Changes In Molecular Size Distribution Research Articles

Deagglomeration of Asphalt Binder Using Kraft Lignin: A Sustainable Valorization of Waste from the Pulping Industry

This study assessed the potential of lignin, derived from black liquor, for deagglomerating asphaltene fractions. Chemical and rheological tests were employed to evaluate the efficacy of the lignin additives. Gas chromatography-mass spectrometry (GC–MS) analyzed the chemical components of lignin (L1) isolated with sulfuric acid and lignin (L2) isolated with hydrochloric acid. Gel permeation chromatography (GPC) examined changes in molecular size distribution and deagglomeration effectiveness of the lignin additives. Fourier transform infrared spectroscopy (FTIR) assessed the chemical functional groups of asphalt, while the rheological performance of asphalt was evaluated through the crossover modulus (|G*c|) and frequency (ωc) analysis. L1, possessing high phenolic content and low pH, demonstrated deagglomeration effectiveness, achieving a 6% reduction in weight-average molecular weight, 3% in polydispersity index (PDI), and 16% in asphaltene fraction. Both L1 and L2 decreased the combination aging index of aged asphalt by 28% and 7%, respectively. Moreover, L1 and L2 enhanced the |G*c| of virgin and aged asphalt, consistent with the reduction in the PDI value. These findings suggest that lignin additives possess the potential to deagglomerate asphaltene fractions. The study highlights the potential of repurposing harmful substances from black liquor, traditionally incinerated to reclaim certain chemicals in pulp mills, for use in the pavement industry, promoting sustainability and resource conservation.

Systematic optimization of exopolysaccharide production by Gluconacetobacter sp. and use of (crude) glycerol as carbon source

The usage of polysaccharides as biodegradable polymers is of growing interest in the context of a sustainable and ecofriendly economy. For this, the production of exopolysaccharides (EPS) by Gluconacetobacter sp. was investigated. Glycerol as carbon source revealed to be beneficial compared to glucose. In addition, pure glycerol could be substituted by a crude glycerol waste stream from biodiesel production. Systematic analysis of the peptone and phosphate concentrations in glycerol-based media indicated a strong effect of peptone. Optimized parameters resulted in a titer of 25.4 ± 2.4 g/L EPS with a productivity of 0.46 ± 0.04 g*(L*h)−1. With decreasing peptone, a variation in the monomer ratios was observed. An accompanying change in molecular size distribution indicated the production of two different polysaccharides. Intensified analysis revealed the main polysaccharide to be composed of glucose (Glc), galactose (Gal), mannose (Man) and glucuronic acid (GlcA), and the minor polysaccharide of Gal, Man, ribose (Rib).

Impact of Pressurized Liquid Extraction and pH on Protein Yield, Changes in Molecular Size Distribution and Antioxidant Compounds Recovery from Spirulina.

The research aims to extract nutrients and bioactive compounds from spirulina using a non-toxic, environmentally friendly and efficient method—Pressurized Liquid Extraction (PLE). In this work, Response Surface Methodology (RSM)–Central Composite Design (CCD) was used to evaluate and optimize the extraction time (5–15 min), temperature (20–60 °C) and pH (4–10) during PLE extraction (103.4 bars). The multi-factor optimization results of the RSM-CCD showed that under the pressure of 103.4 bars, the optimal conditions to recover the highest content of bioactive compounds were 10 min, 40 °C and pH 4. Furthermore, the compounds and antioxidant capacity of PLE and non-pressurized extraction extracts were compared. The results showed that under the optimal extraction conditions (10 min, 40 °C and pH 4), PLE significantly improved the antioxidant capacity (2870.5 ± 153.6 µM TE), protein yield (46.8 ± 3.1%), chlorophyll a (1.46 ± 0.04 mg/g), carotenoids (0.12 ± 0.01 mg/g), total polyphenols (11.49 ± 0.04 mg/g) and carbohydrates content (78.42 ± 1.40 mg/g) of the extracts compared with non-pressurized extraction (p < 0.05). The protein molecular distribution of the extracts was analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and the results showed that there were more small-molecule proteins in PLE extracts. Moreover, Liquid Chromatography Triple Time of Flight Mass Spectrometry (TOF–LC–MS–MS) was used to analyze the phenolic profile of the extracts, and the results showed the extracts were rich on phenolic compounds, such as p-coumaric acid and cinnamic acid being the predominant phenolic compounds in the PLE extract. This indicates that PLE can promote the extraction of bioactive compounds from Spirulina, which is of great significance for the application of PLE technology to obtain active substances from marine algae resources.

