中文摘要：

烟草叶是目前惟一可以通过生物合成得到大量异构和反异构烷烃的植物，然而许多文献对化合物中甲基碳位和奇偶碳数的变化成因未提及。对其成因进行了文献追踪，并对温室不同生长条件（温度、CO2和光照）下培育的烟草叶中的长链正构烷烃、异构和反异构烷烃的单体稳定C同位素组成进行了分析，以期对烟草叶中特殊脂类化合物的生物化学合成同位素分馏效应进行探索研究。与正构烷烃和异构烷烃相比，反异构烷烃的δ^13C值平均相对偏重2‰，证明反异构烷烃确实具有不同的合成前身物（2-甲基丁酰辅酶A）。比起甲基基团，脱羧反应很可能使得乙酰辅酶A的羧基基团δ^13C值偏轻。本研究中异构烷烃和正构烷烃δ^13C值的相似性，说明乙酰辅酶A和前身物2-甲基丙酰辅酶A（C4-CoA）之间的C同位素组成很可能是相似的。用水量越多，烟草叶中异构、反异构烷烃和正构烷烃稳定C同位素组成越偏轻。与强光条件比，弱光条件下培育的烟草叶中正构烷烃的稳定C同位素组成偏轻约2．2‰，异构和反异构烷烃稳定C同位素组成也分别偏轻1．8‰和1．9‰。温度对烟草叶正构烷烃、异构烷烃和反异构烷烃的稳定C同位素组成影响甚小，与Farquhar（1980）有关植物体C同位素组成的报道相吻合。

英文摘要：

Tabacco is the only plant known to biosynthesize large quantities of 3-methyl alkanes and 2-methyl alkanes. We investigated the δ^13C values of long-chain n-alkanes, 3-methyl and 2-methyl alkanes extracted from tobacco grown under chamber conditions to confirm the pathway used by the tobacco plant to synthesize these particular lipids. Tobacco was also grown under different environment （temperature, water availability and light） in order to obtain their δ^13C values for purpose of evaluating the isotope fractionation effects. The 3-methyl alkanes were found to have a predominant even-carbon number distribution （maximum at C32）, whereas the 2-methyl alkanes exhibited an odd-carbon number distribution （maximum at C31）. The 2-methyl alkanes were relatively more abundant than the 3-methyl alkanes and only two 3-methyl alkanes （ aiC30 and aiC32） were observed. The δ^13C of the 3-methl alkanes were found to be enriched in ^13C by an average of 2‰ compared to the n-alkanes and 2-methyl alkanes supporting a different biosynthetic precursor （2-methylbutanoyl C5-CoA precursor） . The decarboxylation step probably led to the loss of a carboxyl group of the coenzyme A being depleted in ^13C compared to the methyl group. The 2-methyl alkanes having an odd-numbered dominance are mainly biosynthesized from a 2-methyl- propanoyl （C4） -CoA biological precursor. The δ^13C of acetyl-CoA and methylpropanoyl （C,） -CoA precursors are probably similar. The similarity in the ^13C of n-alkanes and the 2-methyl alkanes are consistent with this suggestion. Higher water availability generally results in more negative ^13C bulk values, which was confirmed in the present study by δ^13C values of 2-methl alkanes, 3-methyl alkanes and n-alkanes lipids extracted from the tobacco leaves. Likewise, light intensity has been shown to influence plant bulk ^13C values. The δ^13C of n-alkanes in tobacco grown under low-light conditions were about 2. 2‰ more depleted in ^13C than those lipids extracted from tobacco g