transport model. From a generic viewpoint, the macroscopic observation of the effect of competing solute (chloride) concentration on the effective intraparticle diffusivity during selective sorption may be used to gather microscopic knowledge about the morphology of the sorbent material. Geosorbents, although not spherical and quite complex in composition, are characteristically similar to macroporous exchangers where clusters of sorption sites are separated by pores filled with liquid. It is postulated that an enhanced understanding of the sorption behaviors of geosorbents can be obtained through the use of surrogate macroporous polymeric materials.

Simultaneous determination of trace metals by solid phase absorptiometry: application to flow analysis of some rare earths

Solid phase absorptiometry was applied to the simultaneous flow analysis of trace metals in combination with a micro black flow-through cell packed with ion exchanger beads and a multi-channel photodetector connected with optical fibers. A 4.0 cm3 sample solution containing five rare earth metal ions (Dy, Ho, Tm, Er and Nd) was introduced into a flow system and these metal ions were concentrated on a cation exchanger (Muromac 50W-X4) in a flow-through cell. The absorbance increases originating from the f-f or d-f transition bands of these metal ions were directly and continuously measured at 910 nm for Dy, at 530 nm for Ho, at 683 nm for Tm, 522 nm for Er and at 790 nm for Nd, respectively. A multi-variable analysis method was combined with the flow analysis because the absorption spectra of the five rare earth metal ions partly overlapped one another. Although no coloring reagents were used, the proposed method was about ¶200 times more sensitive than the corresponding solution method. The reproducibility of this method was less than ± 5%. The detection limits were 0.03, 0.40, 0.30, 0.35 and 0.23 mg dm–3 for Dy, Ho, Tm, Er and Nd, respectively. Five rare earth metal ions could be precisely determined in practical samples such as yttrium concentrate.

Preparation of Organic-Inorganic Composite Adsorbent Beads for Removal of Radionuclides and Heavy Metal Ions

Composite ion exchanger beads were prepared to remove the strontium and silver ions in acidic solution. Potassium titanate and nickelferrocyanate powder, which are acid resistant inorganic ion exchangers were synthesized and then mixed with polyacrylonitrile (PAN) binder to form a PAN-potassium titanate and a PAN-nickelferrocyanate composite ion exchanger beads. Spherical composite beads could be obtained by adjusting the viscosities of the composite dope in the range of 700–1000 cP. The composite beads porosities such as macropore volume and pore size were increased in proportion to the contents of PVP (polyvinylpyrrolidone) which was used as the porosity modifying chemical. The synthesized composite ion exchangers were evaluated on their adsorption characteristics for the Ag1 and Sr21 ion solutions of pH 2.

An integrated process for biomolecule isolation and purification.

Biomolecule isolation and purification from a fermentation broth usually involve centrifugation, filtration, adsorption, and chromatography steps. Each step contributes to the product cost and product loss. In this research, a cyclic process integrating commercially available ultrafiltration membranes and chromatographic resin beads was developed to achieve the same goal in one device. The device consisted of ion exchange beads on the shell side of a hollow fiber ultrafiltration module. Loading of proteins on the stationary phase on the shell side was carried out for a period of 5-20 min from the permeate on the shell side produced from tube-side feed in ultrafiltration. The eluent was then introduced either from the shell-side inlet or tube-side inlet; the chromatographic fractions were collected from the shell-side outlet. The column was regenerated/washed next to start a new cycle. Systems studied in this cyclic process include the following binary mixtures: myoglobin and beta-lactoglobulin; hemoglobin and bovine serum albumin; and myoglobin and alpha-lactalbumin. Excellent resolutions of the proteins were obtained. A yeast-based cellular suspension containing a mixture of myoglobin and alpha-lactalbumin was also applied to this device. The target proteins were recovered and purified successfully. The cyclic process-based device integrates clarification, concentration, and chromatographic purification of biomolecules and is suitable for both extracellular and intracellular products.

