Visible light-induced degradation of the chlorophyll phytyl side chain was studied in senescent cells of two phytoplanktonic strains (Skeletonema costatum and Thalassiosira weissflogii). Particular attention was paid to the induction of autoxidation processes on the phytyl chain and its photoproducts by photochemically produced hydroperoxides. The combination of photochemical oxidation and autoxidation reactions resulted in the production of several acyclic isoprenoid compounds that have been unambiguously identified by comparison of their retention times and mass spectra with those of appropriate standards. Various mechanisms are proposed to explain the formation of these oxidation products. These processes appear to be potential sources of numerous oxidized acyclic isoprenoids that previously have been detected in lacustrine and marine environments. Some oxidation products newly described or whose presence in natural samples was never reported in the literature were then sought in particulate matter, sediment, and microbial mat samples. The results obtained supported the significance of photochemical oxidation and autoxidation of phytoplanktonic chlorophyll phytyl side chain in the marine environment.

The electron ionization (EI) mass spectra of Z-and E-2,3-epoxy-3,7,11,15-tetramethylhexadecan-1-ol trimethylsilyl ethers are reported. These compounds present interesting fragmentation processes involving notably a transannular cleavage of the epoxide ring after transfer of the trimethylsilyl group. These pathways have been substantiated by deuterium labelling and comparison with the EI mass spectra of the isomeric 1,2-epoxy-3,7,11,15-tetramethylhexadecan-3-ol trimethylsilyl ether and of the corresponding methyl ethers. Copyright # 2001 John Wiley & Sons, Ltd.Although the electron ionization (EI) mass spectra of aliphatic epoxides have been studied extensively, 1,2 very little information has been reported on the related a,b-epoxy ethers (i.e. glycidic ethers). To our knowledge, only partial fragmentation pathways of allyl and n-butyl mono-and diglycidic ethers have been determined by metastable linked-scan techniques. 3 In the framework of our studies of the EI mass spectral fragmentation of isoprenoid compounds, 4±7 we investigated the fragmentation pathways of Z-and E-2,3-epoxy-3,7,11,15-tetramethylhexadecan-1-ol trimethylsilyl ethers. It is in fact important to determine whether polyfunctional molecules (notably those in which the functional groups are in close proximity to each other) display separately fragmentation typical of each functionality or whether the polyfunctional molecule behaves sui generis. 2 In the EI mass spectra of these compounds, we observed interesting typical fragmentations and in particular an intense peak at m/z 341 corresponding to a transannular cleavage of the epoxide ring with transfer of the trimethylsilyl group. A pathway involving an initial transfer of the trimethylsilylyl group from the ether group to the epoxy oxygen is proposed in order to explain this interesting cleavage. EXPERIMENTAL Chemicals SilylationCompounds to be silylated were taken up in 400 mL of a mixture of pyridine and BSTFA (3:1, v/v) and allowed to react at 50°C for 1 h. After evaporation to dryness, the residue was dissolved in ethyl acetate and analyzed by gas chromatography/mass spectrometry (GC/MS). Mass spectrometryAnalyses by gas chromatography/electron ionization mass spectrometry were performed with a Hewlett Packard HP 5890 series II plus gas chromatograph connected to a HP 5972 mass spectrometer. ; carrier gas (He) pressure maintained at 1.04 bar until the end of the temperature program and then programmed from 1.04 bar to 1.5 bar at 0.04 bar min À1 ; injector (on column with a retention gap) temperature, 50°C; electron energy, 70 eV; source temperature, 170°C; mass range, 50±700; mass scan cycle time, 1.5 s. RESULTS AND DISCUSSIONElectron ionization (EI) mass spectra of the trimethylsilyl ethers of Z-and E-2,3-epoxy-3,7,11,15-tetramethylhexadecan-1-ols are reported in Fig. 1. In addition to fragment ions at m/z 73, 103 and [M-15] , which are typical of silylated primary alcohols, 11 these EI mass spectra show an intense peak at [M À 43] (m/z 341) and other interesting fragment