Soluble Polyacrylamide Research Articles

Ultrasonic Chain Scission of Polyacrylamide in Solution: Online Monitoring Results and Comparison with Theoretical Models

Ultrasonic processing of a water soluble polymer, polyacrylamide, is investigated and molecular weight evolution during ultrasonic degradation was monitored online by light scattering and viscometry. The resulting Data were compared to various theoretical models. Initially the scission was very rapid but as the limiting molecular weight was approached it slowed down. This slowing down was too pronounced to be described by a first order model. Only the Giz, Ovenall/Harrington/Madras and Tang models, adequately accounted for this slowing down and gave good fits to the data.

Effect of Polyacrylic Acid and Polyacrylamide on Clouding Behavior of Triton-X-100 in Aqueous Medium

Abstract The cloud point (CP) of Triton-X-100 have been studied at various concentration of surfactant in pure and mixed system with water soluble polymers polyacrylic acid (PAA) and polyacrylamide (PAM). The CP of Triton-X-100 was found to be increases with increasing surfactant concentration. The CP of Triton-X-100 has found to decrease with increase of mass of the additives PAA and PAM. The same trend was observed for all surfactant concentrations in presence of PAA and PAM. Considering CP as a threshold temperature of the solubility, the thermodynamic parameters of the clouding process (ΔG0 cl, ΔH0 cl and ΔS0 cl) have been evaluated using “Phase Separation Model”. The results indicate that the clouding phenomenon of Triton-X-100 is exothermic and in presence of polymer additives the process is endothermic and non-spontaneous.

Turbulent flow of oil-water emulsions with polymer additives

The article outlines direct and reverse oil-water emulsions. Microphotography study of these emulsions was carried out. The effect of water-soluble and oil soluble polymers on the emulsion structure and their turbulent flow velocity in cylindrical channel was investigated. It has been experimentally proven that if the fluid being transported is not homogeneous, but a two-phase oil-water emulsion, only the polymer that is compatible with dispersion medium and capable of dissolving in this medium can reduce the hydrodynamic resistance of the fluid flow. Thus, the resistance in direct emulsions can be reduced by water- soluble polyacrylamide, while oil-soluble polyhexene can be applied for reverse emulsions.

Investigation of temperature sensitivity behaviors of water soluble polyacrylamides

AbstractTemperature sensitive polymers with a lower critical solution temperature (LCST) are used in a variety of industries such as the pharmaceutical, cosmetic, food, and paint. These polymers are generally of the poly(N‐alkylacrylamide) type, of which poly(N‐isopropylacrylamide) (PNIPA) is the most commonly used. More novel poly(N‐alkylacrylamide)s have also been the subject of much attention recently. In this study, N‐alkylacrylamides containing different alkyl groups were synthesized by nucleophylic substitution reactions of various amines with acryloyl chloride. They were polymerized using the solution polymerization method, and the temperature sensitivities of the polymers were investigated. For this purpose, three monomers, N,N‐diethylacrylamide, N‐cyclopropylacrylamide, and 4‐piperidineethanolacrylamide, were synthesized using diethylamine, cyclopropylamine, and 4‐piperidineethanol, as the amines, respectively. The obtained polymers, poly(N,N‐diethylacrylamide) (PDEA), poly(N‐cyclopropylacrylamide) (PCPA), and poly(4‐piperidineethanolacrylamide) (PPEA), were found to be thermoresponsive, particularly PPEA is a potential novel material that can be utilized as an alternative to the common temperature sensitive polymers. The effects of several conditions on the LCST and the critical flocculation temperature (CFT) of the polymers were also investigated. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Distribution and Function of Macrophage Galactose-type C-type Lectin 2 (MGL2/CD301b): EFFICIENT UPTAKE AND PRESENTATION OF GLYCOSYLATED ANTIGENS BY DENDRITIC CELLS

