Sedimentary purpurins: oxidative transformation products of chlorophylls

Examination of sediments from three contemporary lakes (Priest Pot, Loch Ness and Lake Baikal) reveals the presence of oxidative transformation products of chlorophyll a in two of the settings. Purpurin-18 phytyl ester, not previously reported in sediments, occurs in the two lakes that have deep water columns. The free acid counterpart, purpurin-18, occurs in only one of the samples, that from Loch Ness. Although, where present, the abundance of purpurin-18 phytyl ester is similar to that of individual chlorin steryl esters there appears not to be a correlation between their abundances in the sediments, suggesting that formation of the purpurins is not mediated by herbivory. The presence of oxidative transformation products of chlorophyll in contemporary lake bottom sediments indicates their formation during the earliest stages in chlorophyll degradation. Their preservation in sediments demonstrates that oxidative transformation reactions in the aquatic environment lead not only to destruction of the chlorophyll macrocycle, but also to structures modified in ring E. This lends support to the view that sedimentary aetioporphyrins originate mainly, though not exclusively, from transformations of chlorophylls that involve oxidative cleavage of the five-membered exocyclic ring.