Phenol release

The rate constants for phenol release (2,4,6-trichlorophenol TCP) were determined spectrophotometrically at 298 nm in both the presence and the absence of the fluorophore (DPA) and were shown to be independent of DPA at the concentrations employed. The time-dependent release of TCP monitored at 298 nm, which is shown in... 

Rhizodegradation Soils, sediments, land application of wastewater Organic compounds (TPH, PAHs, BTEX, pesticides, chlorinated solvents, PCBs) Phenolics releasers (mulberry, apple, osage orange) Grasses with fibrous roots (rye, fescue, Bermuda) for contaminants 0-3 ft deep Phreatophyte trees for 0-10 ft Aquatic plants for sediments... 

Fig. 5. Graph of the logarithm of the acceleration of the rate of phenol release due to 0.01 M cycloamylose against the Hammett substituent constant,

An additional example is the observed moderate acceleration in the cleavage of particular phenyl esters in the presence of a cyclodextrin. In such cases, the bound ester is attacked by an hydroxyl group on the cyclodextrin to yield a new ester. There was found to be a significant enhancement of phenol release from meta-substituted phenyl acetate on interaction with cyclodextrin (relative to other esters which do not fit the cavity so well) (Van Etten, Clowes, Sebastian Bender, 1967). During the reaction, the acyl moiety transfers to an hydroxyl group on the... 

Very few data concerning the presence of phenol in soils were found. Phenol generally does not adsorb very strongly to soils and tends to leach rapidly through soil, which may account for the lack of monitoring data, since any phenol released to soils is likely to leach to groundwater. 

In proton-catalyzed hydrolysis (specific acid catalyzed hydrolysis), protonation of the carbonyl O-atom leads to polarization of the carbonyl group, facilitating addition of the nucleophile, i. e., a H20 molecule (Fig. 3.1, Pathway a). The acid-catalyzed hydrolysis of esters is reversible because the neutral alcohol or phenol released is nucleophilic, whereas hydrolysis of amides is irreversible because the amine released is protonated in the acidic medium and, hence, has considerably reduced nucleophilicity. 

Unsubstituted cycloamyloses have been used to catalyze a number of reactions in addition to acyl group transfer. Brass and Bender (8) showed that cycloamyloses promoted phenol release from diphenyl and bis(p-nitro-phenyl) carbonates and from diphenyl and bis(m-nitrophenyl)methyl phos-phonates. Breslow and Campbell (10,11) showed that the reaction of anisole with HOCL in aqueous solution is catalyzed by cyclohexaamylose and cycloheptaamylose. Anisole is bound by the cyclodextrins and is chlorinated exclusively in the para position while bound. Cycloheptaamylose has been used to promote regiospecific alkylation followed by the highly selective oxidation shown in reaction (3) (95). In addition cycloheptaamylose effec-... 

For example, when cell walls of maize stem were treated with sodium hydroxide (0.1M) at 20°C for various times to release different amounts of phenolics, a highly significant correlation (r = 0.98) was found between the amount of phenolics released and wall biodegradability (measured by cellulase ) (5). It is of interest to note that alkali treatment of poor quality graminaceous forages (e.g., cereal straw) is used commercially to increase their biodegradability, and thus their feed value for the animal (1). 

FIGURE 3. Typical emission (A) and absorbance (B) profiles observed in kinetic experiments with the peroxyoxalate system of oxalic esters reacting with base. A Light emission in the presence of ACT. B Absorption change due to phenol release in the absence of ACT... 

Few examples have been described of nucleophilic cleavage of carbonate- or carbamate-linked alcohols from insoluble supports. A serine-based linker for phenols releases the phenol upon fluoride-induced intramolecular nucleophilic cleavage of an aryl carbamate (Entry 2, Table 3.36). A linker for oligonucleotides has been described, in which the carbohydrate is bound as a carbonate to resin-bound 2-(2-nitrophen-yl)ethanol, and which is cleaved by base-induced 3-elimination (Entry 3, Table 3.36). Trichloroethyl carbonates, which are susceptible to cleavage by reducing agents such as zinc or phosphines, have been successfully used to link aliphatic alcohols to silica gel (Entry 4, Table 3.36). These carbonates can also be cleaved by acidolysis (Table 3.22). 

In practice it is often more convenient to measure the release of a phenol from an aryl phosphomonoester. Standard serum phosphatase methods employ phenyl phosphate (188), p-nitrophenyl phosphate (189), phenolphthalein monophosphate (140), or thymolphthalein monophosphate (141) where the phenol released can be determined spectrophoto-metrically [only the Bodansky method (13) uses a Pi determination]. A number of fluorogenic substrates have been used for phosphatase studies, e.g., jS-naphthyl phosphate (30, 148), 4-methylumbelliferyl phosphate (143), and 3-O-methylfluorescein phosphate (144) The main advantage here is the much greater sensitivity of fluorescence as compared with spectrophotometric assays as little as 1 pmole of 4-methyl-umbelliferone can be detected in continuous assay. 