Dendritic cells (DCs) express cell surface lectins that are potentially involved in the recognition, uptake, and presentation of glycosylated foreign substances. A unique calcium-type (C-type) lectin, the macrophage galactose (Gal)-type C-type lectin (MGL/CD301) expressed on DCs, is thought to participate in the recognition of molecules from both altered self and pathogens due to its monosaccharide specificity for Gal and N-acetylgalactosamine (GalNAc). Although mice have two MGL genes, Mgl1 and Mgl2, their distinct roles have not been previously explored. The present report characterizes the properties of MGL2 by examining its distribution and its role in antigen presentation by DCs. We generated an MGL2-specific monoclonal antibody and examined MGL2 expression in tissues by immunohistochemistry and in isolated cells by flow cytometry. The cells reactive with this antibody were shown to be a portion of MGL1-expressing cells, mostly conventional DCs. Internalization of soluble polyacrylamide polymers (PAA) with alpha-GalNAc residues (GalNAc-PAA) by bone marrow-derived DCs (BM-DCs) was mediated by MGL2, as revealed by a comparison of Mgl1(-/-) and Mgl2(-/-) BM-DCs with wild-type BM-DCs. Biotinylated GalNAc-PAA conjugated to streptavidin (SAv) was more efficiently presented to SAv-primed T cells by BM-DCs than beta-N-acetylglucosamine-PAA conjugated to SAv or SAv alone as shown by thymidine uptake and cytokine production. This is the first report that demonstrates the involvement of GalNAc residues in antigen uptake and presentation by DCs that lead to CD4(+) T cell activation.

Use of a library of mutated Maackia amurensis hemagglutinin for profiling the cell lineage and differentiation

Thirty-five variant lectins were prepared by mutations of two amino acids within the carbohydrate-recognition domain of Maackia amurensis hemagglutinin (MAH). Each lectin showed unique carbohydrate specificity according to their bindings to soluble polyacrylamide with various mono- and oligosaccharides and to glycophorin A. The relative intensity of the bindings of carcinoma, myeloid, fibroblastic, and melanoma cells to immobilized MAH variant lectins was examined. Each cell line showed distinct profiles regarding the number of cells bound to wild-type and 35 MAH variants and the differences and the similarities in these binding profiles were quantitatively documented by the cluster analysis. The cell lines were classified into several groups and these groups surprisingly corresponded to the lineage of the cells. These results indicated that a library of mutated MAH is useful as a tool for the profiling of various cells based on the variations of the surface glycans.

Synthesis and characterization of telechelic polymers obtained by micellar polymerization

The synthesis, characterization and rheological properties in aqueous solution of two telechelic polymers of different structures and size are reported: they are water soluble polyacrylamides, hydrophobically modified with linear and di-substituted hydrophobic initiators. The polymers were prepared via free radical micellar polymerization. The results of static light scattering (SLS), showed similar weight-average molecular weights (Mw) in all of the synthesized polymers. No signal corresponding to the presence of the hydrophobic group of the initiator could be observed by 1H NMR; however, the presence of these hydrophobic groups was demonstrated using rheological measurements.

Low Temperature Initiation by 3-Mercaptopropionic Acid-Ce(IV) or -KMnO4 Redox System for Polymerization of Acrylamide Monomer

ABSTRACT Polymerization of acrylamide monomer was performed at low temperatures using 3-mercaptopropionic acid-cerium(IV) sulfate and 3-mercaptopropionic acid-KMnO4 redox systems in acid aqueous medium. Water soluble polyacrylamides containing 3-mercaptopropionic acid end groups were synthesized. The effects of mole ratio of acrylamide to initiator(nMSA= nCe(IV)), polymerization time, temperature, and concentration of sulfuric acid on the yield and molecular weight of polymer were investigated. The decrease in the mole ratio of acrylamide/Ce(IV) at constant monomer concentration resulted in an increase in the yield but a decrease in molecular weight of polymer. The increase of reaction temperature from 20° to 70°C resulted in a decrease in the yield but indicated generally a constant value for the molecular weight of polymer. With increasing of polymerization time, the yield and molecular weight of polymer did not change mainly. Ce(IV) and Mn(VII) ions are reduced to Ce(III) and Mn(II) ions, respectively in the polymerization reaction. The existence of Ce(III) ion bonded to polymer was investigated by UV-visible spectrometry and fluorescence measurements. The amount of Mn(II) that is incorporated to the polymer was determined using graphite furnace atomic absorption spectrometry. The mechanism of this phenomenon is discussed.