Effect of mineral filler on changes in molecular size distribution of asphalts during oxidative ageing

Asphalt binder is always present in contact with fine and coarse mineral particles in the pavement and roofing shingles. Therefore, the mechanisms of oxidative ageing of binders are expected to be influenced not only by the characteristics of the binder but also by the properties of the mineral surface due to the molecular interaction between these two components. The purpose of this study is to develop a fundamental understanding of the mechanisms of interaction between binder and filler and the resulting changes in the molecular size distribution (MSD) after oxidative ageing. In this study, a method to characterise the behaviour of mastics (binder mixed with mineral dust) with ageing was developed by using the Pressure Ageing Vessel. The changes due to this ageing is measured by monitoring the mastics |G*| ageing index (ratio of complex modulus before and after ageing). The results clearly indicate that fillers can significantly change the effect of oxidative ageing of binders. Gel Permeation Chromatography testing results supported mentioned findings regarding |G*| changes, as the presence of mineral filler appears to decelerate the rate of production of larger molecular size oxidation products in the binder phase of mastics. Implication of the findings is that change in MSD of asphalts during ageing – a controlling factor in mechanical and thermo-volumetric behaviour – can be engineered by proper selection of fillers in the mastic phase. It is thus concluded that by selecting a proper type and concentration of filler, ageing of pavement, or roofing shingles and, consequently, durability and performance, can be controlled.

The application of the perfluorinated bonded alumina phase for natural organic matter catalytic ozonation

An investigation into the efficiency of natural organic matter ozonation in the presence of alumina modified with perfluorooctanoic acid (PFOA) was the goal of this paper. UV254-absorbance analysis indicated that the PFOA/O3 system increases the efficiency of colour removal from treated water when compared with ozonation alone and ozonation in the presence of alumina. The total organic carbon (TOC) analysis proved that the PFOA/O3 system is also responsible for natural organic matter (NOM) degradation in water due to two significant processes that take place during the ozonation step: adsorption and reaction with ozone. Both an increase of carbonyl compounds formation and changes in molecular size distribution of NOM treated with ozone in the presence of PFOA were found to be another evidence for catalytic activity of the PFOA/O3 system. Key words: ozone, drinking water treatment, catalytic ozonation, perfluorooctyl alumina, PFOA, natural organic matter.

Polymerization of dissolved humic substances catalyzed by peroxidase. Effects of pH and humic composition

High performance size exclusion chromatography (HPSEC) was used to follow the changes in molecular size distribution of three dissolved humic materials of various origins brought about by oxidative coupling of humic constituents under the combined action of hydrogen peroxide and horseradish peroxidase (HRP). Increase in weight-average molecular weight ( M w) occurred invariably for all humic substances with the oxidative polymerization catalyzed by HRP. Polymerization was found to occur to a further extent at pH 7 than at pH 4.7. This was attributed to the larger mobility of reacting molecules in the hydrated and smaller humic associations stabilized only by weak dispersive forces at pH 7. Conversely, hydrogen bonds confer a conformational rigidity to humic associations at pH 4.7 and depress molecular reactivity. Comparison with chromatograms and M w values obtained by treating humic solutions with acetic acid to pH 3.5 before HPSEC injection confirmed that the increase in molecular size by HRP catalysis was stable and due to the formation of covalent bonds among reacting humic molecules. Humic polymerization was somewhat inhibited when humic substances were rich in alkyl carbons and poor in carboxyl carbons. These molecular characteristics are conducive to thermodynamically stable hydrophobic domains in aqueous solution and may limit the mobility of potentially reactive humic molecules towards the catalytic sites of HRP. Control of the conformational structure of dissolved humic substances may be of importance in regulating their reactivity in the environment.

