Spectroscopic Investigation and Nanoscale Characterization of Epinephrine Autooxidation under Alkaline Conditions

Melanins are intriguing biomaterials with unique physical and chemical properties. Due to the insoluble nature of the synthetic melanins prepared from different precursors, such as 3,4-dihydroxy-phenylalanine (DOPA) and dopamine (DA), it is still challenging to reveal the structure-property relationships. In this work, the autoxidation of epinephrine (EP) under basic conditions was investigated from the perspective of supramolecular chemistry, and the formed soluble epinephrine-melanin (EPM) was characterized on the nanoscale. The supramolecular aggregate nature of oxidation products has been identified on the basis of spectroscopic investigations. A two-dimensional sheet-like morphology with highly ordered in-plane stacking structures was observed for the first time, and the thickness of the nanosheet increased with increasing EPM concentration. More importantly, in contrast to the well-known monotonic absorption profiles of synthetic melanins, EPM shows featured and unusual pH-responsible absorption profiles in the near-ultraviolet region (UVA). The decrease in pH can induce the disappearance of the absorption in the lower-energy band and the reduction of aggregate size. The oxidative and aggregation kinetic processes of EP were investigated in three different alkaline systems by the combination of absorption and fluorescence spectroscopies. The oxidation process of EP shows concentration- and buffer-dependent behaviors. The unusual absorption properties of EPM were exploited for the fabrication of transparent UV-shielding chitosan biofilms and gelatin hydrogels. Extensive research on the molecular structures, supramolecular exciton coupling, and material-oriented property exploitation of EPM is highly anticipated.