At the root of l-lysine emission in aqueous solutions.

l-lysine is an essential amino acid whose peculiar optical properties in aqueous solutions are still in search of a comprehensive explanation. In crystalline form l-lysine does not emit, but when in an aqueous solution, as the concentration increases, emits in the blue. The origin of such fluorescence is not yet clear. In the present article, we have combined quantum mechanics and classical simulations with experimental techniques to demonstrate that optical absorption and excitation-dependent fluorescence are directly correlated with the formation of aggregates, their dimensions and intermolecular interactions. The nature of the aggregates has been studied as a function of the pH and concentration of the amino acid. At low concentrations, fluorescence intensity increases linearly with molarity, while at high concentrations a new condition is established in which emitting and non-emitting molecular species coexist. The l-lysine aggregation and the formation of intermolecular H-bonding are at the ground of the emission in the blue range.