Novel methyl-branched alkenones with up to five double bonds in saline lakes

摘要

Long-chain alkenones, a class of highly specific and widely used lipid biomarkers found in ocean and lake sediments, have been so far found as straight-chain alkyl ketones with 2 to 4 double bonds. Jaraula et al. (2010) reported assignments of a series of tri- to penta-unsaturated alkenones as straight-chain C_(38) methyl (C_(38)Me) and C_(39) ethyl (C_(39)Et) alkenones in Lake Fryxeli, Antarctica. The same series of compounds were later found in sediments from Lake Van (Randlett et al., 2014). The structure assignments by Jaraula et al (2010) were primarily based on strong ions at [M-43]~+ for C_(38) alkenones and [M-57]~+ for C_(39) alkenones, which were interpreted as a loss of CH_3CO and CH_3CH_2CO groups. However, such fragmentation is atypical for common straight-chain methyl and ethyl alkenones. In this study, we reanalyzed Lake Van sediment samples. We show these new C_(38) and C_(39) alkenones elute earlier than the common straight-chain C_(38)Me and C_(39)Et alkenones on a mid-polarity GC column. After hy-drogenation, mass spectra of these new alkenones show distinct peaks at m/z 72 or 86 caused by McLafferty rearrangement, indicating a methyl substitution at the α position of the carbonyl group in these C_(38) and C_(39) alkenones (i.e., α-methyl-branched C_(38)Me and C_(39)Et). Our new assignments as methyl-branched alkenones are further confirmed by the synthesis of an analog α-methyl C_(25) methyl ketone and comparison of mass spectra. Double bond positions for branched C_(38:5)Me (brC_(38:5)Me) are found to be △~4, △~7, △~(14), △~(21) and △~(28) based on the mass spectrum of corresponding dimethyl disulfide adducts. Analysis of Lake Van alkenone data reveals that U_(br38Mc)~(K*) based on brC_(38) Me shows a trend similar to U_(37)~K for the past 270 ka, suggesting that the degree of unsaturation of branched alkenones is also sensitive to temperature.