Branched-Chain and Cyclic Fatty Acids

The Author: Gerhard Knothe, National Center for Agricultural Utilization Research, Agricultural Research Service, U.S. Department of Agriculture, Peoria, IL, USA.

Gunstone (1993) reported the 13C-NMR spectra of a large number of branched chain acids/esters differing in chain length (C6-C18) and in the nature and position of the branching group (methyl to hexadecyl). Some of the results are summarised in Table 1. Table 2 gives generalised data for mid-chain, iso, and anteiso acids/esters.


Table 1. Some chemical shifts (ppm) for 2-, 3-, and 4-methyl branched acids/esters

2-methyl 18:0 A 183.67 39.47 33.59 27.20 nc nc 16.82
2-methyl 16:0 E 173.39 39.50 33.88 27.29 nc nc 17.08
3-methyl 16:0 E 173.77 41.70 30.39 36.78 26.96 nc 19.77
4-methyl 16:0 E 174.53 31.91 31.95 32.45 36.70 26.97 19.30

A = acid, E = ester, nc = not cited.


Table 2. Chemical shifts (ppm) at and close to branched methyl groups*

mid-chain 19.7-19.8 32.9 37.2 27.1 30.1 37.2 27.1 30.1
iso 22.7 28.0 39.1 27.4 nc 22.7 na na
anteiso 19.3 34.5 36.7 27.2 30.1 nc 11.4 na
na = not applicable; nc = not cited.
* α, β, and γ refer to carbon atoms between the point of branching and the acid/ester function; α', β', and γ' refer to carbon atoms between the point of branching and the end methyl group.


Chemical shifts have been reported for the following acids and derivatives: 14-methyl 11c-15:1 (LePivert et al., 1995), 12-methyl-18:0 (Black, 1989), 11-methyl 12-18:1 (Carballeira et al., 1998), 2-methyl 20:0 (ethyl ester, Besra et al., 1993, 6-methyl 4t-24:1 (ethyl ester, Besra et al., 1993), 9,12-dimethyl-18:0 (Black, 1989), 2,4-dimethyl 14:0 (acid, methyl and ethyl ester, Wallace and Minnikin, 1996a), and 2,4,6-trimethyl 2-24:1 (Wallace and Minnikin, 1996b).



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