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 <b>1</b>) 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-<b>1a</b> (2S,3R,1''R) and <b>1b</b> (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, <b>1a</b> exhibited significant neuroprotective effects against rotenone-induced cytotoxicity in SH-SY5Y neuroblastoma cells, while <b>1b</b> was inactive. Compound <b>1a</b> attenuated oxidative stress by reducing intracellular and mitochondrial reactive oxygen species and elevating glutathione levels. Mechanistically, only <b>1a</b> activated nuclear factor erythroid 2-related factor 2 (Nrf2), a key regulator of antioxidant defenses. Molecular docking studies further indicated a stronger interaction of <b>1a</b> 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.