Retention behavior of very large biomolecules in ion-exchange chromatography

Hepatitis B virus surface antigen (HBsAg) particles were efficiently adsorbed (retained) on a Sulfate–cellulose (S–C) bead column, and then desorbed with sodium chloride solutions (0.5–3.0 M). The HBsAg particles were not efficiently retained onto either sulfopropyl–agarose (SP–A) or quaternary amine–agarose (Q–A) at pH 4.5, 6 and 8. The size-exclusion curve showed that proteins of molecular mass higher than ca. 20 000 cannot penetrate into the pores of S–C beads. The dynamic binding capacity (DBC) values of lysozyme (ca. 7 mg/ml-gel) and of γ-globulin (ca. 3 mg/ml gel) for S–C did not depend on the flow velocity while the DBC of γ-globulin for SP–A decreased sharply with an increase in flow velocity. These results indicated that very large molecules are adsorbed only onto the surface of S–C, which resulted in fast adsorption–desorption rates although the equilibrium adsorption capacity is lower than conventional porous gel beads. Because of the rapid adsorption rate, the DBC values of γ-globulin for S–C at high flow-rate regions are similar to those for SP–A. Bovine serum albumin was not adsorbed onto S–C. As this can not be explained by a simple electrostatic interaction mechanism, molecular recognition of S–C might be different from the agarose-based ion-exchange beads.

Localized, positive charge mediates adhesion of rhodosporidium toruloides to barley leaves and polystyrene

The physicochemical forces that mediate attachment of yeasts to the phylloplane are unknown. Cell surface charge and hydrophobicity and adhesion to polystyrene, glass, and barley were assessed for wild-type Rhodosporidium toruloides and attachment-minus (Att-) mutants. Cells were grown under conditions promoting (excess carbon) or not promoting (excess nitrogen) capsule production. Hydrophobicity was measured by adhesion to xylenes, and surface charge characteristics were assessed by attachment to either DEAE (positive)- or carboxymethyl (CM) (negative)-Sephadex ion-exchange beads. Hydrophobicity and adhesiveness of nonencapsulated, wild-type R. toruloides decreased from mid-log to late stationary phase. Encapsulated wild-type R. toruloides cells were more hydrophobic and more adhesive than nonencapsulated cells. However, two encapsulated Att- mutants were more hydrophobic than the wild type and levels of adhesion of R. toruloides were similar on polystyrene and less hydrophobic glass surfaces. Adhesion of wild-type yeast to barley and polystyrene was correlated with attachment to CM-Sephadex beads, indicating a positive cell surface charge. Sixteen Att- mutants did not exhibit a positive cell surface charge, and wild-type yeast cells that did not attach to CM-Sephadex did not adhere to either polystyrene or barley. Wild-type R. toruloides attached to CM-Sephadex beads by the poles of the cells, indicating a localization of positive charge which was also visualized with India ink. We conclude that localized, positive charge, and not hydrophobic interactions, mediates attachment of R. toruloides to barley leaves.

Biodegradable positively charged ion exchange beads: a novel biomaterial for enhancing soft tissue repair.

Previous work in the area of vulnerary agents is extensive. One material of focus has been positively charged ion exchange beads, which have been shown to promote a variety of wound-healing responses in several models. The goal of this work was to improve upon the clinical utility of positively charged dextran beads by creating a biodegradable version that maintains the material's inherent efficacy. A chemical method consisting of a sodium periodate oxidation was used to create a degradable diethylaminoethanol crosslinked dextran bead. The ability of this process to create a degradable bead was verified in vitro and in vivo. Furthermore, efficacy was shown in a rat linear incision model for a variety of beads exhibiting different degradation rates. The results show that efficacy is maintained by a degradable bead, but there is a diminution of the magnitude of the response as the mass loss rate is increased. Efficacy also was investigated for a moderate but completely degrading bead material over time and dose. Wound breaking strength was evaluated at days 7, 10, 14, 21, and 28 for degradable beads at doses of 10 mg/mL and 50 mg/mL. Although little difference in efficacy was noted for the increased dose, statistically significant increases over control were seen at days 7, 10, and 14 for the 10 mg/mL dose and at days 10 and 14 for the 50 mg/mL dose. At days 21 and 28 there were no differences between treated and control wounds.

Automated solid phase extraction of theophylline by sequential injection on renewable column

A miniaturized and fully automated solid phase extraction system was developed based on sequential injection onto a renewable microcolumn of an ion exchanger. Extraction of theophylline from caffeine solutions was used as an example of sample preparation. An important feature, compared to commercial sorbent extraction systems, was the elimination of prepackaged sorbent cartridges by using an automatically renewable microcolumn. The method has evolved from recent innovations in sequential injection analysis using a jet ring cell that entraps ion exchanger beads and discards them after completion of the monitoring cycle.

