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The Dimerised Fatty Acids and its Derivatives category covers C16 - C18 unsaturated fatty acids derived monomers, dimers and trimers, as well as their hydrogenated products in different proportions and in accordance with their corresponding production and purification processes. They are all prepared by the dimerisation of C16 - C18 unsaturated fatty acids. As UVCB substances derived from natural sources, members of this category are chemically similar as they are all essentially a complex mixture of C16 - C18 unsaturated and saturated, branched and linear fatty acids, their monomers, dimers and trimers with varying structural geometric isomers. In the category family, all substances have an overlap in regard to their composition. With reference to information of existing category, the category of Dimerised Fatty Acids and Its Derivatives is based on similarities in physicochemical and toxicological properties and 2 sub-categories were further defined on the basis of their environmental fate and environmental toxicity. The first sub-category covers three monomeric (by-) products of the dimerization process (readily biodegradable substances). The second sub-category covers the predominately oligomers (dimeric and trimeric products) of dimerization based on their lack of biodegradability and the environmental fate.

Sub-category 1: predominantly monomers

ID No.

CAS No.

Common Name

Chemical Name

#5

68955-98-6

Monomer acid

Fatty acids, C16-18 and C18-unsaturated, branched and linear,

#6

68201-37-6

Hydrogenated monomer acid

Octadecanoic acid, branched and linear”

#7

30399-84-9

 

Isooctadecanoic acid

 

Sub-category 2: predominantly oligomers (dimers, trimers)

ID No.

CAS No.

Common Name

Chemical Name

#4

71808-39-4

Crude dimer

Fatty acids, C16-C18 and C18-unsaturated, dimerized”

#1

61788-89-4

Dimer

Fatty acids, C18-unsaturated, dimers, “”

#3

68783-41-5

Hydrogenated dimer

Fatty acids, C18-unsaturated, dimers, hydrogenated

#2

68937-90-6

Trimer

Fatty acids, C18-unsaturated, trimers

Derived from the same starting substance, all substances in this category have a homologous composition of fatty acids with a C16 - C18 carbon chain in diverse forms, that is susceptible to oxidation of metabolic process. In view of the results of various QSAR analyses, the toxic hazard of the substances mainly depends on the number of the carbons, on the chain “structure”, such as branching, unsaturation, grade of cyclics and aggregation, as well as their position in the molecular structures. Whereas, the number of the function group “carboxylic acids” has no significant influence on the tox- and ecotoxicological profiles.

Sub-category 1: predominantly monomers

As aforementioned, the similarity of the sub-category 1 members is justified , in accordance with the specifications listed in Regulation (EC) No. 1907/2006 Annex XI, 1.5 Grouping of substances and read across, on basis of scope of variability and overlapping of composition, representative molecular structure, physico-chemical properties, tox-, ecotoxicological profiles and supported by various QSAR methods. There is no convincing evidence that any one of these chemicals might lie out of the overall profile of this sub-category, respectively. The key points that the members share are:
  • Common origin of C16-18 unsaturated fatty acids
  • Similar/overlapping structural features (no hydrolysable groups, all members have a homologous composition of fatty acids with a C16 - C18 carbon chain in diverse forms, that are susceptible to oxidation of metabolic process)
  • Similar metabolic pathways (same ADME pathways of fatty acids, absorbed fatty acids undergo rapid metabolism (via ß- or ω-oxidation) and excretion either in the expired CO2 or as a hydroxylated or conjugated metabolite in the urine in the case of cyclic fatty acids)
  • Similar physico-chemical properties (log Koc >4 < 5, the log Kow is judged to be > 4, the poor solubility in water)
  • Common properties for environmental fate & eco-toxicologcial profile of the two sub-categories (readily biodegradable, no toxicological effects up to the water solubility limit for aquatic organisms)
  • Common levels and mode of human health related effects

Environmental fate Sub-category 1 (predominantly monomers):

In accordance with Regulation (EC) No 1907/2006 Annex VIII 9.2.2.1 column 2 the testing for hydrolysis is not required for substances that are readily biodegradable. In addition as Fatty acids, C16-18 and C18-unsaturated., branched and linear (CAS No. 68955 -98 -6) does not have a functional group that is susceptible to hydrolysis, it is expected not to hydrolyse under environmental conditions. Photodegradation of Fatty acids C16-C18 and C18 unsaturated branched and linear in air is not a relevant degradation pathway as the vapour pressure under ambient conditions is neglible. In the biodegradation screening test Fatty acids C16-C18 and C18 unsaturated branched and Linear was determined as readily biodegradable (67 % after 28 d, Sewell (1994)).

Adsorption potency is predicted by QSAR modeling (EPISUITE KOCWIN v2.00). The main factor determining the log Koc value of fatty acids is the chain length. Each carbon atom added contributes to Koc ca. 1/4 of the logarithmic unit. Taking into account that both predicted and experimental values for acetic acid CC(=O)O are close to 0, we see that adding 16 carbon atoms increases the log Koc to ca. 16x1/4 = 4. This is the minimum value to be expected for the category members. Introducing double bonds and branching may decrease this value, but not very significantly For monomers, C16 to C18, the calculated log Koc values are between 3.2 (heptamethylnonanoic acid, MCI method) and 4.7 (octadecanoic acid, Kow method) and therefore the overall contribution of monomers fulfils the condition: 4 < log Koc < 5. The calculated results for potential dimeric constituents are higher. As the content of oligomers should be maintained as small as possible, it can be assumed that this contribution is negligible. By higher content of residual dimeric components (> ca. 5%), the worst case log Koc > 5 has to be considered. For "predominantly oligomers", log Koc > 4 < 5. In both cases, adsorption to organic carbon in soils and sediments is expected.