SDS-insoluble glutenin polymer formation in developing grains of hexaploid wheat: the role of the ratio of high to low molecular weight glutenin subunits and drying rate during ripening

The accumulation of polymeric proteins and the changes in molecular size distribution of these proteins were followed during grain filling and/or premature desiccation. The accumulation behavior of polymeric proteins and their constituent polypeptides (high and low molecular weight glutenin subunits, HMW-GS and LMW-GS) was determined by reversed phase-high performance liquid chromatography using a NaI/propanol purification procedure. With this new extraction and separation procedure, we have demonstrated that there was a coordinated initiation of storage protein biosynthesis, even if the accumulation rate varied greatly between the two main classes of proteins (i.e. monomeric and polymeric fractions). Moreover, the glutenin subunit composition was largely modified during glutenin accumulation. Both the HMW-GS/LMW-GS and HMW-GS-x/HMW-GS-y ratios increased significantly during the whole cell enlargement phase (from 16 to 37 d after anthesis). By applying premature grain desiccation during this physiological phase, we demonstrated that the polymerization index (SDS-insoluble polymers/total polymers) of the glutenin polymers was closely related to the HMW-GS/LMW-GS ratio of these proteins. An increase in the relative proportion of HMW-GS in glutenins caused the proportion of SDS-insoluble polymers to rise during grain desiccation. From these studies, it appears that the modification of the desiccation rate (grain desiccation at a constant temperature with variable relative humidity levels) induced a parallel modification of the glutenin insolubilization rate but did not affect the polymerization index of the glutenins at maturity.

Accumulation and changes in molecular size distribution of polymeric proteins in developing grains of hexaploid wheats: role of the desiccation phase

Two varieties of wheat differing in high molecular weight glutenin subunit composition (Soissons, 5+10, Glu-D1a allele; Thésée, 2+12, Glu-D1a allele) were examined to follow the accumulation of polymeric proteins and the changes in molecular size distribution of these proteins during grain filling. The accumulation behaviour of polymeric proteins was determined by size-exclusion-HPLC, multistacking SDS-PAGE and the constituent polypeptides (high molecular weight and low molecular weight glutenin subunits) by reversed-phase-HPLC. For both cultivars, the accumulation of each class of protein was highly asynchronous, especially between the early deposition of SDS-soluble polymers and the late deposition of SDS-insoluble polymers, such that the average molecular size of polymeric protein increased in the period from 30 to 45 days after anthesis in natural conditions. By applying premature grain desiccations during the cell enlargement phase, it was demonstrated that the SDS-insoluble polymers formation was closely related with the process of water loss from the grain. Moreover, the rapid accumulation of SDS-insoluble polymers coincided with a rapid decrease in mass of both SDS-soluble polymers and monomers, suggesting an aggregative mechanism. Over the same period, the molecular size distribution of the polymers which can be used to differentiate the two genotypes studied, is highly correlated with the percentage of high molecular weight glutenin subunits in glutenins present in kernels when desiccation occurred. The formation of SDS-insoluble fraction is discussed in connection with the specific contribution of high molecular weight glutenin subunits to the formation of polymers (subunits linked by disulfide bonds).

Assessment of biological activity during temperature changes in a bench-scale sequencing batch reactor fed with synthetic medium containing lignin

The influence of temperature on activated sludge in a bench-scale SBR fed with a synthetic medium simulating paper mill effluents was determined using biological methods based on the measurement of enzymatic activities. Redox activity inside cells and in the extracellular polymeric substances as well as the proportion of active cells in the sludge were measured by the reduction of the tetrazolium salts XTT and CTC, respectively. A significant shift in the size distribution of lignin at 30°C correlated with a maximum of extracellular redox activity. The percentage of active cells in the biocoenosis was independent of temperature. α-glucosidase and L-alanine-aminopeptidase activities were steady at 20, 30 and 40°C. After shifting the temperature to 50°C there was an enormous short-time increase of both enzyme activities because of the change from mesophilic to thermophilic conditions. Of the biological parameters tested, only extracellular redox activity correlated with degradative activity as detected by a change in molecular size distribution of lignin molecules.

Assessment of biological activity during temperature changes in a bench-scale sequencing batch reactor fed with synthetic medium containing lignin

The influence of temperature on activated sludge in a bench-scale SBR fed with a synthetic medium simulating paper mill effluents was determined using biological methods based on the measurement of enzymatic activities. Redox activity inside cells and in the extracellular polymeric substances as well as the proportion of active cells in the sludge were measured by the reduction of the tetrazolium salts XTT and CTC, respectively. A significant shift in the size distribution of lignin at 30°C correlated with a maximum of extracellular redox activity. The percentage of active cells in the biocoenosis was independent of temperature. α-glucosidase and L-alanine-aminopeptidase activities were steady at 20, 30 and 40°C. After shifting the temperature to 50°C there was an enormous short-time increase of both enzyme activities because of the change from mesophilic to thermophilic conditions. Of the biological parameters tested, only extracellular redox activity correlated with degradative activity as detected by a change in molecular size distribution of lignin molecules.

