ESI-protonated natural curcumin (1) undergoes gas-phase cyclization and dissociates via competitive expulsions of 2-methoxy phenol and C4H4O2 (diketene or an isomer). Evidence from mechanistic mass spectrometry and from Density Functional Theory (DFT) reveals that a two-step sequential cyclization occurs for the protonated molecule prior to the unusual loss of the elements of 2-methoxy phenol. Furthermore, the presence of the methoxy group at postion-3 is essential for the second cyclization. The transformation of curcumin upon protonation in the gas phase may be predictive of its solution chemistry and explain how curcumin plays a protective role in biology.

中文翻译：

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姜黄素的质子化触发气相中的连续双环化：电喷雾质谱和 DFT 研究

ESI 质子化的天然姜黄素 (1) 进行气相环化，并通过 2-甲氧基苯酚和 C4H4O2（双烯酮或异构体）的竞争性排出而解离。来自机械质谱和密度泛函理论 (DFT) 的证据表明，在 2-甲氧基苯酚元素异常丢失之前，质子化分子发生了两步顺序环化。此外，位置 3 处甲氧基的存在对于第二次环化是必不可少的。姜黄素在气相质子化时的转化可以预测其溶液化学，并解释姜黄素如何在生物学中发挥保护作用。