Chlorophyll absorption coefficients

In 1941, Mackinney ° published the first specific absorption coefficients for chlorophyll a and chlorophyll b in 80% acetone, quickly followed by other reports citing different solvents. Chlorophylls form aggregates in various organic solvent-water mixtures that may interfere strongly with the absorbance maximum wavelength and the shapes of spectra. 

The extraction of chlorophylls and carotenoids from water-containing plant materials requires polar solvents, such as acetone, methanol, or ethanol, that can take up water. These extracts must then be transferred to a solvent such as diethyl ether in order to be stored stably. Samples with very high water content, such as juices and macerated plant material, are usually freeze-dried first, and can then be extracted directly with diethyl ether. After extraction, solutions are clarified and diluted to an appropriate volume to measure chlorophyll content by UV-VIS spectrophotometry. Absorption coefficients and equations needed for quantitative determination are given in unitf4.3. 

For other solvents, use the chlorophyll maxima, absorption coefficients, and equations found in UNITF4.3 and in Lichtenthaler(1987). 

Presents redetermined absorption coefficients for chlorophylls and total carotenoids, which allows the determination of all three in the same pigment extract of leaves or fruits... 

In both treatments, the in vivo light absorption characteristics were invariable over the day (data not shown). Maximum values in the chlorophyll-specific absorption coefficient were observed near 675 nm (Chi a), 470 nm (carotenoids) and 430 nm (Chi a) (Fig. 4). The absorption cross section was 0.057 m2 mg Chi a 1 under iron limitation and... 

Fujiki T, Tagushi S (2002) Variability in chlorophyll a specific absorption coefficient in marine phytoplankton as a function of cell size and irradiance. J Plankton Res 24(9) 859-874... 

Interestingly, only two types of pigments appear to be involved in all known photochemical reactions in plants and algae. These are the carotenoids and the tetrapyrroles, the latter class including the chlorophylls, the phycobilins, and phytochrome. The maximum absorption coefficients for the most intense absorption bands are slightly over 104 m2 mol-1 in each case, with 7 to 12 double bonds in the main conjugated system. Cytochromes, which are involved in the electron transport reactions in chloroplasts and mitochondria, are also tetrapyrroles (considered later in this chapter). Table 5-1 summarizes the relative frequency of the main types of photosynthetic pigments. 

D. Assuming that the chlorophyll is uniformly distributed throughout the cell, what is the maximum absorbance by one cell in the red and the blue regions Use absorption coefficients given in Figure 5-3. 

Such a high foliar absorption coefficient applies to horizontal leaves with at least 0.5 g chlorophyll m-2, which can occur for crops such as potato, soybean, sunflower, and white clover (Fig. 9-6). 

Table 1. Redetermined specific absorption coefficients (SAC) for chlorophyll a and b at the peak maxima of each (underlined) and at 47Qnm plus the eJIui values at 470 nm for total leaf carotenoids (xanthophylls = x plus carotenes = c) in different solvents.

The coefficients given in Table 1 represent mean values from measurements performed with two recording spectrophotometers, a Shimadzu UV 200 and a Unicam SP 8000. However there are variations in spectrophotometer response especially in the relative height of the absorption in the blue-green as compared to that in the red region as well as a shift of the peak maxima of chlorophylls of between 0.5 to 1 nm. By using pure chlorophyll a and b solutions prepared in the manner described, the values at 470 nm and at the peak maximum of the other relative to the specific absorption coefficient at the peak maximum given in Table 1 can be re-adjusted, if necessary, for any spectrophotometer. 

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. 

---