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Positive,
J
-type
Negative,
H
-type
Negative,
H
-type
Positive,
J
-type
FIGURE 3.7 (See color insert following page 336.)
Molecular arrangements for
H
- and
J
-aggregates.
Tetrameric lysophospholipid (
R
)-
3.15
forms predominantly
H
-aggregates in addition to a small percentage of
J
-aggregates. The calculated VIS absorption of the tetramer (
R
)-
3.15
is in accordance with the experimental
VIS spectra. (Reprinted from Foss, B.J. et al.,
Chem. Eur. J.
, 11, 4103, 2005b. With permission.)
the specii c employed experimental conditions. It may be possible that the aggregate preference
for the investigated carotenoids changes when the conditions are reversed, by adding water to the
carotenoid-acetone solution. When water was successively added in small increments to a metha-
nolic solution of the astaxanthin oximium hydrochloride,
3.38
,
H
-aggregates were formed, how-
ever when the hydrochloride,
3.38
, was immediately dispersed in water,
J
-aggregates were found,
Figure 3.8. Measured in the laboratory of synthesis, the phospholipid,
3.15
, gave an
H
-aggregate
with
380 nm. When another sample of
3.15
was later dispersed in the spectroscopy labora-
tory, the
H
-aggregates absorbed at 390 nm. Afterward,
3.15
was again dispersed in the laboratory
of synthesis and now formed
H
-aggregates with absorption at 400 nm. Measurements with subse-
quently synthesized batches of
3.15
demonstrated the same alternation among the three varieties
of aggregate absorption. The different aggregate dispersions were stable and did not convert to a
common absorption value over time. The dependence of aggregate absorption on the method of
dilution had been previously observed with a dihydroxycarotenone (Simonyi et al. 2003). Obviously,
the formation and size of specii c aggregates is neither exactly reproducible nor predictable. The
preparation of carotenoid aggregates with predei ned dimensions has to rely on other methods (E.M.
Sandru, Trondheim, unpublished).
The absorption band of a monomolecular dissolved molecule expresses the energy between the
molecule's ground and its excited state. In dispersions, the number of molecules associated in an
aggregate can be quite high, e.g., in aggregates of palmitoylglycerophosphocholine
N
λ
max
=
=
900 (Hayashi
et al. 1994), and in heterogeneous inclusion aggregates
N
10,000 carotenoid molecules (Horn and
Rieger 2001). Does the exciton band represent the interaction of all the many chromophores in the
aggregate? Similar to a crystal, in which the crystal unit determines the properties regardless of the
crystal's size, a small aggregation unit may express the properties of aggregates regardless of
N
. So
far, only a couple of carotenoid aggregates have been studied with the intention to locate a simple
molecule arrangement. The calculated aggregation spectra of capsorubin,
3.48
, Scheme 3.14, are
considered reliable from an exciton interaction of four molecules. The absorption maxima of cap-
sorubin tetramer, pentamer, hexamer, and heptamer are well resolved, and the octamer absorption
is quite similar to that of the nonamer. The aggregate absorption for the decamer, undecamer, and
dodecamer are practically identical, indicating a convergence value (Köpsel 1999), Figure 3.9. In a
detailed investigation with aggregates of enantiomeric zeaxanthin,
3.3
, astaxanthin,
3.6
, capsoru-
bin,
3.48
, and other carotenoids not only the absorption, but also the circular dichroism (CD) spec-
tra were calculated (Köpsel 1999). It was found that the spectra for astaxanthin,
3.6
, are represented
by an octamer of the
H
-aggregate type. Possible higher oligomers could not be dei ned, the octamer
reaching the convergence value (Köpsel et al. 2005).
=
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