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Physical & Chemical properties

Partition coefficient

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Endpoint:
partition coefficient
Type of information:
experimental study
Adequacy of study:
key study
Study period:
30 January 2014 to 27 February 2014
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
guideline study without detailed documentation
Qualifier:
according to guideline
Guideline:
EU Method A.8 (Partition Coefficient)
Qualifier:
according to guideline
Guideline:
OECD Guideline 107 (Partition Coefficient (n-octanol / water), Shake Flask Method)
GLP compliance:
yes (incl. QA statement)
Type of method:
other: calculation method
Partition coefficient type:
octanol-water
Specific details on test material used for the study:
- Name of test material (as cited in study report): L(+)-lactic acid
- Physical state: clear colourless liquid
- Analytical purity: 90%
- Lot/batch No.: 1208002033
- Expiration date of the lot/batch: 2015-01-01
- Stability under storage conditions: 2015-01-01
- Storage condition of test material: at room temperature in the dark
Analytical method:
high-performance liquid chromatography
Key result
Type:
log Pow
Partition coefficient:
ca. -0.54
Temp.:
25 °C
pH:
7

It was found that n-octanol influences the enzyme reaction and that the tested UV-Vis spectrophotometric method involving enzymatic reactions is not a suitable method to determine the test substance concentration in samples taken from the partition coefficient (shake flask method) at low concentration levels.

Since an analytical method for the determination of L(+)-lactic acid concentrations in samples from the shake flask method and/or estimation method could not be developed, these methods are not applicable for the determination of log Pow of the test substance.

The HPLC method is also not applicable as it covers log Pow values in the range of 0 to 6, and because the calculation method indicated a log Pow value outside this range. Based on this, it was decided to report the result of the calculation method.

Conclusions:
A study was performed in accordance with EU method A.8 and OECD guideline 107 and in compliance with GLP. The Pow of the test substance was calculated to be 2.9E-01 (log Pow -0.54) using the Rekker calculation method.
Executive summary:

Due to the poor UV-absorption ability of the test item an enzymatic reaction in combination with UV-detection was performed. However, no suitable method for the detection of lactic acid in n-octanol and/or water in the required concentration range could be developed (technically not feasible). Therefore, the partition coefficient of the test substance was calculated using the Rekker calculation method by the software PrologP 6.0, module in Pallas 3.0, CompuDrug International, San Francisco, CA, USA.

The calculated log Pow is –0.54.

Endpoint:
partition coefficient
Type of information:
(Q)SAR
Adequacy of study:
supporting study
Study period:
2007
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
accepted calculation method
Justification for type of information:
QSAR prediction
Qualifier:
no guideline required
Principles of method if other than guideline:
QSAR calculation
GLP compliance:
no
Type of method:
other: calculation method
Partition coefficient type:
octanol-water
Specific details on test material used for the study:
No details on test material required, since endpoint addressed via in silico methods. The calculation refers to the pure substance.
Type:
log Pow
Partition coefficient:
ca. -0.65
Temp.:
20 °C
Remarks on result:
other: QSAR calculation
Conclusions:
The log octanol-water partition coefficient of lactic acid, calculated by a QSAR method (KOWWin, EpiSuite), is -0.65.
Executive summary:

The log octanol-water partition coefficient of lactic acid, calculated by a QSAR method (KOWWin, EpiSuite), is –0.65.

In support of this calculation, publicly available values for lactic acid are quoted in the expert statement:

