Pigments light absorption

Light absorption of quinones depends on their skeletons and is also strongly influenced by the presence of various substituents. The absorption spectra of 1.4-benzoquinone presents an intense absorption band (band I) at 240 nm, a medium-intensity absorption band (band II) around 285 nm, and a weak absorption band (band III) at 434 nm. Due to their weak absorption levels in the visible region, unsubstituted benzoquinones are not important as pigments. 

Different photoreceptor pigments may reasonably be expected to undergo different primary photoprocesses upon light absorption, aside from possessing different spectral characteristics. Specifically, light absorption in flavins fairly easily leads to electronic excitation of the molecule to the triplet state, while this process does not readily occur in carotenoids87). 

The light absorption of rhodopsin is in the visible range, with a maximum at about 500 nm. The absorption properties of the visual pigment are thus optimally adjusted to the spectral distribution of sunlight. 

Accessory Pigments Extend the Range of Light Absorption... 

The light absorption coefficient of black pigments determines their optical quality. Their color intensity and hiding power depend on the particle size and particle size distribution. The most important black pigments are carbon blacks, iron oxides (Section 3.1.1), and mixed metal oxides (Section 3.1.3). 

Figure 8-5 The fluid mosaic model of Singer and Nicolson.61 Some integral membrane proteins, which are shown as irregular solids, are dissolved in the bilayer. Transmembrane proteins protrude from both sides. One of these is pictured as a seven-helix protein, a common type of receptor for hormones and for light absorption by visual pigments. Other proteins adhere to either the outer or the inner surface. Many membrane proteins carry complex oligosaccharide groups which protrude from the outer surface (Chapter 4). A few of these are indicated here as chains of sugar rings.

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