Chlorophyll absorbance properties

Chlorophyll, plastoquinone, and cytochrome are complicated molecules, but each has an extended pattern of single bonds alternating with double bonds. Molecules that contain such networks are particularly good at absorbing light and at undergoing reversible oxidation-reduction reactions. These properties are at the heart of photosynthesis. 

Chlorophyll a and chlorophyll b are distinguishable by their typical spectral properties (Figure 6.1.lA). Each one shows a specific absorbance coefficient and E... 

Direct spectroscopic measurements of absorptions could provide substantial and much-needed complimentary information on the properties of BLMs. Difficulties of spectroscopic techniques lie in the extreme thinness of the BLM absorbances of relatively few molecules need to be determined. We have overcome this difficulty by Intracavity Laser Absorption Spectroscopic (ICLAS) measurements. Absorbances in ICLAS are determined as intracavity optical losses (2JI). Sensitivity enhancements originate in the multipass, threshold and mode competition effects. Enhancement factor as high as 106 has be en reported for species whose absorbances are narrow compared to spectral profile of the laser ( 10). The enhancement factor for broad-band absorbers, used in our work, is much smaller. Thus, for BLM-incorporated chlorophyll-a, we observed an enhancement factor of 10 and reported sensitivities for absorbances in the order of lO- (24). 

It is obvious that for radiant energy absorbed by any one of the chromophores to be transferred efficiently along the rod toward the core and eventually to the chlorophyll in the thylakoid complex, it is necessary that the hexamer-linker complexes along the rod have appropriate spectral properties. Since all three hexamer disks in some rods may contain only PC, its spectral properties must be appropriately modulated to fulfill the condition for directional energy transfer. In Synechococcus 6301, for example, the spectral properties of the three PC hexamer disks along the rod are indeed modified by their linker polypeptides from the outer end of the rod to the core, as shown here from left to right ... 

Fig. 1. Three difference spectra for P680 photooxidation. See text for discussion. (A) from DOring, Renger, Vater and Witt (1969) Properties of the photoactive Chlorophyll-a f in photosynthesis. Z Naturforsch 24B 1139 (B) from van Gorkom, Pulles and Wessels (1975) Light-induced changes of absorbance and electron spin resonance in small photosystem II particles. Biochim Biophys Acta 408 336 (C) from Gerken, Dekker, Schlodder and Witt (1989) Studies on the multiphasic charge recombination between chlorophyll a/ (P680) and plastoquinone in photosystem II complexes. Ultraviolet difference spectrum of Chl-a / Chl-a. Biochim Biophys Acta 977 57.

Quite pronounced photovoltaic effects have been observed in Mx 1150 nm -chlorophyll a M2 sandwich cells, where Mx = A1 or Cr and M2 = Hg or Au.7e>77 These are ascribed to a Schottky barrier at the junction with metal Mx which has a lower work function than M2. If the Mi junction is the front (illuminated) electrode then the photovoltaic action spectrum is identical with the absorption spectrum of the chlorophyll. If it is at the rear, the action spectrum shows an inner filter effect, because only light absorbed in the region of the barrier is effective. Figure 11 shows the performance of a typical cell, which has a power conversion efficiency of ca. 10-3% at 745 nm. The best efficiency, 5 x 10-a%, was achieved by a Cr chlorophyll a Hg cell. Photovoltaic properties have also been reported in the cell A11 Mg phthalocyanine Ag, which has a Schottky barrier of height ca. 0.6 eV at the A1 junction.78 At 690 nm, the power conversion efficiency was ca. 10-2%. It has been shown that oxidized A1 contacts to Cu phthalocyanine are blocking.79... 

---