Oxidative polymerization of acrylamide in the presence of thioglycolic acid

Abstract Chemical polymerization of acrylamide at room temperature was examined by using thioglycolic acid-cerium (IV) sulfate and thioglycolic acid-KMnO4 redox systems in acid aqueous medium. Water soluble polyacrylamides containing thioglycolic acid end groups were synthesized. The effects of the molar ratio of acrylamide to Ce(IV) nAAm/nCe(IV), the polymerization time, the temperature, the monomer concentration, the molar ratio of cerium (IV) sulfate to thioglycolic acid and the concentration of sulfuric acid on the yield and molecular weight of polymer were investigated. Lower molar ratios of acrylamide/Ce(IV) at constant monomer concentration resulted in an increase in the yield but a decrease in molecular weight of polymer. The increase of reaction temperature from 20 to 70°C resulted in a decrease in the yield but generally resulted in a constant value for the molecular weight of polymer. With increasing polymerization time, the yield and molecular weight of polymer did not change substantially. Ce(IV) and Mn(VII) ions are reduced to Ce(III) and Mn(II) ions respectively in the polymerization reaction. The existence of Ce(III) ion bound to polymer was investigated by UV-visible spectrophotometry and fluoresce measurements. The amount of Mn(II) incorporated into the polymer was determined using graphite furnace atomic absorption spectrometry. The mechanism of this phenomenon is discussed.

Neoglycoconjugates: trade and art.

This chapter deals with the tendencies in design of multivalent neoglycoconjugates for glycobiology research and high-throughput profiling technologies, including cellular phenotyping. Soluble polyacrylamide (PAA) conjugates are remarkable owing to a variety of possibilities for synthesis and application. PAA is soluble and stable, and the molecule is flexible, PAA-tethered ligands are capable of adjusting to a receptor during multiple-point interactions and PAA does not bind to the cell surface. Synthesis provides unlimited diversity of the probe types (biotin, fluorescein, allyl, digoxygenin, 3H, radiolabelled I), glyco-particles, glyco-surfaces, multiarrays, immunogens etc. Several examples illustrate the most advanced applications. (i) Dynamic systems: the selectin ligands immobilized on the surface as sugar-PAA conjugates made the study of the kinetics of rolling in the model system possible. Carbohydrate ligands that are covalently attached to the chip as sugar-PAA conjugates are of use with the surface plasmon resonance method. (ii) Pseudoglycoprotein: some questions arise regarding the biologically active glycoproteins, e.g.: is a carbohydrate or peptide fragment responsible for the activity? We have proposed the approach that promotes to answer this and other questions. The pool of oligosaccharides that were spitted off a glycoprotein is attached to PAA resulting in a pseudoglycoprotein. (iii) Virtual (dynamic) glycotope: receptor-ligand recognition, such as that of P-selectin with its receptor P-selectin glycoprotein ligand 1, frequently involves molecular interactions at two distinct sites. Using P-selectin as a model, we developed an approach to discover novel ligands. PAA was synthesized with multiple ligands; a marked synergistic inhibitory effect was observed.

Molecular Cloning and Characterization of a Novel Mouse Macrophage C-type Lectin, mMGL2, Which Has a Distinct Carbohydrate Specificity from mMGL1

A novel mouse macrophage galactose-type C-type lectin 2 (mMGL2) was identified by BLAST analysis of expressed sequence tags. The sequence of mMGL2 is highly homologous to the mMGL, which should now be called mMGL1. The open reading frame of mMGL2 contains a sequence corresponding to a type II transmembrane protein with 332 amino acids having a single extracellular C-type lectin domain. The 3'-untranslated region included long terminal repeats of mouse early transposon. The Mgl2 gene was cloned from a 129/SvJ mouse genomic library and sequenced. The gene spans 7,136 base pairs and consists of 10 exons, which is similar to the genomic organization of mMGL1. The reverse transcriptase-PCR analysis indicates that mMGL2 is expressed in cell lines and normal mouse tissues in a macrophage-restricted manner, also very similar to that of mMGL1. The mMGL2 mRNA was also detected in mMGL1-positive cells, which were sorted from thioglycollate-induced peritoneal cells with a mMGL1-specific monoclonal antibody, LOM-8.7. The soluble recombinant proteins of mMGL2 exhibited carbohydrate specificity for alpha- and beta-GalNAc-conjugated soluble polyacrylamides, whereas mMGL1 preferentially bound Lewis X-conjugated soluble polyacrylamides in solid phase assays. These two lectins may function cooperatively as recognition and endocytic molecules on macrophages and related cells.

Macrophage C-type lectin on bone marrow-derived immature dendritic cells is involved in the internalization of glycosylated antigens.

