Specific molar extinction coefficient

Sea salt aerosol particles form a unique chemical enviroiunent, exposed to much higher sunlight photon fluxes than found in the surface waters of the ocean and potentially the site of intense photochemical activity. Species that may interact with sunlight in marine aerosol particles are outhned in Table 2, which hsts known constituents and concentration ranges based on and inferred from previous studies. Obviously, these species and levels will vary significantly from study to study, but this merely serves to summarize the most important chromophores commonly found in marine aerosol particles. Based on estimated levels and absorbance cross-sections, specifically molar extinction coefficients and absorptivities, DOM will be the most dominant chromophore in sea salt aerosol particles (as it is in the surface ocean). The photochemical reactions that may occur in the marine aerosol are discussed in more detail in the following section. 

The physical meaning of the quantities Z(A) and s(A) for a given molarity of solute is as follows the reciprocal of the Napierian coefficient of absorption a(A) corresponds to the thickness of the medium for which the incident intensity is divided by the base of the Napierian logarithm (i.e., e = 2.718281828...), while the reciprocal of the decadic coefficient of absorption e(A) corresponds to the thickness of the solution for which the incident intensity is divided by ten. In addition, note that in analytical chemistry, the solution is in a rectangular cell with optical path denoted I, expressed in centimeters, while the molarity of the solute is expressed in mol.dm hence in this particular case the molar extinction coefficient is expressed in mor. dmlcm . Moreover, in some textbooks (e.g., pharmacy), a derived quantity called the specific molar extinction coefficient, denoted E is defined for a mass concentration of solute of 1 wt.% and a cell having an optical path 1 cm in length. 

Pectin lyase (PNL) activity was measured spectrophotometrically by the increase in absorbance at 235 nm of the 4,5-unsaturated reaction products. Reaction mixtures containing 0.25 ml of culture filtrate, 0.25 ml of distilled water and 2.0 ml of 0.24% pectin from apple (Fluka) in 0.05M tris-HCl buffer (pH 8.0) with ImM CaCl2, were incubated at 37 C for 10 minutes. One unit of enzyme is defined as the amount of enzyme which forms Ipmol of 4,5-unsaturated product per minute under the conditions of the assay. The molar extinction coefficients of the unsaturated products is 5550 M cm [25]. Also viscosity measurements were made using Cannon-Fenske viscometers or Ostwald micro-viscosimeter, at 37°C. Reaction mixtures consisted of enzyme solution and 0.75% pectin in 0.05 M tris-HCl buffer (pH 8.0) with 0.5 mM CaCl2. One unit is defined as the amount of enzyme required to change the inverse specific viscosity by 0.001 min under the conditions of reaction. Specific viscosity (n p) is (t/to)-l, where t is the flow time (sec) of the reaction mixture and t is the flow time of the buffer. The inverse pecific viscosity (n p ) is proportional to the incubation time and the amount of enzyme used [26]. Units of enzyme activity were determined for 10 min of reaction. 

The specific surface area of the fresh and used catalysts was measured by nitrogen adsorption method (Sorptometer 1900, Carlo Erba Instruments). The catalysts were outgassed at 473 K prior to the measurements and the Dubinin equation was used to calculate the specific surface area. The acidity of investigated samples was measured by infrared spectroscopy (ATI Mattson FTIR) by using pyridine (>99.5%, a.r.) as a probe molecule for qualitative and quantitative determination of both Bronstcd and Lewis acid sites (further denoted as BAS and LAS). The amounts of BAS and LAS were calculated from the intensities of corresponding spectral bands by using the molar extinction coefficients reported by Emeis (23). Full details of the acidity measurements are provided elsewhere (22). 

The ratio /spectrophotometric measurement, and the value of a is then calculated from eq. (3.5) to yield the desired absorption constant. The numerous absorption constants found in the literature arise from the choice of quantities incorporated in the constant b. Some of the terms most commonly used to express absorption in minerals are summarized in table 3.2. Note that optical densities (O.D.), representing the direct output from many spectrophotometers, lack specificity about sample thickness and element concentrations. Absorption coefficients (a) indicate that sample thicknesses have been measured or estimated. Molar extinction coefficients (e) require chemical analytical data as well as knowledge of sample thicknesses. 

