Natural Deuterium Distribution in Branched-Chain Medium-Length Fatty Acids is Nonstatistical: A Site-Specific Study by Quantitative 2H NMR Spectroscopy of the Fatty Acids of Capsaicinoids

Quantitative 2H NMR spectroscopy has been used to determine the natural abundance site‐specific 2H isotopic content of 6,7‐dihydrocapsaicin (1) and capsaicin (2). Prior to analysis, the fatty acyl moieties were released as methyl 8‐methylnonanoate (3) and methyl E‐8‐methylnon‐6‐enoate (4), respectively. A marked and similar nonstatisitical isotopic distribution of 2H is observed for both fatty acids. Notably, it can be seen that: 1) the isobutyl portion of 3 is more impoverished in 2H than the methylenic portion; 2) the isobutyl portion of 4 is more impoverished than that of 3; 3) an alternating pattern occurs in the (2H/1H)i between the C3 to C7 positions; and 4) the ethylenic hydrogens at C6 and C7 of 4 are, respectively, impoverished and unchanged relative to these positions in 3. These observations are compatible with the proposed biosynthetic origins of the different parts of 1 and 2, and with the view that 1 is a proximal precursor of 2. Furthermore, it can be suggested that, firstly, the hydrogen atoms at C3 to C7 originate alternatively from the substrate and from the environment and, secondly, that the Δ6‐E desaturation is introduced by a mechanism closely mimicking that of the Z desaturation of higher plants. Kintetic isotope effects and differences in biosynthetic origin cause a marked nonstatistical distribution of 2H in the fatty acid moieties of capsaicinoids. The measured site‐specific isotopic 2H signatures (expressed as the (2H/1H)i, the 2H/1H ratio at a given carbon atom i; see graph) are interpreted in terms of the enzymes involved, notably a possible E‐desaturase.