Soil organic matter mobilization by root exudates of three maize hybrids

In order to study the macromolecular behaviour of humic substances, we treated humic material (from the Ah horizon of an Eutric Cambisol) either with exudates from 10 day-old seedlings obtained in axenic conditions from three maize cultivars (Zea mays L.; cv. Paolo, cv. Polaris and cv. Sandek) or with the organic acids (succinic and fumaric) present in the exudates and followed the change in molecular size distribution. Succinic acid, Polaris and Sandek exudates shifted the humic absorbance from high to low molecular weight in size exclusion chromatograms, while Paolo exudates were not able to produce the same shift and gave an absorbance profile like that of humic substance alone. Our results are evidence of the micellar behaviour of humic substances in solution and of the importance of hydrophobic bonds in holding humic molecular together. Exudates were also used to separate soil organic fractions from the same soil. Soil organic extracts were characterized by low pressure gel permeation chromatography, hormone-like activities, and 1H-NMR. The data, compared with water extract and KOH extract, indicate that maize exudates, fumaric and succinic acids mobilize soil organic fractions in small quantities, but these extracts are endowed with auxin- and/or gibberellin-like activity. Water extract and maize exudates exhibit no hormone-like activity, while KOH extract shows a negligible one. Moreover, only Polaris and Sandek exudates and succinic acid are able to disaggregate an organic fraction with low apparent molecular size (<13KDs) from the bulk of the soil. The 1H-NMR data indicate that water and succinic extracts are largely composed of carbohydrate and aliphatic constituents. Sandek exudate and fumaric acid can liberate aromatic components, Polaris exudate releases carbohydrates from soil, while Paolo slightly influences the aliphatic constituents. The role of organic acids released by maize seedlings in mobilising soil organic fractions is presented.

Micelle-1ike conformation of humic substances as revealed by size exclusion chromatography

To gain further insight into the macromolecular behaviour of humic substances we treated a humic material with simple organic compounds and followed the change in molecular size distribution. Monocarboxylic, dicarboxylic, and tricarboxylic acids shifted the hunric absorbance from high to low molecular sizes in size exclusion chromatograms. Mineral acids, phenol, alcohols, were not able to produce the same shift and gave total absorbance at the column void volume as in the case of hunric substances alone. Our results are evidence of the micellar behaviour of humic substances in solution and of the importance of hydrophobic bondings in holding humic molecules together. The organic acids enter in the interior of the humic micelle and alter its stereochemical hydrophobic arrangement. In alkaline conditions, the developed negative charges disrupt the apparent high molecular size configuration and disperse the humic material into small-size micelles. This macromolecular property, that we report for the first time, may be of great importance in understanding the biological activity and the overall environmental behaviour of humic substances.

Biodegradation and adsorption in refractory leachate treatment by the biological activated carbon fluidized bed process

A real leachate from a landfill site of old landfill-age was treated by the biological activated carbon fluidized bed (BACFB) process. Steady-state removal of dissolved organic carbon (DOC) and changes in molecular size distribution, adsorption characteristics and amount of humic substances of the leachate were evaluated as a function of the hydraulic retention time (HRT). The steady-state removal of the leachate DOC was enhanced from 42 to 58% when the HRT was increased from 24 to 96 h, indicating that biological oxidation was indeed responsible for the removal of the leachate organics. This increase was found to be due primarily to the biodegration of the leachate organics of low molecular weight (LMW); the high-molecular-weight organics may be highly refractory. Activated carbon preferentially adsorbed the LMW organics of both the leachate and the BACFB-treated effluents. This preferential adsorption was consistent with the removal of the LMW organics by the BACFB process. Adsorption may act as a prerequisite step toward biodegradation. Humic substances accounted for about 30% of the leachate DOC. The BACFB process removed about 70% of the humics with a HRT of 24 h; however, the concentration of the humics remained constant (about 10 mg l −1) with increasing HRT.

