Light energy reactions, carotenoids

A group of competing reactions takes place when carotenoids are exposed to light. When the incidence of light energy overlaps the carotenoid absorption region. 

Hence, the presence of light is essential for this step. In this reaction, chlorophyll, carotenoids, and several other pigments that are organized in clusters absorb light energy and store it in the form of ATP and nicotinamide adenine diphosphate (NADP) in its reduced state (NADPH). 

Fig. 1 (B) illustrates the second major function of carotenoids, namely, photoprotection of chlorophyll, either by direct triplet-triplet energy transfer [labeled T-T j from Chl to the carotenoid or by indirect transfer via the formation of singlet oxygen. The reaction steps for each of the two mechanisms are shown in Fig. 1, bottom, right. Further details on both the light harvesting by carotenoids and photoprotective role of carotenoids will be described in the following sections.

Figure 19.28 Energy transfer from accessory pigments to reaction centers. Light energy absorbed by accessory chlorophyll molecules or other pigments can be transferred to reaction centers, where it drives photo induced charge separation. The green squares represent accessory chlorophyll molecules and the red squares represent carotenoid molecules the white squares designate protein.

Photosynthesis. In the photosynthetic process light energy is absorbed by carotenoids and bacteriochlorophylls and transferred to photosynthetic reaction centers. Time-resolved vibrational methods, including transient RR and TR techniques, are well-suited to elucidate the structure and dynamics of the excited (singlet and triplet) states of these chromophores. Nanosecond and picosecond transient RR studies of carotenoid excited states have been conducted by Atkinson and co-workers ... 

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