Chlorophyll extinction coefficients

Amotf was the first to develop a set of equations for acetone to simultaneously calculate chlorophyll a and chlorophyll b in 1949. Several authors later proposed different new equations based on more adjusted and accurate extinction coefficients due to the development of higher resolution spectrophotometers adapted to each special condition. Moreover, besides 80% acetone, coefficients for diethyl ether and ethanol were also established and their respective equations developed, as reviewed by Schwartz and Lorenzo and Eder. Solvents chosen should be those for which specific absorbance coefficients have been published to derive equations and updates should be carefully tracked for new values. 

Porra, R.J., Thompson, W.A., and Kriedemann, P.E., Determination of accurate extinction coefficients and simultaneous equations for assaying chlorophylls a and b extracted with four different solvents verification of the concentration of chlorophyll standards by atomic absorption spectroscopy, Biochim. Biophys. Acta, 975,384, 1989. 

The beautiful colors associated with porphyrin and chlorophyll systems are manifest in their characteristic electronic absorption spectra. The most intense band in the spectra, around 400 nm, is known as the Soret band it is intrinsic to the large macrocyclic conjugated pathway and has molar extinction coefficients usually between 150 000-400 000. This extinction value is lower in chlorins than in porphyrins, and the band is absent in porphyrinogens (6) and ring-opened bile pigments. 

The problem relating to chlorophylls can be overcome to some extent by saponification of the sample, which will remove the chlorophylls however, care must be taken in the choice of conditions, as some carotenoids, particularly the xanthophylls, may be degraded (see UNITF2.1). On the other hand, it is possible to use an alternate wavelength for the carotenoids. For example most of the major carotenoids of interest in foods have an absorption peak around 480 nm, where any absorption of chlorophylls causes less interference however, it is then necessary to use alternate extinction coefficients (e.g., 2180 for (3-carotene). 

Spectrophotometric assessment of chlorophyll content is based on the strong electronic absorption spectra of these pigments. Arnon (1949) developed an early method measuring 80% acetone/20% water plant extracts based on the electronic absorption spectra of chlorophylls a and b. Absorbance of the extract was measured at different wavelengths, and simultaneous equations were constructed based on extinction coefficients for each derivative s unique electronic absorption maxima. Over the... 

Dyes such as erythrosin B [172], eosin [173-177], rose bengal [178,179], rhodamines [180-185], cresyl violet [186-191], thionine [192], chlorophyll a and b [193-198], chlorophyllin [197,199], anthracene-9-carboxylate [200,201], perylene [202,203] 8-hydroxyquinoline [204], porphyrins [205], phthalocyanines [206,207], transition metal cyanides [208,209], Ru(bpy)32+ and its analogs [83,170,210-218], cyanines [169,219-226], squaraines [55,227-230], and phe-nylfluorone [231] which have high extinction coefficients in the visible, are often employed to extend the photoresponse of the semiconductor in photoelectro-chemical systems. Visible light sensitization of platinized Ti02 photocatalyst by surface-coated polymers derivatized with ruthenium tris(bipyridyl) complex has also been attempted [232,233]. Because the singlet excited state of these dyes is short lived it becomes essential to adsorb them on the semiconductor surface with... 

Figure 19.6. Light Absorption By Chlorophyll A. Chlorophyll a absorbs visible light efficiently as judged by the extinction coefficients near 10 M i cm f... 

Using the known extinction coefficient of 32 mM cm" at the peak of red band for pheophytin in solution, the amount ofpheophytin undergoing photoreduction relative to total chlorophyll is estimated to be 1/30-40. Since the Chl/P680 ratio in TSF Ila is known to be -40/1, the reactant P680/[Pg.308]

Fig. 5. (A) Structure of the chlorophyll-a epimer (B) The in vitro oxidized-minus-reduced difference spectrum of dimeric Chi a compared with that of P700 (of spinach). Figure source Watanabe, Kobayashi, Hongu, Nakazato, Hiyama and Murata (1985) Evidence that a chlorophyll a dimer constitutes the photochemical reaction centre (P700) in photosynthetic apparatus. FEBS Lett 191 2255. The light-induced difference spectrum for P700 was originally in Hiyama and Ke (1972) Difference spectra and extinction coefficient of P700. Biochim Biophys Acta 267 163.

To summarize the analysis to this point, the saturated growth rate K has been estimated from available data and its temperature dependence established. The reduction to be expected from nonoptimum light intensities has been quantified and used to calculate the reduction in growth rate, to be expected in each volume element V as a function of the extinction coefficient and the depth of the segment. The mechanism of self-shading has been included by specifying the chlorophyll dependence of the extinction coefficient. It remains to evaluate the effect of nutrients on the growth rate. 

The heterocyclic five-ring system that surrounds the Mg has an extended polyene structure, with alternating single and double bonds. Such polyenes characteristically show strong absorption in the visible region of the spectrum (Fig. 19-41) the chlorophylls have unusually high molar extinction coefficients (see Box... 

In searching for a suitable electron acceptor, it seems reasonable to just mimic what is known about the initial acceptors in vivo. In photosynthetic bacteria it has been well established that bacteriopheophytin a is one of the first electron acceptors. In photosystem I of green plants a chlorophyll a dimer or monomer have been proposed as the acceptor. Either chlorophyll a or pheophytin a would be excellent choices as electron acceptors. However, the singlet lifetime of the pyrochlorophyll a dimer in toluene is only 4 ns. " For a diffusion-controlled electron transfer reaction between the dimer and one of the in vivo acceptors to take place in a few nanoseconds would require a 10 to 10 molar concentration of pheophytin a or chlorophyll a. The molar extinction coefficient of these molecules is on the order of 40,000. At a concentration of 10 M, the absorption of pheophytin a (or the chlorophyll a monomer) would be much too high. The solution of this dilemma is to link an electron acceptor such as pheophytin or chlorophyll to the dimer. Linking the dimer to an electron acceptor not only solves the diffusion problem, but also begins to mimic the photosynthetic reaction center. 

R being the relative photosynthesis, determined empirically, for the appropriate value of surface radiation, k the extinction coefficient per metre of the water, C the amount of chlorophyll in g m, and the factor 3.7 a mean value for the assimilation number. Values estimated by this method and those determined by the oxygen technique agreed reasonably well for a number of sea areas in which primary productivity varied by an order of magnitude (Fogg, 1975). 

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