Bone marrow-derived dendritic cells (DCs) were examined for the expression of the murine macrophage C-type lectin specific for galactose and N-acetylgalactosamine (mMGL). Flow cytometric analysis after double staining for MHC class II and mMGL with specific monoclonal antibodies indicated that mMGL was expressed on immature DCs with low to moderate levels of MHC class II and down-regulated during maturation. Immature DCs bound and internalized alpha-N-acetylgalactosaminides conjugated to soluble polyacrylamide (alpha-GalNAc polymers), whereas mature DCs and bone marrow cells did not. The two-color flow cytometric profiles indicated that the degree of alpha-GalNAc polymer bindings exactly coincided with the intensity of the binding of a mMGL-specific monoclonal antibody LOM-14. The internalized alpha-GalNAc polymers seemed to be transported to MHC class II compartments. Thus, mMGL is transiently expressed on bone marrow-derived DCs during their development and maturation and suggested to be involved in the uptake of glycosylated antigens for presentation.

Neoglycoconjugates: trade and art

This chapter deals with the tendencies in design of multivalent neoglycoconjugates for glycobiology research and high-throughput profiling technologies, including cellular phenotyping. Soluble polyacrylamide (PAA) conjugates are remarkable owing to a variety of possibilities for synthesis and application. PAA is soluble and stable, and the molecule is flexible, PAA-tethered ligands are capable of adjusting to a receptor during multiple-point interactions and PAA does not bind to the cell surface. Synthesis provides unlimited diversity of the probe types (biotin, fluorescein, allyl, digoxygenin, 3 H, radiolabelled I), glyco-particles, glyco-surfaces, multiarrays, immunogens etc. Several examples illustrate the most advanced applications. (i) Dynamic systems: the selectin ligands immobilized on the surface as sugar–PAA conjugates made the study of the kinetics of rolling in the model system possible. Carbohydrate ligands that are covalently attached to the chip as sugar–PAA conjugates are of use with the surface plasmon resonance method. (ii) Pseudoglycoprotein: some questions arise regarding the biologically active glycoproteins, e.g.: is a carbohydrate or peptide fragment responsible for the activity? We have proposed the approach that promotes to answer this and other questions. The pool of oligosaccharides that were spitted off a glycoprotein is attached to PAA resulting in a pseudoglycoprotein. (iii) Virtual (dynamic) glycotope: receptor-ligand recognition, such as that of P-selectin with its receptor P-selectin glycoprotein ligand 1, frequently involves molecular interactions at two distinct sites. Using P-selectin as a model, we developed an approach to discover novel ligands. PAA was synthesized with multiple ligands; a marked synergistic inhibitory effect was observed.

Fluorescent carbohydrate probes for cell lectins

Fluorescein labeled carbohydrate (Glyc) probes were synthesized as analytical tools for the study of cellular lectins, i.e. SiaLe x-PAA-flu, Sia 2-PAA-flu, GlcNAc 2-PAA-flu, LacNAc-PAA-flu and a number of similar ones, with PAA a soluble polyacrylamide carrier. The binding of SiaLe x-PAA-flu was assessed using CHO cells transfected with E-selectin, and the binding of Sia 2-PAA-flu was assessed by COS cells transfected with siglec-9. In flow cytometry assays, the fluorescein probes demonstrated a specific binding to the lectin-transfected cells that was inhibited by unlabeled carbohydrate ligands. The intense binding of SiaLe x-PAA- 3H to the E-selectin transfected cells and the lack of binding to both native and permeabilized control cells lead to the conclusion that the polyacrylamide carrier itself and the spacer arm connecting the carbohydrate moiety with PAA did not contribute anymore to the binding. Tumors were obtained from nude mice by injection of CHO E-selectin or mock transfected cells. The fluorescent SiaLe x-PAA-flu probe could bind to the tumor sections from E-selectin positive CHO cells, but not from the control ones. Thus, these probes can be used to reveal specifically the carbohydrate binding sites on cells in culture as well as cells in tissue sections. The use of the confocal spectral imaging technique with Glyc-PAA-flu probes offered the unique possibility to detect lectins in different cells, even when the level of lectin expression was rather low. The confocal mode of spectrum recording provided an analysis of the probe localization with 3D submicron resolution. The spectral analysis (as a constituent part of the confocal spectral imaging technique) enabled interfering signals of the probe and intrinsic cellular fluorescence to be accurately separated, the distribution of the probe to be revealed and its local concentration to be measured.

