Fatty acid identifiers

[Anonymous]

Fatty acids are specified using the common convention Fx:y where x is the number of carbon atoms and y is the number of unsaturated C=C bonds.  For branched-chain fatty acids, the number x is the total number of carbons and thus, for example, the isomer F15:0I is13-methyltetradecanoic acid, systematically named as a C₁₄ compound.

n-Notation and monounsaturated fatty acids

In nutritional contexts, the position of double bonds in unsaturated fatty acids is commonly specified using n-notation shorthand, which counts from the methyl end of the chain.  The system is widely used in the Thesaurus for fatty acid descriptors and identifiers.  However, its scope has strict limitations, which the book Food lipids (pp 8-9) states as:

Shorthand nomenclature cannot be used for fatty acids with trans or acetylenic bonds, for those with additional functional groups (branched, hydroxy, etc.), or for double bond systems (2 or more double bonds) that are not methylene interrupted ([i.e. that are not] isolated or conjugated).

Wider usage of n-notation for fatty acid identifiers depends on extra information provided before the shorthand notation, which must always terminate the identifier.   Configuration is shown explicitly, for example F16:1CN7 is used for palmitoleic acid and F16:1TN7 for its trans isomer palmitelaidic acid.  Although rarely occurring, substitution needs to be considered and should be recorded specifically.  The current identifier for ricinoleic acid (12-hydroxy-cis-9-octadecenoic acid) is F18:1N9O, which would be better as F18:1O12CN9.  This would distinguish it from the trans form ricinelaidic acid, as well as accommodating any alternative locations of the hydroxy group.

Polyunsaturated fatty acids

Polyunsaturated fatty acids provide further complications as the positions and configurations of the several double bonds must be specified unambiguously.  The termination CNx is used to indicate strict n-notation, i.e.the methylene separated (=CH-CH₂-CH=), all-cis form; for example gamma-linolenic acid is F18:3CN6.  However, because the term trans-fatty acid is used when one or more bonds is trans, the explicit identifier F18:3TTTN3 is used for the all-trans structure linolenelaidic acid.  The F18:3TN3 would be used for the total of n-3 trans isomers, whereas overall cis or trans totals for a fatty acid are indicated by the 3-letter terminations CIS or TRS, e.g. F18:3TRS for total trans-octadecatrienoic acids.

Conjugated fatty acids

The total of conjugated forms for a fatty acid use the 3-letter terminator CON; at present only F18:2CON is defined.  Where specific positional information is included, this is done using the chemical convention of counting from the functional group, i.e. the carboxylate end.  Thus F18:2C9T11 is defined for rumenic acid (9-cis,11-trans-octadecadienoic acid) and F18:2T10C12 for the other most common isomer.  However, these may co-occur with the 9-cis,11-cis and 10-cis,12-cis isomers, so it may be necessary to identify further isomers and mixtures of them.

Totals, remainders and unidentifieds

A wide range of terms is available for reporting totals of fatty acids and of their isomers.  Some summate isomers of mono- and polyunsaturated acids, e.g. fatty acids, total n-3 [FAN3] and fatty acids, total cis n-3 [FACN3].  Others represent totals within the different types of fatty acid, e.g. fatty acids, polyunsaturated, total n-3 [FAPUN3].  This overall total has more specific related totals, e.g. for the (all-)cis isomers [FAPUCN3], the long-chain acids [FAPUN3LC] and fatty acids derived from particular food sources such as fish [FAPUN3FI].

Terms are included for various remainders and "others".  The one term now using "other" is fatty acids, polyunsaturated, other (= PUFA-linoleic-linolenic) [FAPUOT] and thus it specifically defines the fatty acids that are included (except that it does not state clearly whether both alpha- and gamma-linolenic acid are included).

 

For discussions of component identifiers for fatty acids that are requested for inclusion in the Component Thesaurus, see the topic Fatty acid coverage.

[Ian Unwin]

n-Notation and monounsaturated fatty acids

Initial discussion points:

• Does the use in component codes of the n-notation specification (Nx) for trans or substituted fatty acids cause any problems in defining the structure represented?
• Are there any exceptions to the rule that the n-notation specification (Nx) should always be the last part of the component code?
• Are there any reasons for not changing the code for ricinoleic acid from F18:1N9O to F18:1O12CN9 (or the tagname from F18D1N9O to F18D1O12CN9)?

[Ian Unwin]

Polyunsaturated fatty acids

Initial discussion points:

• The use of n-notation (Nx) for trans polyunsaturated fatty acids must be limited to the methylene separated isomers, for example with F18:2TTN3 representing 12-trans,15-trans-octadecadienoic acid.  The code for 9-trans,15-trans-octadecadienoic acid would be F18:2T9T12.  Is this policy logical?
• If all positions cited before the final n-notation (Nx) are counted from the carboxylate end, F18:3TCCN3 (for example) would identify 9-trans,12-cis,15-cis-octadecatrienoic acid.  Is this an acceptable approach?
• Is the use of short forms (e.g. F18:3CN3) for the all-cis isomers, explicit forms (e.g. F18:3TTTN3) for the all-trans isomers and forms such as F18:3TN3 for total trans isomers the best approach to take?

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