EFFECT OF THE HETEROGENEITY OF MACROPOROUS STYRENE-DVB RESINS ON ION-EXCHANGE EQUILIBRIA

ABSTRACT Equilibrium isotherms at high total ionic concentration have been measured for exchange of light cations and hydrogen, as reference ion. with a sulphonic highly-crosslinked polystyrene-divinylbenzene resin. All but one of the isotherms show an inflexion point, even at low solution concentrations, and none of them can be fitted with the constant-selectivity model. This behaviour can be explained assuming that ion-exchange beads are heterogeneous. In fact, the copolymerization process of macroporous styrene-divinylbenzene resins leads to particles composed by highly crosslinked microparticles pasted together with less crosslinked material. Moreover, the sulphonic styrene may show a different selectivity to that of the sulphonic divinylbenzene. A mathematical model was developed based on the postulation of two areas inside the resin beads, having different selectivity coefficients. The volumetric fraction of these areas is supposed to be the degree of cross-linking. Therefore, the model has only two adjustable parameters: the selectivity coefficient of both areas. In spite of its simplicity, it was fitted quite successfully to the whole set of laboratory data and also predicted the data collected by other authors for resins covering the complete range of cross-linking.

Reaction-diffusion waves Homogeneous and inhomogeneous effects

Experimental evidence for the spontaneous formation of spiral waves and crossing wave patterns for the Belousov–Zhabotinsky reaction in solution are presented. Also observed are so-called ‘lateral instabilities’ with the spontaneous deformation of circular fronts in systems of low excitability. Lateral instabilities in resin-based systems, with the redox catalyst immobilised on ion-exchange beads, and the effect of the ageing of solutions, are also reported.

Efficacy of positively charged ion exchange beads in radiation-impaired models of wound healing.

We have previously demonstrated in a rat surgical incision model that charged dextran beads enhance wound repair in a charge-specific manner: positively charged beads increased wound breaking strength (WBS), whereas neutral or negatively charged beads were ineffective. The present work extends these observations into two models of radiation-impaired healing. One hundred five rats were divided into three groups: group 1, controls, no irradiation; group 2, total body irradiation; group 3, surface irradiation. Three days after irradiation, 0.1 ml of a suspension of either positively charged beads, neutral beads, or vehicle alone was applied to paired 6-cm incisions on the dorsum of the rats. Ten days later, wounds were excised and sample strips were harvested for determination of WBS and for histological analysis. Following total body irradiation, positively charged beads enhanced WBS 84% compared to beadless controls. When results with positively charged beads were compared to those with neutral beads, a 39% increase in WBS was noted. Healing, as measured by WBS, was more impaired in surface-irradiated animals, but when results with positively charged beads were compared with those obtained with neutral beads, a similar degree of healing (+38%) was noted. Neutral beads proved ineffective. Histology revealed that neutral beads evoked a modest foreign body response, yet there was a consistently greater clustering of giant cells around positively charged beads in wound sites. This paralleled in vitro experiments demonstrating increased monocyte aggregation around positively charged beads in tissue culture. The results suggest that positively charged ion exchangers, such as these positively charged beads, perhaps through their recruitment and/or activation of wound macrophages, may have a potential clinical role in the treatment of impaired wounds.

Strong Cation and Anion Exchange Membranes and Beads for Protein Isolation from Whey

Abstract Protein sorption from raw rennet whey at pH 3.0 onto S-type (strong acid), and at pH 7.2 onto Q-type (strong base), ion exchange beads and membranes was measured. The data were used to analyze protein binding capacities, percentage recoveries, production rates, and design parameters for a commercial-scale process. Performance of S-type beads was superior to Q-type beads, but Q-type and S-type membranes performed comparably, except that the S-type membranes lost capacity during repeated cycling without cleaning, while the Q-type membranes did not. Performance of the S-type membranes was increased by microfiltration of the whey before loading, but this was partially offset by the lower protein content of the microfiltered whey. Increasing the flow rate through the cartridge increased productivity, but the percentage protein recovery decreased. This was attributed to the pore size of the membrane, which may have been too large for protein to reach the pore wall by diffusion before it passed through the membrane. A smaller pore size may increase the capture of protein and the binding capacity, but at the expense of plugging if the pores are too small.

