Properties of Carotenoid Radicals and Excited States and Their Potential Role in Biological Systems - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 286

HO

Asteroidenone

O

OH

HO

Adonixanthin

O

O

HO

Adonirubin

O

FIGURE 14.1

Structures of three asymmetrical xanthophylls.

TABLE 14.2
Singlet Oxygen Quenching Rate Constants for Asymmetric
Carotenoids in Benzene and Methanol

k q (×10 9 M −1 s −1 )
(Benzene)

k q (×10 9 M −1 s −1 )
(MeOD) Carotenoid n

Asteroidenone 11 (+1 C=O) 14.8 18.2

Adonixanthin 11 (+1 C=O) 12.3 18.2

Adonirubin 11 (+2 C=O) 10.4 13.2

Source: Burke, M., Pulsed radiation studies of carotenoid radicals and excited
states, PhD thesis, University of Keele, Keele, U.K., 2001.

As can be seen in homogeneous environments such as benzene quenching of singlet oxygen by car-
otenoids is nea r to d iff usion cont rolled ( k q ~ 1 × 10 10 M −1 s −1 ) and the rate constants given in Table 14.1
indicate that the ability of the carotenoids to quench singlet oxygen increases with the increasing
number of conjugated double bonds ( n ). This data are in agreement with Devasagayam et al. (1992)
who noted that the quenching efi ciency increases with increasing wavelength of

* absorption
maximum. This principle suggests that the energy transfer from excited 1 O 2 becomes more exo-
thermic as the conjugation of the carotenoid increases. Of course, simple Hückel theory predicts
a lowering of singlet state energy (of the carotenoid) on increasing conjugation, accompanied by
a decrease in the triplet energy level. In fact, several research groups have demonstrated a linear
relationship between

ππ

λ max of the ground state and the triplet state, which is the state involved in the
quenching process. Farmilo and Wilkinson (1973), and Wilkinson and Ho (1978) demonstrated that
electron exchange energy transfer is the principal mechanism by which carotenoids accept excita-
tion energy from 1 O 2 , producing the carotenoid triplet state (Equation 14.1).
The three unsymmetrical carotenoids have also been studied in methanol (Burke 2001) and all
are very efi cient singlet oxygen quenchers. This may be attributable to the polarity of the mole-
cules. These asymmetrical XANs will possess a permanent dipole and their solvent interaction will