Nitration of phenolic compounds leads to the formation of 3-nitrotyrosine (molar extinction coefficient = 14 400 at 428 nm) (Figure 7.11). The reaction is very specific for phenolic compounds. Chemical nitration of functionally important tyrosine residues by tetranitromethane has often been found to inactivate or alter the enzyme properties. It was only after the detection of in vivo nitrotyrosine formation under inflammatory conditions that the physiological aspects of nitrotyrosine metabolism came to light. Abundant production (1-120 /uM) of nitrotyrosine has been recorded under a number of pathological conditions such as rheumatoid arthritis, liver transplantation, septic shock, and amyotrophic lateral sclerosis (Balabanli etal. 1999). 

If the concentration is expressed in molarity, a becomes the molar absorption coefficient or molar extinction coefficient, or JE. If the concentration is given in g/liter, a becomes the "specific absorption coefficient, a, ] = a, X MW. If the concentration is expressed in % w/v, then the... 

The method described by Morrison and Bayse (1970) for the enzymic iodination of tyrosine can be readily adapted to the modification of proteins. The reaction mixture contains, in order of addition, L-tyrosine (8.1x10 M), KI (1.0 xlO M), lactoperoxidase (7.4 X 10 M), in 0.05 M K-phosphate buffer, containing 1 x 10 M EDTA, at pH 7.4. The iodination is initiated by the addition of H2O2 to a concentration of 1.0 x 10 M. The specific activity observed for lactoperoxidase under these conditions was 1.05 x 10 moles of L-3-iodotyrosine per min per mole of enzyme at 25°C. At pH 7.4, the rate of enzymatic conversion of L-3-iodotyrosine to L-3,5-diiodotyrosine was 0.34 that of monosubstitution (Morrison and Bayse 1970). The desired level of iodination can be attained by successive equimolar additions of KI and HjOj to the reaction mixture. In this manner, only a low concentration of H2O2 is maintained, minimizing oxidation reactions. The concentration of lactoperoxidase may be calculated from the millimolar extinction coefficient of 114 at 412 run, while the concentrations of stock H2O2 solutions may be determined from the absorbance at 230 nm and a molar extinction coefficient of 72.4 (Phillips and Morrison 1970). 

The D-xylanase system of Stereum sanguinolentum,199 the only D-xylanase for which an amino acid composition has as yet been published, was found to contain a high proportion of acidic and aromatic amino acid residues. The M.W., as determined from the amino acid composition, is 23,900, compared with 21,600 as calculated from ultracentrifugation data. Other physical parameters that have been determined199 for this D-xylanase include the sedimentation coefficient [2.8S, which is similar to that reported for a D-xylanase isolated from Trichoderma viride,203 namely, 2.IS], the partial specific volume (0.71 cm3.g 1), and the molar extinction coefficient (6.25 X 104). Activation energies have been reported for D-xylanases from Schizophyllum commune233 (EA 28.6 kj.mol-1) and from a commercial cellulase preparation229 (EA 34.0 kj.mol-1). 

It is useful to collect here a few more definitions normally used in the discussion of optical properties. One is the molar extinction coefficient Gm = g/c, where c is the concentration in moles/liter of the absorbing material which implies the assumption that the absorption of light is due to specific light absorbing species. Absorbance (A) and optical density (D) are the other two quantities. They are related as follows ... 

Carotenoids are generally light sensitive, easily oxidised by air and are affected by traces of acid, e.g. in solvents. These cause the polyenes to bleach or polymerize. The necessary precautions are therefore required to minimize these effects during isolation, purification and storage. They are identified by their UV-VIS spectra, and their molar extinction coefficients at specific wavelengths ( , max) have been used for characterisation and for quantitation. More recently ORD, CD, NMR, IR and mass spectroscopy have been used extensively. 

NO (reactant) and CO (product) molecules were used as IR probes of the copper oxidation state in H-Cu-ZSM-5 catalysts. CO adsorption is specific to Cu sites. Its characteristic band at 2158 cm provides quantitative results on integrating its molar extinction-coefficient (e) NO decomposes oxidizing Cu to Cu. Propane and oxygen in a special IR reactor cell always yielded chemisorbed CO. Use of e indicates the NO influence on Cu state. 

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