Carotenoids absorption bands

The carotenoid absorption band of each purified preparation of antenna complexes was similar, in shape and location, to that of the membranes from which that particular preparation was derived. Besides, the similitude was kept unchanged after organic solvent extraction of the preparations (not shown). Then, it seems that the incorporation of carotenoid pigments to the antenna complexes in vivo is a nonspecific process. The detection of spirilloxanthin as the sole carotenoid of previously analyzed antenna preparations of the same microorganism seems to be accidental and, as suggested by Duysens (in Cogdell and Thomber, 1979, pp. 77), probably due to the use of old, spirilloxanthin enriched cultures as the starting material for complex solubilization. 

Fig. 2 Evolution of the carotenoid absorption band of R. rubram cells. Continuous line, cells from midexponential cultures broken line, carbon starved cells...

In 1969, Jackson and Crofts demonstrated that the carotenoid absorption-band shifts in bacterial chromatophores can be induced in the dark by an ion gradient generated using uncouplers and ionophores. Fig. 21 (A) shows the light-induced absorption-band shifts of carotenoids mRp. sphaeroides chromatophores that were incubated in 2 mM KCl before being suspended in a choline chloride medium. If a pulse of the... 

Electronic Absorption Spectroscopy. A review has been published of the absorption spectra of a range of natural carotenoids." A model has been presented which attributes the widths of carotenoid absorption bands to conformational... 

Methods to measurethe position of the isosbestic point of the long-wavelength carotenoid absorption band, X- , the band-center of that band, Z A, the light-induced absorbance changes and the Bchl spectrum are described in ref (5). 

Solvent — The transition energy responsible for the main absorption band is dependent on the refractive index of the solvent, the transition energy being lower as the refractive index of the solvent increases. In other words, the values are similar in petroleum ether, hexane, and diethyl ether and much higher in benzene, toluene, and chlorinated solvents. Therefore, for comparison of the UV-Vis spectrum features, the same solvent should be used to obtain all carotenoid data. In addition, because of this solvent effect, special care should be taken when information about a chromophore is taken from a UV-Vis spectrum measured online by a PDA detector during HPLC analysis. 

Numerous studies of carotenoid aggregates have focused on the molecular organization of the aggregates (Simonyi et al. 2003), but little is known about aggregation-induced effects on carotenoid excited states. Classical exciton theory can qualitatively explain the aggregation-induced shifts of absorption bands (Section 8.3.1), but a detailed understanding of the parameters governing the... 

While excited-state properties of monomeric carotenoids in organic solvents have been the subject of numerous experimental and theoretical studies (Polfvka and Sundstrom 2004), considerably less is known about excited states of carotenoid aggregates. Most of the knowledge gathered so far stems from studies of aggregation-induced spectral shifts of absorption bands of carotenoid aggregates that are explained in terms of excitonic interaction between the molecules in the aggregate. 

Unfortunately, the above analysis can never be widely applicable to the determination of excited-state geometries since so few molecules and ions exhibit vibronically structured absorption bands and excitation profiles, even at low temperatures. Moreover, some questions arise as to the possible breakdown of the Condon approximation. Other types of molecule for which similar analyses have been carried out include 3-carotene, carotenoids (9) and certain carotenoproteins such as ovorubin (10). In these cases the excitation profiles of three skeletal a bands are monitored, and estimates for the change in C-C and C=C bonds lengths ( 0.02 A) have been made. 

The absorption bands or peaks reported for the all-fraws conformers of unsubstituted polyenes29-31, a,ct>-dimethylpolyenes30 32 33, a,ct>-di-ferf-butylpolyenes34, a,diphenyl-polyenes35,36 and a,ct>-dithienylpolyenes37 are complied in Tables 1-5, respectively. The data for carotenoids are described in a previous review38. These absorptions are attributed to the 1 <— 11Ag transitions (jt-tt transitions). 

The characteristic absorption spectrum of each carotenoid is determined by a series of conjugated double bounds, the so-called chromophore. Usually the spectrum shows three absorption bands, which are affected by the length of the chromophore, the nature of the double bound, and the taking out of conjunction of one double bond. Several absorption spectra of some common carotenoids are shown in Fig. 2. A change of solvent may, however, cause a shift of the absorption bands. Owing to the extensive double-bond system, carotenoids exist in many geometrical isomeric forms (Z or E isomers). In nature most carotenoids occur in the all-trans form (E isomers) cis isomers (Z isomers) are frequently present in small amounts (6). Cis isomers can be distinguished from trans isomers by a characteristic absorption band ( cis peak ) that appears at 300-360 nm (7). 

In vivo RRS spectroscopy of the macula can take advantage of favorable anatomical features of the tissue structures encountered in the excitation and light scattering pathways. The major site of macular carotenoid deposition is the Henle fiber layer, which has a thickness of only about 100 pm, and to a lesser extent the plexiform layer (Fig. 12.8). Considering that the optical density of MP in the peak of the absorption band is typically quite a bit smaller than 1, as determined from direct absorption measurements of MP in excised... 

The carotenoids take their name from the major pigments of carrot Daucus carota). The color is the result of the presence of a system of conjugated double bonds. The greater the number of conjugated double bonds present in the molecule, the further the major absorption bands will be shifted to the... 

The three absorption bands that are characteristic of absorption spectra of carotenoids (Fig. 5-5) are about 17 kJ mol-1 apart, a reasonable energy spacing between adjacent vibrational sublevels (Fig. 5-7). Specifically, the... 

Figure 5-7. Energy level diagram including vibrational sublevels, indicating the principal electronic states and some of the transitions for carotenoids. The three straight vertical lines represent the three absorption bands observed in absorption spectra, the wavy lines indicate possible radiationless transitions, and the broad arrows indicate deexcitation processes (see Fig. 4-9 for an analogous diagram for chlorophyll).

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. 

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