Visible-Light Activated [2+2] Cycloaddition Reaction Enables Pinpointing Carbon-Carbon Double Bonds in Lipids - 陈素明课题组（质谱研究组）

The precise location of C=C bonds in bioactive molecules is critical for deep understanding the relationship between their structures and biological roles. However, the traditional ultraviolet light-based approaches exhibited great limitations. Here, we discovered a new type of visible-light activated [2+2] cycloaddition of carbonyl with C=C bonds. We found that carbonyl in anthraquinone showed great reactivities towards C=C bonds in lipids to form the oxetanes under the irradiation of visible-light. Combining with tandem mass spectrometry, this site-specific dissociation of oxetane enabled precisely locating the C=C bonds in various kinds of monounsaturated and polyunsaturated lipids. The proof-of-concept applicability of this new type of [2+2] photocycloaddition was validated in the global identification of unsaturated lipids in complicated human serum sample. 86 of monounsaturated and polyunsaturated lipids were identified with definitive positions of C=C bonds, includes phospholipids and fatty acids even with up to 6 C=C bonds. This study provides new insights both on the photocycloaddition reactions and the structural lipidomics.