Octadecanoic acid, branched and linear

Endpoint summary

Additional information

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 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. However as Octadecanoic acid, branched and linear (CAS No. 68201-37-6) does not have a functional group that is susceptible to hydrolysis, it is expected not to hydrolyse under environmental conditions. Photodegradation of Octadecanoic acid, branched and linear in air is not a relevant degradation pathway as the vapour pressure under ambient conditions is neglible. In accordance to Regulation (EC) No. 1907/2006 Annex XI, 1.5 1.2. Weight of evidence, there is sufficient data from several independent sources of information of monomeric fatty acids C16-18 saturated and unsaturated leading to the assumption/conclusion that Octadecanoic acid, branched and linear is readily biodegradable. As summarized in the category justification, the members of the fatty acids will predominantly rapidly biodegrade. Individual degradation rates reported may vary to some degree depending on their physico chemical properties (e.g. water solubility), bioavailability and test methods employed. Consistent positive test, i.e. readily biodegradable, results from both Fatty acids, C16-18 and C18-unsaturated, branched and linear and Isooctadecanoic acid in addition to two further 2 analogue substances: Octadecan-9-oic acid and strearic acid supersede negative results as reported in the HPV documentation (EPA, 2009). In conclusion, aliphatic fatty acids comprising C6-C22 carbon chain length are shown to be predominantly rapidly biodegradable providing a sufficient bioavailability is given. Further taking into account that Isooctadecanoic acid is also highly branched and has a lower water solubility than the Octandecanoic acid, branched and linear one can conclude that Octandecanoic acid , branched and linear is also readily biodegradable.

Adsorption potency is predicted by QSAR method. Using QSARs PCKOCWIN an estimated adsorption coefficient of log Koc > 4 < 5, was calculated for Isooctadecanoic acid, branched and linear (CAS No. 30399-84-9), due to its relatively well defined structure. 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). Therefore the general estimated result for the category is that the log Koc is > 4 < 5.

The value for partition coefficient log Kow of UVCB substance is based on a weight of evidence approach using experimental and QSAR results for the components of the substance. For the evaluation of the influence of the dimerisation and branching, hypothetical structures were examined with use of QSAR model (KOWWIN). The log Kow was also experimentally determined (HPLC method, OECD 117) with Octadecanoic acid, branched and linear (CAS 68201-37-6). In unbuffered media, a partition coefficient range of 4.0 to 5.9 was determined. The pH of this sample was measured as ca 5-6. In media adjusted to pH 2, a partition coefficient range of 5.6 to >6 (6.1) was determined. The results from this testing need to be viewed with caution as the method only covers materials with partition coefficient values in the range of 0 to 6. Some components within the mixtures had estimated values out of this range. All available results show that the logKow of Octadecanoic acid, branched and linear (CAS 68201-37-6) is high and well above 4. Therefore a log Kow >4 cut-off value 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 Octadecanoic acid branched and linear (CAS No. 68201-37-6) the potential of bioaccumulation is expected to be low. The water solubility of Octadecanoic acid, branched and linear, in its entirety as a complex mixture is 2.5 mg/L at 20°C. The water solubility of Octadecanoic acid, branched and linear based on its major constituents is 1.1 mg/L at 20°C. Considering this, one can only expect to have a very low water concentration and consequently, exposure to the aquatic environment would be extremely low. This would automatically reduce, if not eliminate, the uptake of the chemical through the aquatic environment.

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 signalling 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 (2001) 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, Octadecanoic acid, branched and linear, 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 the intrinsic potential of the substance, but does 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 and therefore not relevant.

In conclusion, 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