Bistable aggregate of all-trans-astaxanthin in an aqueous solution

The temperature dependence of the optical absorption spectra for astaxanthin aggregate has been studied between 2 and 32°C. Red-shifted absorption bands as compared to the monomer absorption band are found above 21°C in addition to the blue-shifted band of the aggregate. The spectra suggest that the molecular arrangement in the aggregate is a bistable one consisting of head-to-tail and card-packed arrangements. A diagram describing the bistability together with the monomer state is proposed in the space defined by the free energy and the quantity of Σi = 1N<θ12 + <σθ12 for the ith molecule in the N-molecule aggregate.