The development of interneurons in the chick embryo spinal cord following in vivo treatment with retinoic acid

To investigate the role of retinoic acid (RA) in the development of interneurons in the spinal cord, we examined the expression of cellular retinoic acid binding protein type I (CRABP I). The earliest developing interneurons in the chick spinal cord can be divided into two major groups: circumferential (C) neurons and primitive longitudinal (PL) neurons. In brachial segments, both types of interneurons began to express CRABP I at stage (st.) 13+ of the V. Hamburger and H.L. Hamilton (1951, J. Morphol. 88:49-92) stage series, which is before the onset of axonogenesis. Subsequently, with the onset of axonal outgrowth, C neurons and PL neurons expressed CRABP I in their cell bodies, axons, and growth cones. The expression of CRABP I was developmentally regulated. CRABP I immunoreactivity gradually decreased after st. 36 (embryonic day [E] 10) such that no interneurons expressed this protein by E21. The transient expression of CRABP I during a period of intensive axonal growth suggested that RA may be involved in the development of interneurons. To test this idea, we implanted an all-trans RA-containing ion exchange bead into either rostral segments of the spinal cord at st. 12-13 or into caudal segments at st. 15-16, all stages that are well before the appearance of CRABP-I-positive neurons in these segments. In the RA-treated spinal cord, increased numbers of pyknotic cells were found predominantly in dorsal regions, presumably reflecting the death of neuroepithelial cells, C neurons and premigratory neural crest cells. Surviving C neurons in the RA-treated spinal cord extended their axons ventrally toward the floor plate as in control embryos. PL neurons also projected their axons rostrally or caudally in the RA-treated spinal cord, similarly to control embryos. However, the proportion of caudally projecting PL neurons was significantly increased in segments rostral to the RA-containing bead. These results suggest that RA may regulate the survival and axonal orientation (directionality) of subpopulations of spinal interneurons.

Distribution of Retinoids Applied Exogenously to Chick Limb Buds: an Autoradiographic Analysis: (Retinoic Acid/Limb Bud/Pattern Formation/Autoradiograph).

An autoradiographic analysis was undertaken to examine the localization of retinoids applied exogenously to chick limb buds. Ion-exchange beads (AGI-X2) containing a tritium-labeled synthetic retinoid, Am80, were implanted to various regions of chick wing buds. This synthetic retinoid is known to induce a duplicated limb pattern as retinoic acid (RA) does. One to 24 hours after the application, wing buds were fixed, sectioned, and prepared for autoradiography. Heavy labeling was observed in the peripheral region of the wing mesoderm, but no gradient along the antero-posterior axis was found. These results suggest that the peripheral region of the limb bud may be important for the morphogenetic function of RA. Tissue-bound retinoids may not form an antero-posterior concentration gradient when retinoids are added to the anterior margin of the chick limb bud.

Relevance of charged groups for the integrity of the S-layer from Bacillus coagulans E38-66 and for molecular interactions.

In this paper, the importance of charged amino and carboxyl groups for the integrity of the cell surface layer (S-layer) lattice from Bacillus coagulans E38-66 and for the self-assembly of the isolated subunits was investigated. Amidination of the free amino groups which preserved their positive net charge had no influence on both. On the other hand, acetylation and succinylation, which converted the amino groups into either neutral or negatively charged groups, and amidation of carboxyl groups were accompanied by the disintegration or at least by the loss of the regular structure of the S-layer lattice. Treatment of S-layer monolayers with the zero-length cross-linker carbodiimide led to the introduction of peptide bonds between activated carboxyl groups and amino groups from adjacent subunits. This clearly indicated that in the native S-layer lattice the charged groups are located closely enough for direct electrostatic interactions. Under disrupting conditions in which the S-layer polypeptide chains were unfolded, 58% of the Asx and Glx residues could be amidated, indicating that they occur in the free carboxylic acid form. As derived from chemical modification of monolayer self-assembly products, about 90% of the lysine and 70% of the aspartic and glutamic acid residues are aligned on the surface of the S-layer protein domains. This corresponded to 45 amino groups and to 63 carboxyl groups per S-layer subunit. Labelling experiments with macromolecules with different sizes and charges and adsorption studies with ion-exchange particles revealed a surplus of free carboxyl groups on the inner and on the outer faces of the S-layer lattice. Since the carboxyl groups on the outer S-layer face were accessible only for protein molecules significantly smaller then the S-layer protomers or for positively charged, thin polymer chains extending from the surface of ion-exchange beads, the negatively charged sites must be located within indentations of the corrugated S-layer protein network. This was in contrast to the carboxyl groups on the inner S-layer face, which were found to be exposed on elevations of the S-layer protein domains (D. Pum, M. Sára, and U.B. Sleytr, J. Bacteriol. 171:5296-5303, 1989).