The value for partition coefficient log Pow of UVCB substance is based on a weight of evidence using experimental and QSAR results for the components of the substance. Due to the complex nature of the UVCB substance not all components of the substance are known. For the evaluation of the influence of the dimerisation and branching, the hypothetical structures were examined with use of QSAR model (KOWWIN v1.68). The experimental literature partition coefficient log Pow for monomers of C16-18 fatty unsaturated and saturated acids are all above 6: log Pow=7.05 for linoleic acid, 8.23 for octadecanoic acid, 7.64 for 9-octadecenoic acid, 6.46 for linolenic acid, 7.17 for palmitic acid and 7.45 for heptadecanoic acid. The partition coefficient of the substance increases with the increase of number the C chain incrementally. Log Pow of the dimer of C18 fatty acids is then expected to be much higher than log Pow of monomers. It is also proven by QSAR prediction for example dimer structure (log Pow = 14.81). The branching does not affect the log Pow substantially and it was calculated by KOWWIN v1.68 for an example structure of C16 acid to be 6.45. The log Pow = 4.9 was also experimentally determined with Fatty acids, C16-18 and C18-unsaturated., branched and linear. All available results show the Fatty acids, C16-18 and C18-unsaturated., branched and linear is high and above 4, therefore a cut off value of log Kow > 4 is accepted for the chemical safety assessment.

If a substance has a low potential for bioaccumulation and/or a low potential to cross biological membranes then one is not required to conduct studies for bioaccumulation. This is the case for the dimerised fatty acids, and in this particular case Fatty acids C16-C18 and C18 unsaturated branched and linear (CAS No. 68955-98-6) as the potential for bioaccumulation is expected to be low. First of all, Fatty acids C16-C18 and C18 unsaturated branched and linear has a poor water solubility. The water solubility of Fatty Acids, C16-18 and C-18, Unsaturated, branched and linear, in its entirety, as a complex mixture (the sum solubility of all the components of the test material) is estimated to be 15.0 mg/L at 20°C. The water solubility, if based on its 2 major constituents, may be considered only to be 0.6 mg/L at 20°C. Considering this, one can expect to have a very low water concentration and consequently, exposure to the aquatic environment would be extremely low.

Due to the potential of these substances to absorb (log Koc > 4 < 5, log Kow > 4), one may assume that the uptake will occur through the ingestion of soil or sediment. From the toxicokinetic behaviour of monomeric acids in mammals it can be assumed that unsaturated monomeric C16-C18 fatty acids are more readily absorbed than saturated fatty acids like octadecanoic and isooctadecanoic acid but less than fatty acids with shorter chain length.

Fatty acids occur naturally in all aquatic organisms and are ubiquitous in the aquatic environment, where fatty acids are predominantly readily biodegraded in an aerobic environment by microorganisms. As nutritional energy source, fatty acids are absorbed by different uptake mechanisms in mammals depending on the chain length. Long chain fatty acids (>C12) are absorbed into the walls of the intestine villi and assembled into triglycerides, which then are transported in the blood stream via lipoprotein particles (chylomicrons). In the body, fatty acids are metabolised by various routes to provide energy. Besides this, fatty acids are stored as lipids in adipose tissue and as precursors for signaling molecules. In addition fatty acids are an integral part of the cell membranes of every living organism from bacteria and algae to higher plants. Fatty acids are known to be easily metabolised. The rate of metabolism of fatty acids was considered to vary in proportion to their water solubility (Lloyd, 1957). Also CIR (1987) and Iwaoka and Perkins (1976) stated that in case of absorption fatty acids will undergo rapid metabolism and excretion (either in the expired CO2 or as hydroxylated or conjugated metabolite in the urine in the case of cyclic fatty acids) as they feed into physiological pathways like the citric acid cycle, sugar synthesis, and lipid synthesis. As fatty acids are naturally stored in the form of triacylglycerols primarily within fat tissue until they are used for energy production (fat storage tactic), it is therefore concluded that there will be no risk to organisms from bioconcentration/biomagnification of fatty acids within the food chain.

Hence, Fatty acids C16-C18 and C18 unsaturated branched and linear, does not pose a risk to organisms in regard to bioaccumulation/biomagnification. Although the logKow values given suggest that dimerised fatty acids may be expected to have tendency of a higher bioaccumulation, this only indicates intrinsic potential of the substance, but not truely reflect its behaviour in the environment. For instance, one must consider the ready biodegradation of the substance, and also its degradation within living organism, (e.g. metabolism). Furthermore, one should keep in mind that the fat storage strategy evolved as adaption in an environment where food supply was uncertain.

In conclusion, dimerised fatty acids are considered do not pose a risk to aquatic organisms from their bioconcentration and biomagnification properties. The bioconcentration factors of dimerised fatty acids are generally below the level of concern. Hence, no classification to chronic hazardous to environment needs to be assigned.

References:

CIR (2001). Final report on the safety assessment of trilaurin, triarachidin, tribehenin, tricaprin, tricaprylin, trierucin, triheptanoin, triheptylundecanoin, triisononanoin, triisopalmitin, triisostearin, trilinolein, trimyristin, trioctanoin, triolein, tripalmitin, tripalmitolein, triricinolein, tristearin, triundecanoin, glyceryl triacetyl hydroxystearate, glyceryl triacetyl ricinoleate, and glyceryl stearate diacetate. Cosmetic Ingredient Review. International Journal of Toxicology. 20 (Suppl. 4): 61-94.

Iwaoka W. T., Perkins E. G. (1976). Nutritional effects of the cyclic monomers of methyl linolenate in the rat. Lipids 11:349-353

Lloyd, L. E. and Crampton, E. W. (1957). The relation between certain characteristics of fats and oils and their apparent digestibility by young pigs, young guinea-pigs and pups. J. Anita.Sci. 16:377.