Changes in Molecular Size Distribution of Cellulose during Attack by White Rot and Brown Rot Fungi

The kinetics of cotton cellulose depolymerization by the brown rot fungus Postia placenta and the white rot fungus Phanerochaete chrysosporium were investigated with solid-state cultures. The degree of polymerization (DP; the average number of glucosyl residues per cellulose molecule) of cellulose removed from soil-block cultures during degradation by P. placenta was first determined viscosimetrically. Changes in molecular size distribution of cellulose attacked by either fungus were then determined by size exclusion chromatography as the tricarbanilate derivative. The first study with P. placenta revealed two phases of depolymerization: a rapid decrease to a DP of approximately 800 and then a slower decrease to a DP of approximately 250. Almost all depolymerization occurred before weight loss. Determination of the molecular size distribution of cellulose during attack by the brown rot fungus revealed single major peaks centered over progressively lower DPs. Cellulose attacked by P. chrysosporium was continuously consumed and showed a different pattern of change in molecular size distribution than cellulose attacked by P. placenta. At first, a broad peak which shifted at a slightly lower average DP appeared, but as attack progressed the peak narrowed and the average DP increased slightly. From these results, it is apparent that the mechanism of cellulose degradation differs fundamentally between brown and white rot fungi, as represented by the species studied here. We conclude that the brown rot fungus cleaved completely through the amorphous regions of the cellulose microfibrils, whereas the white rot fungus attacked the surfaces of the microfibrils, resulting in a progressive erosion.

Degradation and formation of refractory DOM by bacteria during simultaneous growth on labile substrates and persistent lake water constituents

Changes in molecular size distribution associated with degradation of refractory DOM (macromolecules, apparent mol wt. ≥1500) by 3 strains of bacteria were investigated by Sephadex G-15 gel permeation chromatography and DOC analysis of the eluates. Macromolecules and bacteria were isolated from the same lake water sample, one taken in summer and one in winter. The decompositional changes of the DOM fraction were compared with respect to substance- and bacterial species-specific differences, and with respect to the action of photolysis and co-substrate supplementation. The metabolite patterns resulting from the simultaneous growth of the bacteria on the persistent DOM fraction and the labile co-substrate, glutamic acid were analyzed. The macromolecules differed in accessible components, and the bacteria degraded most effectively the DOM fraction of the parent lake water sample. Photolysis was the prerequisite for the reduction of the inaccessible bulk of the macromolecules. Glutamic acid enhanced the degradation of the macromolecules. The enhancement effect was impaired by the build up of waste products which balanced the losses of the DOM fraction. Three formation modes of refractory metabolites could be distinguished: formation of intensely UV absorbing small sized products which were poor in DOC during degredation of (1) the macromolecules of winter, and (2) of glutamic acid, and formation of (3) apparent high molecular weight substances from glutamic acid in cultures containing the macromolecules of summer which probably results from a stable linkage between the small sized metabolites of the amino acid and the DOM fraction.

Changes in Molecular Size and Size Distribution and Their Effects on Mechanical Properties of Nylon-6 Yarns During Texturizing

The changes in average intrinsic viscosity and its distribution and their effect on mechanical properties of nylon-6 (poly caprolactam) multifilament yarn during texturizing were studied. It was found that change in average intrinsic viscosity is not the only indicator of the changes taking place in the, mechanical properties of the yarn. The changes in the molecular size distribution, on the other hand, can be regarded as a better indicator in this regard. The reduction in the breaking stress of the textured yarn is dependent on texturizing variables— e.g. : heater temperature, contact time twist, inherent moisture content of the feed yarn, yarn tension in the texturizing zone, etc. The elongation percentage at break, on the other hand, may either decrease or increase. The changes have been found to be dependent on the changes in the low molecular weight, high molecular weight species and broadness of the distribution curve.

Mathematical Theory of Polymer Gelation

A new general theoretical treatment is given concerning the change in molecular size distribution due to crosslinking. Mass conservation and gelation are discussed by this fundamental treatment. The behaviors of the number and weight average molecular size of sol after gelation point are shown for many initial molecular size distributions. When a polymer has such a molecular size distribution as has two peaks, the weight average molecular size of sol may have a maximum and a minimum in the region after gelation point. Polymers which contain gel fraction initially are discussed by the same fundamental treatment as those which contain no gel fraction initially. The latter is the limiting case of the former. New quantities are derived which conserve in the region prior to gelation.