The synthesis and characterization of acrylamide polymer containing R-(+)-cysteine end groups

Water soluble polyacrylamides which contain R-(+)-cysteine end groups were synthesized by using R-(+)-cysteine–Ce(IV) sulfate and R-(+)-cysteine–KMnO 4 redox systems in acid–aqueous medium. The dependence of polymerization yields and the molecular weight of polymers on the mole ratio of acrylamide to R-(+)-cysteine ( n Cys= n Ce(IV)) at different temperatures, the polymerization time, the temperature, and the concentration of sulfuric acid was investigated. The increase in the mole ratio of acrylamide to R-(+)-cysteine resulted in an increase in the molecular weight but a decrease in the yield. The increase of reaction temperature from 30°C to 60°C resulted in an increase in the molecular weights and slight increase in the yield of polymer. Ce(IV) and Mn(IV) ions are reduced to Ce(III) and Mn(II) respectively in polymerization reaction. The existence of Ce(III) ion in polymer was investigated by UV–Vis spectrometry and fluorescence measurements. The amount of Mn(II) which is incorporated in the polymer was determined. The mechanism of this phenomenon is discussed.

Carbohydrate-Based Probes for Detection of Cellular Lectins

Carbohydrate (spacered saccharide residue, Glyc) probes with various tags were synthesized as analytical tools for study of cellular lectins, i.e., Glyc–polyacrylamide–3H, Glyc–PAA–biotin, Glyc–PAA–fluorescein (flu), and Glyc–PAA–digoxigenin, where PAA is a soluble polyacrylamide carrier of ≈30 kDa. Binding of all types of probes, where Glyc is the sialyl Lewis X (SiaLeX) tetrasaccharide or a blank saccharide, was assessed using Chinese hamster ovary (CHO) cells either transfected with the E-selectin cDNA or mock-transfected. High binding of SiaLeX–PAA–3H to E-selectin-transfected cells and absence of binding to control cells (both native and permeabilized) allowed the conclusion that the polyacrylamide carrier and the spacer arm do not contribute significantly to the binding. The biotinylated probe showed a high level of nonspecific binding in cell enzyme-linked assays. A similarly built digoxigenin-labeled probe was significantly better. In flow cytometry assays, the fluorescein probe demonstrated a specific binding to E-selectin-transfected cells of a similar level to that given by an anti-E-selectin antibody. In addition, it could be inhibited by the anti-E-selectin antibody, further demonstrating specificity. Tumors were obtained from nude mice by injection of CHO E-selectin or mock-transfected cells. The fluorescent SiaLeX–PAA–flu probe could bind to tumor sections from E-selectin-positive CHO cells, but not from control CHO cells. These probes can thus be used to reveal specifically complex carbohydrate-binding sites on cells either in culture or on tissue sections.

Carbohydrate composition and immunomodulatory activity of different glycoforms of alpha1-acid glycoprotein.

The acute phase protein, alpha1-acid glycoprotein (AGP), is a normal constituent of human blood (0.2-1 mg ml(-1)) and its glycosylation and concentration in the blood change during inflammation. In this review of our recent work, we discuss the immunomodulatory properties of AGP in connection with the structure of its carbohydrate chains. AGP samples prepared from normal donor serum (nAGP), serum obtained during abortion (fAGP), serum of cancer patients (cAGP), and ascitic fluid of patients with stomach cancer (sAGP) were subjected to analysis. All the samples except for fAGP had five N-linked chains of the 'complex' type, however, the numbers of bi-, tri-, and tetra-antennary chains, as well as glycan structures terminating these chains, were different. fAGP had three N-linked chains of the lactosamine and polylactosamine type and three O-chains which were not present in AGP isolated from the other sources. The glycoforms of nAGP and sAGP that were isolated using a ConA affinity column were similar in respect to their branching, but differed in their terminal oligosaccharides. sAGP was enriched in units ending in Le(x) and asialoagalacto (GlcNAc-terminating) forms. Immunomodulatory activity of different AGP preparations was tested in vitro by measuring their effect on the proliferative response of human lymphocytes stimulated by PHA, and by determining their influence on the production of IL-1, IL-2, IL-6, and TNF in the stimulated cells. nAGP was less active compared to cancer or fetal AGP in the proliferation test, but more active in affecting cytokine production. Some AGP glycoforms had opposite immunomodulatory effects. A new approach was developed in order to clarify the role of carbohydrate chains in the biological activity of AGP. A pool of N-linked oligosaccharide chains were attached to a soluble polyacrylamide matrix. This 'pseudoglycoprotein' was similar to AGP in its molecular weight; in its relative amounts of tetra-, tri-, and bi-antennary chains; and in the content of mono-, di-, tri-, and tetra-sialylated-oligosaccharides. This pseudo-AGP displayed a similar activity to its parent AGP in the biological tests. Analytical flow cytometry of leukocyte subpopulation from human peripheral blood showed that monocytes and granulocytes but not lymphocytes were the main targets for the binding of AGP and pseudo-AGP. This binding was inhibited by synthetic glycoconjugates containing mannose or sialic acid. The binding curve data suggested that there are two monocyte and granulocyte populations. These may have different carbohydrate specificities. All the evidence provided by these studies indicate that it is the carbohydrate chains on AGP that are important in modulating the immune system and not the AGP molecule itself.

