Hydrophilic carotenoids: surface properties and aggregation of an astaxanthin-lysine conjugate, a rigid, long-chain, highly unsaturated and highly water-soluble tetracationic bolaamphiphile

The surface and aggregation properties of a synthetic, highly water-soluble carotenoid, the tetracationic astaxanthin-lysine conjugate (Asly), have been examined through measurements of surface tension, optical absorption and dynamic light scattering. The following parameters were determined: critical aggregation concentration cM, surface concentration Γ, molecular area am, free energy of adsorption and aggregation (ΔGad° and ΔGM°, respectively), and the aggregate size rH. The compound forms true monomolecular solutions in water below cM; aggregates emerge only at rather high concentrations (≥2.18 mM).