| Summary: | 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 Fryxell, 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 3 CO and CH 3 CH 2 CO 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 hydrogenation, 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 Δ 4 , Δ 7 , Δ 14 , Δ 21 and Δ 28 based on the mass spectrum of corresponding dimethyl disulfide adducts. Analysis of Lake Van alkenone data reveals that based on brC 38 Me shows a trend similar to for the past 270 ka, suggesting that the degree of unsaturation of branched alkenones is also sensitive to temperature.
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