Screening of umbilical cord blood hemoglobins by isoelectric focusing in capillaries.

Separation and quantitation of the three main hemoglobin components of umbilical cord blood (fetal, acetylated fetal and adult hemoglobins; Hb F, Fac, A) by capillary isoelectric focusing (IEF) in a pH 6-8 gradient is reported. Even in coated capillaries (with covalently bound chains of linear acrylamido derivatives, notably N-acryloylaminoethoxyethanol), no base line separation is obtained between Hb F and A, although this is routinely achieved in gel slab IEF. However, when the carrier ampholyte buffers were added to 3% short-chain liquid linear polyacrylamide, base line resolution and stabilization of peak transit times were obtained. This suggests that even in the best coating procedures, patches of the inner capillary surface could still be naked, so that the static coating is complemented by a dynamic coating on the unoccupied sites. An additional improvement in separation occurs if the above mixture, comprising 5% carrier ampholytes in the pH 6-8 range and 3% soluble polyacrylamide, is made to contain 50 mM beta-alanine, a "separator" known to flatten the pH gradient around pH 7. In the normal newborns analyzed (n = 30), the following average values were obtained: Hb F, 70.1% (range 65-75%); Hb A, 20.2% (range 15-25%); and HbFac, 9.5% (range 7-11%).

POLYACRYLAMIDE CHARACTERISTICS RELATED TO SOIL APPLICATIONS

Water soluble polyacrylamides (PAMs) have been proposed as soil amendments for various agricultural purposes. Recent interest has centered on very high molecular weight (10–20 g mol−1), low to moderate charge (10–20 mol %) anionic materials, added to irrigation water to prevent silt loss from irrigated fields. These PAMs are within a broad spectrum of commercially available compounds employed for a variety of applications in other industries. These include solid-liquid separations in clarification of potable and waste waters, dewatering of sludges, mining separations, food processing and paper making, as well as petroleum recovery, textile additives, friction reduction, personal care products, and cosmetics. This paper reviews PAM characteristics (chemistry, synthesis, molecular weight, product form), residual monomer concentrations, toxicology, regulations, and fate and effects in soil and plant systems. Anionic PAMs for soil systems have residual acrylamide (AMD) concentrations of <0.05%. AMD has been found to be readily biodegradable in soil and aquatic environments. Anionic PAMs exhibit low toxicity to mammalian systems (oral and dermal LD50 > 5 mg Kg−1) and fish (LC50 > 100 mg L−1)

Water soluble polyacrylamide as embedding medium for electron microscopy

We have developed a method for embedding biological materials for electron microscopy in the water soluble polymer polyacrylamide (PAM). The fixed samples are infiltrated with an aqueous solution of acrylamide, which is then polymerized to form a hydrated gel. The gel is ready for sectioning after insolubilization by cross-linking with glutaraldehyde. Water is retained throughout the whole embedding process and contact of the samples with organic solvents is completely avoided. Sections are best stained with silicotungstic acid. The potential of this technique has been evaluated by detailed examination of several representative tissues. Observation of the native periodicity of 17 nm in PAM embedded myelin indicates a very good structural preservation. The method gives optimal results with samples which are easily infiltrated, such as pellets of cells.