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 C₃₈ methyl (C₃₈Me) and C₃₉ ethyl (C₃₉Et) 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 C₃₈ alkenones and [M−57]⁺ for C₃₉ alkenones, which were interpreted as a loss of CH₃CO and CH₃CH₂CO 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 C₃₈ and C₃₉ alkenones elute earlier than the common straight-chain C₃₈Me and C₃₉Et 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 C₃₈ and C₃₉ alkenones (i.e., α-methyl-branched C₃₈Me and C₃₉Et). Our new assignments as methyl-branched alkenones are further confirmed by the synthesis of an analog α-methyl C₂₅ methyl ketone and comparison of mass spectra. Double bond positions for branched C_38:5Me (brC_38:5Me) are found to be Δ⁴, Δ⁷, Δ¹⁴, Δ²¹ and Δ²⁸ based on the mass spectrum of corresponding dimethyl disulfide adducts. Analysis of Lake Van alkenone data reveals that ${\mathrm{U}}_{\mathrm{b}\mathrm{r}38\mathrm{M}\mathrm{e}}^{\mathrm{K}\ast }$ based on brC₃₈Me shows a trend similar to ${\mathrm{U}}_{37}^{\mathrm{K}}$ for the past 270 ka, suggesting that the degree of unsaturation of branched alkenones is also sensitive to temperature.

长链烯酮是在海洋和湖泊沉积物中发现的一类高度特异性且广泛使用的脂质生物标记，迄今为止，已发现其为具有2至4个双键的直链烷基酮。Jaraula等。（2010年）报道了在南极州弗莱克塞尔湖上分配的一系列三到五不饱和烯酮作为直链C ₃₈甲基（C ₃₈ Me）和C ₃₉乙基（C ₃₉ Et）烯酮。后来在范湖的沉积物中发现了相同系列的化合物（Randlett等，2014）。Jaraula等人（2010）的结构分配主要基于C ₃₈烯酮的[M-43] ⁺和C _39的[M-57] ^+的强离子。烯酮，这被解释为CH ₃ CO和CH ₃ CH ₂ CO基团的损失。然而，这种断裂对于常见的直链甲基和乙基烯酮而言是非典型的。在这项研究中，我们重新分析了范湖沉积物样本。我们在中极性GC色谱柱上显示了比常见的直链C ₃₈ Me和C ₃₉ Et烯酮更早洗脱的这些新C ₃₈和C ₃₉烯酮。氢化后，这些新的烯酮的质谱图显示由McLafferty重排引起的m / z 72或86处的明显峰，表明在这些C ₃₈和C中羰基的α位置有甲基取代_39个烯酮（即，α-甲基支链的C ₃₈ Me和C ₃₉ Et）。通过合成类似的α-甲基C ₂₅甲基酮并比较质谱，进一步证实了我们作为甲基支链烯酮的新定义。为支链C双键的位置_5：38我（BRC _38：5 Me）中被发现是Δ ⁴，Δ ⁷，Δ ¹⁴，Δ ²¹，Δ ²⁸基于对应二甲基二硫化物加合物的质谱。对范湖烯酮数据的分析表明${\mathrm{ü}}_{\mathrm{b}\mathrm{[R}38\mathrm{中号}\mathrm{Ë}}^{\mathrm{ķ}\ast }$基于brC ₃₈ Me的趋势类似于${\mathrm{ü}}_{37}^{\mathrm{ķ}}$ 对于过去的270 ka，表明支链烯酮的不饱和度也对温度敏感。