  Carotenoids in hydrated polar solvents form aggregates characterized by
dramatic changes in their absorption spectra with respect to monomers. Here we
analyze absorption spectra of aggregates of the carotenoid astaxanthin in
hydrated dimethylsulfoxide. Depending on water content, two types of aggregates
were produced: H-aggregates with absorption maximum around 390 nm, and
J-aggregates with red-shifted absorption band peaking at wavelengths >550 nm.
The large shifts with respect to absorption maximum of monomeric astaxanthin
(470-495 nm depending on solvent) are caused by excitonic interaction between
aggregated molecules. We applied molecular dynamics simulations to elucidate
structure of astaxanthin dimer in water, and the resulting structure was used
as a basis for calculations of absorption spectra. Absorption spectra of
astaxanthin aggregates in hydrated dimethylsulfoxide were calculated using
molecular exciton model with the resonance interaction energy between
astaxanthin monomers constrained by semi-empirical quantum chemical
calculation. The intramolecular vibrations of astaxanthin are modeled by a line
shape function corresponding to two characteristic C-C and C=C stretching modes
with frequencies of 1150 cm^-1 and 1520 cm^-1. The solvent is represented by a
spectral density of harmonic vibrations. The spectral changes at increasing
concentrations of water were assigned to formation of aggregates with
decreasing exposure of the astaxanthin hydrophobic chain to the solvent.