Metabolism of ribonucleic acid during respiratory adaptation of yeast

1. (1) The RNA metabolism of non-growing yeast undergoing oxygen-induced respiratory adaptation was compared to that of non-adapting cells. 2. (2) No difference in the rate of RNA synthesis was observed. A marked stimulation in the rate of [ 14C]uracil incorporation into RNA during adaptation was attributed to increased rate of entry of the precursor into the nucleotide pool. 3. (3) No qualitative differences in the most rapidly labelled RNA fraction were found. In both cases the RNA was extremely heterogeneous in molecular size (mean value 10–22 S) and distinct from ribosomal and soluble RNA. The technique for isolation of non-degraded yeast RNA was given. The base composition resembled neither the bulk RNA nor the DNA, nor was it an intermediate type; it was characterized by low G/A and U/A ratios. 4. (4) Differences were detected in the fate of the RNA when exposure to precursors was prolonged. (a) In both cases the RNA underwent rapid changes in molecular size distribution towards that of the ribosomal and soluble RNA. (b) In adapting cells a rapid change in base composition towards that of the bulk RNA was observed; there was evidence for marked heterogeneity within the accumulating fraction. In non-adapting cells the base-composition change was more restricted; intermediate RNA forms were detected, resembling ribosomal RNA in molecular size but with quite different base composition. (c) There was an increased transfer of RNA to the ribosomal fraction in adapting cells. (d) In non-adapting cells much of the RNA found was unstable. No instability was detected in adapting cells, the first formed RNA acting as a precursor of the more stable forms.

Metabolism of ribonucleic acid during respiratory adaptation of yeast

(1) The RNA metabolism of non-growing yeast undergoing oxygen-induced respiratory adaptation was compared to that of non-adapting cells. (2) No difference in the rate of RNA synthesis was observed. A marked stimulation in the rate of [14C]uracil incorporation into RNA during adaptation was attributed to increased rate of entry of the precursor into the nucleotide pool. (3) No qualitative differences in the most rapidly labelled RNA fraction were found. In both cases the RNA was extremely heterogeneous in molecular size (mean value 10–22 S) and distinct from ribosomal and soluble RNA. The technique for isolation of non-degraded yeast RNA was given. The base composition resembled neither the bulk RNA nor the DNA, nor was it an intermediate type; it was characterized by low G/A and U/A ratios. (4) Differences were detected in the fate of the RNA when exposure to precursors was prolonged. (a) In both cases the RNA underwent rapid changes in molecular size distribution towards that of the ribosomal and soluble RNA. (b) In adapting cells a rapid change in base composition towards that of the bulk RNA was observed; there was evidence for marked heterogeneity within the accumulating fraction. In non-adapting cells the base-composition change was more restricted; intermediate RNA forms were detected, resembling ribosomal RNA in molecular size but with quite different base composition. (c) There was an increased transfer of RNA to the ribosomal fraction in adapting cells. (d) In non-adapting cells much of the RNA found was unstable. No instability was detected in adapting cells, the first formed RNA acting as a precursor of the more stable forms.

Metabolism of ribonucleic acid during respiratory adaptation of yeast

(1) The RNA metabolism of non-growing yeast undergoing oxygen-induced respiratory adaptation was compared to that of non-adapting cells. (2) No difference in the rate of RNA synthesis was observed. A marked stimulation in the rate of [14C]uracil incorporation into RNA during adaptation was attributed to increased rate of entry of the precursor into the nucleotide pool. (3) No qualitative differences in the most rapidly labelled RNA fraction were found. In both cases the RNA was extremely heterogeneous in molecular size (mean value 10–22 S) and distinct from ribosomal and soluble RNA. The technique for isolation of non-degraded yeast RNA was given. The base composition resembled neither the bulk RNA nor the DNA, nor was it an intermediate type; it was characterized by low G/A and U/A ratios. (4) Differences were detected in the fate of the RNA when exposure to precursors was prolonged. (a) In both cases the RNA underwent rapid changes in molecular size distribution towards that of the ribosomal and soluble RNA. (b) In adapting cells a rapid change in base composition towards that of the bulk RNA was observed; there was evidence for marked heterogeneity within the accumulating fraction. In non-adapting cells the base-composition change was more restricted; intermediate RNA forms were detected, resembling ribosomal RNA in molecular size but with quite different base composition. (c) There was an increased transfer of RNA to the ribosomal fraction in adapting cells. (d) In non-adapting cells much of the RNA found was unstable. No instability was detected in adapting cells, the first formed RNA acting as a precursor of the more stable forms.