BioPHYSICAL chemistry

In practice, this transition is not observed because it is forbidden as we have

not yet considered factors that influence the strength of the transition,

such as symmetry (Chapter 14).

Carotenoids have several essential functions in photosynthetic organisms.

This calculation is useful for understanding how the length of a carotenoid

would influence its absorption spectrum. However, to understand the role

of carotenoids in the conversion of light into chemical energy requires a

detailed analysis of the electronic structure. For example, during the after-

noon on a sunny day, the amount of sunlight available is in significant excess

and plants dispose of the excess light by a process called nonphotochemical

quenching (see below). One role of carotenoids in plants is to help avoid

over-excitation of the photosynthetic system by dissipating energy when

light levels are very high (Deming-Adams 1990; Kulheim et al. 2002; Li

et al. 2002). This is called the xanthophyll cycle because the carotenoid com-

position in the plant changes as the light level changes (Figure 10.6).

Under low light levels the light energy causes excitation of the chloro-

phyll followed by electron transfer, forming a charge-separated state

involving oxidized chlorophyll and a reduced quinone. These reactions

are part of the overall process known as the Z scheme involving many

different proteins.

Low light: Chl Q→→Chl* Q Chl Q+− (10.38)

hν

206 PART 2 QUANTUM MECHANICS AND SPECTROSCOPY

Limiting HO Zeaxanthin

hν

Limiting

hν

Excess

hν

Excess

hν

OH

HO Antheraxanthin

O

OH

HO Violaxanthin

O

O

OH

Figure 10.6The carotenoid composition in plants changes in response to light intensity in the
xanthophyll cycle.