Relative rates of hydrolysis were determined with 0.5 ml reaction mixtures in 0.1 M sodium acetate buffer, pH 5.0, at 37°. Liberated phosphate was measured by the method of C. H. Fiske and Y. SubbaRow [JBC 66, 375 (1925)]. The amounts of enzyme used were 0.22 unit of crystalline enzyme and 0.24 unit of peak II enzyme. The concentration of substrate and inhibitor was 1.0 mM. For inhibitor study, 1.0 mM p-nitrophenyl phosphate was used as substrate. Inhibition was calculated from the amount of p-nitro-phenol released and expressed as fractional inhibition. 

Similar to catalytic antibodies, we observed some product inhibition. In the case mentioned, the reaction rate was calculated from the amount of released acid. If the calculation is based on phenol release, the rate enhancement turned out to be nearly doubled. Hydrolysis of carbonates should avoid this difficulty. Therefore, diphenyl phosphate was used as template, and the hydrolysis of diphenyl carbonate was then investigated [13]. Compared to solution an enhancement of 982-fold was obtained and typical Michaelis-Menten kinetics were observed (K ,ax = 0.023 mM/min, = 5.01 mM, = 0.0115/min, kaalK = 2.30/min/M). 

Measurement of NTE activity has classically been done by colorimetric determination of phenol released by hydrolysis of the substrate, phenyl valerate [89,90]. The absorbance maximum of the red phenol chromophore overlaps substantially with that of whole blood homogenates and dilution of the blood to remove the interfering absorbance decreases NTE activity below the detection limit of the colorimetric assay [88], Thus, the colorimetric assay cannot be used to assay NTE in whole blood. The problems inherent in a colorimetric NTE assay can be eliminated by using an amperometric technique to detect phenol. 

Since the reaction was followed by phenol release, Jfcim corresponds to the rate constant of acylation of the imidazole unit. 

Relating toxicity thresholds (section 9.2) to the estimated environmental concentrations of phenol (section 9.1) makes it possible to determine the likelihood of adverse effects in the target populations, and, thus, to make an ecological risk assessment. For the example of phenol release into the River Rhine, the risk for the most sensitive species was evaluated using the quotient method and the quantitative probabilistic procedure (Table 9.6) for four different communities. 

No differences occur if for the same scenario either experimental data or QSAR predictions of the compound-specific parameters are used. Possible influences of parameter inaccuracies are by far outweighed by the predominance of the environmental factors in this example. The realistic assumption of a phenol release rate of 10 kg/h, resulting in an exposure level of about 70 xg/l, is unlikely to conflict with the effective concentrations (> 1 mg/l). Compared with the values based on the small data set (n = 5 species), the increased width of the sensitivity distribution for the fish and macroinvertebrate community n = 29 species) yields considerably lower LC5Q estimates for the most sensitive species. Consequently, the ratio of environmental and... 

The example of phenol release into river ecosystems clearly illustrates that the risk assessment procedures currently used are far from perfect. The major limitations concern the issues of... 

Pinelo, M Amous, A Meyer, AS. Upgrading of grape skins significance of plant cell-wall structural components and extraction techniques for phenol release. Trends Food Sci. Technol. 2006,17, 579-590. 

The kinetics of the phenol release were studied on ca. 1 g samples immersed in 100 ml pH=7.5 phosphate buffer at 37.5 °C. 1 ml aliquots of the buffer solution were then extracted with diethylether and the phenol concentration was determined by UV... 

The kinetics of the phenol release was also studied on a completely soluble system, i.e. a PMHS modified with 80% PEO grafts (DPn =8) (18) and 20% BPC... 

Figure 3. Kinetics of the phenol release in pH=7.5 buffered medium at 37.5°C from different polymeric systems, PSO 1 1 PSO/PC sIPN O 1 4 PSO/PC sIPN Bill PSIb/PC IPN.

The ki and k2 rate constants were determined by studying the kinetics on systems where either of them could be eliminated in turn. Thus k2 was determined from the rate constants calculated from the kinetics of the phenol release from a series of... 

The validity of this simple model which was derived from measurements on PSO/PC s-IPNs on one hand and on single PSI networks on the other hand was checked on the full IPNs by comparing calculated values to experimentally determined ones. The best fit was obtained between 5 and 40% phenol release showing that truly tailor-made delivery systems can be designed for the specific application. 

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