Oxidation of A2E occurs during sample handling. A) MS/MS spectrum of singly-oxidized A2E (m/z = 608) from 100 fmol of synthetic A2E standard. Asterisks indicate fragment ions for which potential structures are shown in panels B-D. As the ions present in the tandem mass spectrum can be produced when the oxygen is in three different locations, the fragmentation pattern of singly-oxidized A2E supports three sites of oxidation, B) a 7, 8 epoxide, C) a 5, 8 monofuranoid, and D) a 9, 10 epoxide. In these structures, placement of the oxygen on the short arm of A2E was done for simplicity; it is likely that the oxygen also occurs on the long arm.