N-Acetyldopamine Dimer from Cicadidae Periostracum Is Enantioselectively Neuroprotective via Antioxidant Property

N-Acetyldopamine oligomers are typically biosynthesized as racemic mixtures, yet enantiomers can differ markedly in pharmacological efficacy and safety. In this study, we isolated a pair of enantiomers of an N-acetyldopamine dimer (compound 1) from Cicadidae Periostracum, a traditional medicinal substance, and characterized their structures using mass spectrometry and 1D/2D NMR spectroscopy. The absolute configurations of the enantiomers-1a (2S,3R,1''R) and 1b (2R,3S,1''S)-were determined for the first time through a combination of electronic circular dichroism and Mosher's esterification analysis. Biological evaluation revealed striking differences in activity between the two enantiomers. Specifically, 1a exhibited significant neuroprotective effects against rotenone-induced cytotoxicity in SH-SY5Y neuroblastoma cells, while 1b was inactive. Compound 1a attenuated oxidative stress by reducing intracellular and mitochondrial reactive oxygen species and elevating glutathione levels. Mechanistically, only 1a activated nuclear factor erythroid 2-related factor 2 (Nrf2), a key regulator of antioxidant defenses. Molecular docking studies further indicated a stronger interaction of 1a with Keap1, the repressor of Nrf2, suggesting a structural basis for the enantioselective activation of the pathway. To our knowledge, this is the first report to assign the absolute configuration at C-1'' of this class of compounds and to demonstrate enantioselective neuroprotective activity mediated by the Nrf2 pathway. These findings underscore the therapeutic potential of insect-derived chiral natural products and provide a rationale for developing stereochemically defined neuroprotective agents for the treatment of neurodegenerative diseases such as Parkinson's disease.