Bacteriochlorophyll absorption band

In contrast to the prompt responses of the bacteriopheophytin absorption bands to the Qa Qb QaQb l ctron transfer, electrochromic shifts in the bacteriochlorophyll absorption bands are initially unaltered by the electron transfer. A through-space electrostatic interaction would suggest that initially both quinone anions produce equivalent electric field increases at the bacteriochlorophylls. Following the formation of the P" "QaQb state, relaxation processes with a response time of about 400 /is are found to attenuate of the electrochromic shifts in the bacteriochlorophyll band region, A similar relaxation is seen for the P" Qa state when its lifetime is extended by blocking electron transfer to Qg. However, the quenching of the electrochromic shift is found to be greater for Qg than Qa". 

Only the smallest carotenoid pool containing 20-30% of the total carotenoid content shows a substantial spectral shift in response to the generation of transmembrane potentials. This field-sensitive pool is associated with the light-harvesting complex LH II which shows bacteriochlorophyll absorption bands at 800 and 850 nm (4). The field-insensitive carotenoids, accounting for 75 % of total carotenoid content, are distributed between both light-harvesting complexes 1/3 is associated with the LH I complex (Bchl absorption at 870 nm) and 2/3 with the LH II complex (4). 

Bacteriochlorophyll in Chromatium has three absorption bands with peak positions at 800, 850, and 890 nm. The last includes the reaction center bacteriochlorophyll and is the only form that fluoresces. Recent studies have established that most if not all chlorophyll is bound to specific proteins, a fact that can account for the various overlapping absorption bands. 

The reduction of ring IV in chlorophylls a or b changes the optical absorption spectrum of the molecule dramatically. Whereas the long-wavelength absorption band of a cytochrome is relatively weak (see fig. 14.4), chlorophyll a has an intense absorption band at 676 nm (fig. 14.5). Chlorophyll b has a similar band at 642 nm. Bacteriochlorophylls a and b have strong absorption bands in the region of 770 nm (see fig. 15.5). The chlorophylls thus absorb red or near-infrared light very well. 

Since the bacteriochlorophyll present in the light-harvesting complex accounts for the majority of all the bacterial pigments, its absorption bands can readily be identified even in the spectrum of the unfractionated membrane. On the other hand, the pigments belonging to the reaction center amount to only "1% of the total BChl and its absorption is often masked by the bulk pigments. The BChl a present in the reaction center may be identified however in a purified reaction-center preparation isolated from the chromatophore membrane. This may be illustrated with Chromatium vinosum following fractionation and isolation of the reaction-center complex and the three antenna complexes from the chromatophore membrane. Fig. 2 (B) shows the absorption spectrum of the unfractionated Chromatium chro-... 

Chlorophylls and bacteriochlorophylls typically exhibit three intense electronic absorption bands in the visible and... 

Bacteriochlorophyll a (Figure 12) is a natural pigment with an absorption band around 780 nm. At this wavelength, the penetration depth of light is approximately three times greater than that reached at 630 nm, the wavelength generally used in clinical PDT with Photofrin [73,74]. 

Fig. 4.5 Dipole strength of the long-wavelength absorption band of bacteriochlorophyll-a, calculated by Eq. (4.16a) from absorption spectra measured in solvents with various refractive indices. Three treatments of the local-field correction factor (/) were used down triangles, f= 1.0 (no correction) filled circles, f is the cavity-field factor empty circles, f is the Lorentz factor. The dashed lines are least-squares fits to the data. Spectra measured by Connolly et al. [148] were converted to dipole strengths as described by Alden et al. [4] and Knox and Spring [5]...

Fig. 4.26 Dependence of the transition energy of the long-wavelength (Qy) absorption band of bacteriochlorophyll-a on the refractive index (n) in nonpolar solvents. Experimental data from Limantara et al. [83] are replotted as a function of (n - l)/(2n + 1). Extrapolating ton = 1 (Oon the abscissa) gives 13,810 cm for the transition energy in a vacuum. A similar plot of the data vs (n — l)/(n + 2) gives 13,600 cm ...

Fig. 6.7 FTIR difference spectrum (light-minus-dark) of the absorbance changes associated with electron transfer from the special pair of bacteriochlorophylls (P) to a quinone (Qa) in photosynthetic reaction centers of Rhodobacter sphaeroides. The negative absorption changes result mainly from loss of absorption bands of P the positive changes, from the absorption bands of the oxidized dimer (P ). These measurements were made with a thin film of reaction centers at 100 K. The amplitudes are scaled arbitrarily. Adapted from [101]...

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