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Environmental fate & pathways

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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. All substances in the category have an overlap in regard to their composition. With reference to information of existing categories, the category of Dimerised Fatty Acids and its derivatives is based on similar physicochemical and toxicological properties and 2 sub-categories are further defined on the basis of their environmental fate and 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 other environmental fate properties.

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 through metabolic processes. In view of the results of various QSAR analyses, the toxic hazard of these substances mainly depends on the number of carbons, on the chain “structure”, such as branching, unsaturation, grade of cyclics and aggregation, as well as on their position in the whole molecular structure. In contrast, the number of the functional group “carboxylic acids” has no significant influence on the tox- and ecotoxicological profiles.

Sub category 2: predominantly Oligomers (dimers, trimers)

As aforementioned the similarity of the sub-category 2 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 the basis of overlap in composition, representative molecular structures, physico-chemical properties, tox-, ecotoxicological profiles and is 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. The key characters 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 by metabolic processes)
  • 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 >5, log Kow is judged to be > 4, insolubility in water)
  • Common properties for environmental fate & eco-toxicologcial profile of the two sub-categories (not 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 2:

Due to the chemical structure of Fatty acids, C18-unsaturated, dimers, hydrogenated (CAS No. 68783-41-5) and the very low water solubility (< 0.52 mg/L, limit of detection) hydrolysis does not contribute to abiotic degradation in the aquatic environment. Photodegradation of Fatty acids, C18-unsaturated, dimers, hydrogenated (CAS No. 68783-41-5) in air is not a relevant degradation pathway as the vapour pressure under ambient conditions is negligible. The biodegradation screening test with Fatty acids, C18-unsaturated, dimers, hydrogenated (CAS No. 68783-41-5) revealed the potential for persistence in the environment with biodegradation rates below 10 % within 28 days (ready biodegradability test).

The adsorption potential is predicted by QSAR modeling (EPISUITE KOCWIN v2.00). The main factor determining the log Koc value of fatty acids is the carbon chain length. For "predominantly monomers", the log Koc is in the range 4 < log Koc < 5. For "predominantly oligomers", the log Koc is > 5. In both cases, a high adsorption to organic carbon in soils and sediments is expected.

The test substance has estimated log Kow values outside the range 0 to 6. Additionally, chromatograms contain peaks at retention times corresponding to partition coefficients outside this range. In the mobile phase adjusted to pH 2, only hydrophilic components (tR< 5 min; ca. 22 % of total area) have been eluted and detected under the chromatographic conditions employed. Corresponding to the judgment of Fatty Acids, C-18 unsaturated, dimers (CAS No. 68783-41-5) with an estimated log Kow value of >3.0 and taking into consideration all the limitations of the test design, the log Kow of Fatty Acids, C-18 unsaturated, dimers, hydrogenated (CAS No. 68783-41-5) should also be judged to be > 4.

If a substance has a low potential for bioaccumulation and/or a low potential to cross biological membranes then studies on bioaccumulation are not required. This is the case for the dimerised fatty acids, and in particular for Fatty acids, C18-unsaturated, dimers, hydrogenated (CAS No. 68783-41-5).

Due to the potential of these substances to adsorb (log Koc > 5, log Kow >4) one may assume that the uptake may occur through the ingestion of soil or sediment. However, uptake of dimers and trimers is expected to be low based on the fact that the Dimerised Fatty Acids are relatively large molecules (C36 as dimers and C54 as trimers) with high molecular weights (dimeric acids 561 g/mol, trimeric acids 838 g/mol). Thus, according to Lipinski’s rule of five, they have a low potential to cross biological membranes (Lipinski et al., 2001).

 

From the toxicokinetic behaviour of mono- and oligomeric fatty 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 readily than fatty acids with shorter chain length. Very low absorption is reported for dimeric and trimeric fatty acids via the gastro intestinal tract and thus, most of the ingested fatty acids will be excreted in the faeces (≥ 80 % for dimeric acid methyl esters (Hsieh and Perkins, 1976; Paschke et al. 1964)). In case of absorption fatty acids will undergo rapid metabolisation 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 concluded that there will be no risk to organisms from bioconcentration/biomagnification of fatty acids.

Finally, as the Dimerised Fatty Acids have a low water solubility (sub-category 2 dimers and trimers: all < 0.52 mg/L) the concentration in water is expected to be low and consequently also the exposure to the aquatic environment, if occurring at all.

Hence, Fatty acids, C18-unsaturated, dimers, hydrogenated (CAS No. 68783-41-5) does not pose a risk to organisms in regard to bioaccumulation/biomagnification.

References:

Hsieh, A. and Perkins, E.G. (1976). Nutrition and Metabolic Studies of Methyl Ester of Dimer Fatty Acids in the Rat. Lipids, 11(10):763-768.

Lipinski et al. (2001) Experimental and computational approaches to estimate solubility and permeability in drug discovery and development settings, Adv. Drug Del. Rev., 2001, 46, 3-26.

Paschke, R.F. et al. (1964). Dimer acid structures. The dehydro-dimer from methyl oleate and Di-t-butyl peroxide. Journal of the American Oil Chemists' Society 41(1):56-60.

U.S.Environmental Protection Agency (2009). Risk-Based Prioritization Document. Initial Risk-Based Prioritization of High Production Volume (HPV) Chemicals – Fatty Acid Dimers and Trimer Category. pp 1-19. Report date: April 2009.