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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 ready 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 (4 < log Koc < 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 to Regulation (EC) No 1907 /2006 Annex VIII 9.2.2.1 column 2 the testing for hydrolysis is not required for substances that are readiyl biodegradable. In addition, due to the chemical structure and the low water solubility (< 0.24 mg/L, limit of detection) of Isooctadecanoic acid, hydrolysis does not contribute to abiotic degradation in the aquatic environment. Photodegradation of Isooctadecanoic acid in air is not a relevant degradation pathway as the vapour pressure under ambient conditions is negligible. The estimated Henry’s Law Constant (25°C) (EPISUITE, HENRYWIN) between 2.93 and 42.1 Pa m³/mol suggests a slow volatilisation of isooctadecanoic acid from the water phase, which is due to the low water solubility of < 0.24 mg/L therefore this is not a relevant distribution pathway.

In the biodegradation screening tests Isooctadecanoic acid was determined as ready biodegradable.

Adsorption potency is predicted by QSAR method. Using EPISUITE, KOCWIN v2.00 an estimated adsorption coefficient of log Koc ≥ 4, was calculated for Isooctadecanoic acid (CAS No. 30399-84-9). The main factor determining the log Koc value of fatty acids is the carbon 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 (in worst cases to the value of ca. 3.5). The substance is defined as UVCB and it is likely that other types of C18 carbon chain may occur in the overall composition. However, the main difference between monomers related to branching is that the methyl group can be attached to the main chain at different position. That has no influence on calculated values 4.0 and 4.5 employing MCI and Kow based models, respectively. With additional assumption that contents of oligomers are negligible, it can be concluded that for Isooctadecanoic acid (CAS 30399 -84 -9) adsorption to organic carbon in soils and sediments is expected. Log Koc is high: 4 < log Koc < 5. No additional tests on adsorption/desorption are to be conducted. The result is accepted for the chemical safety assessment. By higher content of residual dimeric components (> ca. 5 %), the worst case log Koc > 5 can be considered.

 

An estimation of partition coefficient was made based on calculations with KOWWIN v1.68, Estimation Programs Interface Suite™ for Microsoft® Windows v4.10. US EPA, United States Environmental Protection Agency, Washington, DC. USA. Representative homologues were taken into account. The calculations confirm that the log Kow is >4.

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 Isooctadecanoic acid (CAS No. 30399-84-9) as the potential for bioaccumulation is expected to be low. Isooctadecanoic acid has low water solubility therefore one can only expect to have a very low concentration of the test substance in water and consequently, exposure to the aquatic environment would be extremely low, if at all.

Due to the potential of the members of sub-category 1 to absorb, 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, Isooctadecanoic acid, does not pose a risk to organisms in regard to bioaccumulation/biomagnification. Although the logKow values given suggests that the dimerised fatty acids may be expected to have tendency of a higher bioaccumulation, this only indicates intrinsic potential of the substance, but not truly reflect its behaviour in the environment. For instance, one must consider the 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 addition, as fatty acids are the end products of carbohydrate metabolism in living organisms muscle tissues, an evaluation of anthropogenic contribution of fatty acids based on the natural concentrations determined in the organs and tissues of aquatic organisms may be over estimated or in fact not revelant.

 

In conclusion, dimerised fatty acids are considered to pose no risk to aquatic organisms from their bioconcentration and biomagnification properties.

References:

CIR (1987) Final report on the safety assessment of oleic acid, lauric acid, palmitic acid, myristic acid, stearic acid.J. of the Am. Coll. of Toxicol.6(3):321-401.

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