Figure 7.

Investigation of necine base biosynthesis by feeding of stable isotope labeled precursors. Feeding of (A) [1,4-¹³C₂]-putrescine and [1-¹³C]-putrescine, (B) [2,3-¹³C₂]-putrescine, (C) [1,2-¹³C₂]-putrescine, (D) [1-¹⁵N, 1-¹³C]-putrescine and (E) [1,9-¹³C₂]-homospermidine. Red dots symbolize ¹³C labels, a red N symbolizes a ¹⁵N label. ¹³C-¹³C double labels are marked with bonds in red. Please mind that the retronecine structures shown in (A), (B), and (C) are composite representations of all labeled species present.