Effects of Self-Assembled Aggregation on Excited States - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 148

et al. 2006). The planarity of their conjugated system allows for better packing of the molecules
within the aggregate. Furthermore, other molecular forces such as stacking interactions, which

may contribute signii cantly to the attractive forces between closely packed carotenoid molecules
(Wang et al. 2004), are stronger when molecules are planar. Nevertheless, the spectral position
of the H-band of astaxanthin aggregates indicates weaker intermolecular interaction than in the
zeaxanthin H-aggregate. The maximum of the astaxanthin H-band also exhibits a large dependence
on the conditions of the experiment, Table 8.1, suggesting a lower stability of the H-aggregates of
astaxanthin compared to zeaxanthin. This effect could be attributed to the presence of carbonyl
groups that may interfere with tight packing of the astaxanthin molecules. It should also be noted
that isomerization prevents the formation of H-aggregates; although all- trans zeaxanthin in 1:2
ethanol/water mixture produces H-aggregates, the absorption spectrum of 9- cis and 13- cis zeaxan-
thin in the same mixture exhibit characteristics of J-aggregate (Milanowska et al. 2003).

π - π

8.3.2.2 Effect of Hydrogen Bonds
The ability to form hydrogen bonds via hydroxyl groups is a decisive factor determining whether
aggregation will be of H- or J-type. The role of hydrogen bonding was extensively studied by
Simonyi et al. (2003) who showed that the esterii cation of hydroxyl groups stimulates the forma-
tion of J-aggregates. While capsanthol acetate, lutein diacetate (Bikadi et al. 2002), and zeaxanthin
diacetate (Zsila et al. 2001c) form exclusively J-type aggregates, their nonesterii ed counterparts
with hydroxyl groups form predominantly H-aggregates (Simonyi et al. 2003). A different approach
to study the role of hydrogen bonding was used by Billsten et al. (2005) who varied pH of the water
added to the ethanolic solution of zeaxanthin. When 40% of water at pH 4 was added to the solution,
it produced the H-aggregate, while the same amount of water at pH 10 generated exclusively the
J-aggregate. The pH dependence is directly related to the ability of zeaxanthin to form a hydrogen
bond, because an increase in pH causes the deprotonation of the hydroxyl groups of zeaxanthin.
At higher pH, zeaxanthin is not able to participate as readily in hydrogen bonding, indicating that
J-aggregates are preferentially formed when hydrogen bonding is prevented. This conclusion is fur-
ther supported by the fact that the nonpolar counterpart of zeaxanthin, -carotene, having the same

structure but lacking the hydroxyl groups, preferentially forms J-aggregates. The J-band at 515 nm
dominates the absorption spectrum of

β

-carotene in acetone/water mixture, with only a hint of a
blueshifted band around 420 nm (Zsila et al. 2001d).
It was also demonstrated that not only the presence of the hydroxyl groups, but also their posi-
tion is an important factor in the formation of hydrogen bonds, and consequently in determining
whether J- or H-aggregates will be produced. As shown by Simonyi et al. (2003), the presence
of a free hydroxyl group on both sides of a carotenoid molecule is necessary for the formation of
H-aggregates. However, even in this case it may eventually happen that J-aggregate is formed, as
observed by comparing of aggregation properties of capsanthol stereoisomers having two hydroxyl
groups either on the same or on the opposite sides of the molecular plane (Zsila et al. 2001e).
The results obtained either from the pH dependence (Billsten et al. 2005) or esterii cation (Simonyi
et al. 2003) support the idea that the card-pack H-aggregates are stabilized via a hydrogen-bonding
network. The ability of hydrogen-bond formation is thus a decisive factor determining whether
J- or H-aggregates are formed. The necessity of hydrogen bonding for H-aggregate formation can
be justii ed by the card-pack structure of the aggregates. Therefore, in the simple case of a dimer,
hydrogen bonding at both sides of the carotenoid molecule helps to keep the two molecules together
lying one on top of the other with their dipoles oriented almost perfectly parallel to each other.

β

8.3.2.3 Other Spectral Features in Absorption Spectra of Carotenoid Aggregates
Besides the main band, H-aggregates also exhibit weaker bands in the red part of the absorption
spectrum (marked by * in Figure 8.5). Although in some cases the position of these bands coin-
cides with the vibrational bands of the monomeric carotenoid and can be therefore assigned to
nonaggregated carotenoid molecules, certain spectral features do not match the vibrational bands