European HPV programme: log Pow = –0.62

IPCS International program on Chemical Safety: log Pow = –0.6

Endpoint:
partition coefficient
Type of information:
other: Literature review
Adequacy of study:
supporting study
Reliability:
4 (not assignable)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Remarks:
Extrapolation from literature data; documentation of the methodology in the original references is scarce, complicating proper assessment of the reliability. Therefore, a Klimisch score of 4 is assigned. Nevertheless, in view of the overall consistency of the reported results the data can be used in a weight-of-evidence approach.
Qualifier:
no guideline required
Principles of method if other than guideline:
Extrapolation from literature data.
GLP compliance:
no
Type of method:
other: Extrapolation from literature data
Partition coefficient type:
octanol-water
Specific details on test material used for the study:
Lactic acid (CAS 50-21-5), i.e. the racemic mixture, was considered in this literature review. L-(+)-lactic acid is one of two possible enantiomers. Enantiomers are totally equivalent regarding physicochemical properties hence "read-across" from the racemic mixture (CAS 50-21-5) to the target molecule L-(+)-lactic acid is not restricted.
Type:
log Pow
Partition coefficient:
>= -0.72 - <= -0.62
Temp.:
20 °C
Remarks on result:
other: Range of values from the publications reviewed
Conclusions:
The log Kow of lactic acid as cited in the reviewed publications ranges between -0.72 and -0.62.
Executive summary:

An overview is given of literature data (measured values) for the log Kow of lactic acid. The reported values consistently range between -0.72 and -0.62. However, documentation of the methodology in the original references is scarce, complicating assessment of the reliability of the individual sources.

Endpoint:
partition coefficient
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
supporting study
Rationale for reliability incl. deficiencies:
other:
Justification for type of information:
This information is used in a read-across approach in the assessment of the target substance. For details and justification of read-across please refer to the read-across report attached to IUCLID section 13.
Reason / purpose for cross-reference:
read-across source
Type:
log Pow
Partition coefficient:
ca. -0.72
Remarks on result:
other: Experimental value by Leonard et al. (1948), included in the KOWWin reference data base, training data set.
Type:
log Pow
Partition coefficient:
ca. -0.65
Remarks on result:
other: KOWWin estimate - not reliable due to autocorrelation
Details on results:
Leonard et al. (1948) quote the partition coefficient (water/n-octanol) as 5.2, which is the reverse of the conventional n-octanol/water partition coefficient. Accordingly, the log Pow can calculated using the formula: log10(1/5.2) = -0.72.
Conclusions:
The log Kow of lactic acid was found to be -0.72.
Executive summary:

Lactic acid is included in the KOWWin reference substance data base, training data set. The quoted figure of –0.72 is claimed to be an experimental value, originating from the publication by Leonard et al. (1948). Although the QSAR estimate generated by KOWWin of –0.65 matches the former values very well it cannot be used for reasons of autocorrelation. Therefore, the experimental value of –0.72 is favoured.

This information is used in a read-across approach in the assessment of the target substance. For details and justification of read-across please refer to the read-across report attached to IUCLID section 13.

Description of key information

All available evidence indicates that the octanol-water partition coefficient of lactic acid is significantly below 0. In a worst-case approach, and supported by weight of evidence from a range of published studies, the highest value of log Pow = -0.54 (QSAR estimate) is selected.

Key value for chemical safety assessment

Log Kow (Log Pow):
-0.54
at the temperature of:
20 °C

Additional information

In a weight-of-evidence approach the partition coefficient of L-(+)-lactic acid is assessed to range between –0.72 and –0.54. Typically the racemic mixture (lactic acid) is referred to, but since enantiomers are optical antipodes they do not differ in their physicochemical properties. Therefore, data on the lactic acid racemate can be used for L-(+)-lactic acid without restriction.

Early publications claim to have measured the concentrations of lactic acid in n-octanol and water, either by titration against phenol red (Leonard et al., 1948), or, in most other cases, by an unspecified method. This limits the individual reliability of the reported values. However, all reported partition coefficients are consistent, and are supported by a recent QSAR calculation using the software PrologP 6.0 (Selditz, 2014; logPow= -0.54). Note that the QSAR estimate generated by KOWWin (EpiSuite) cannot be used since the experimental value by Leonard et al. (1948) is included in the training data set (autocorrelation).

In conclusion, adopting a worst-case approach (highest predicted distribution into biota, if any) the highest reported partition coefficient of log Pow= -0.54 is selected as the key value for chemical safety assessment. The weight of evidence from all other identified literature sources indicates that this value is fully appropriate for risk assessment.