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Partition coefficient

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Reference
Endpoint:
partition coefficient
Type of information:
(Q)SAR
Adequacy of study:
key study
Study period:
03 April 2018
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model, but not (completely) falling into its applicability domain, with adequate and reliable documentation / justification
Justification for type of information:
1. SOFTWARE
EPI Suite Version 4.11

2. MODEL (incl. version number)
KOWWIN v1.68

3. SMILES OR OTHER IDENTIFIERS USED AS INPUT FOR THE MODEL
CAS: 73246-85-2: CCCCCCCCCCCCOC(=O)CS[Sn](CCCCCCCC)(CCCCCCCC)SCC(=O)OCCCCCCCCCCCC
CAS: 79330-84-0: O=C(CS[Sn](SCC(=O)OCCCCCCCCCCCCCC)(CCCCCCCC)CCCCCCCC)OCCCCCCCCCCCCCC

4. SCIENTIFIC VALIDITY OF THE (Q)SAR MODEL
- KOWWIN uses a "fragment constant" methodology to predict log P.  In a "fragment constant" method, a structure is divided into fragments (atom or larger functional groups) and coefficient values of each fragment or group are summed together to yield the log P estimate.   KOWWIN’s methodology is known as an Atom/Fragment Contribution (AFC) method.  Coefficients for individual fragments and groups were derived by multiple regression of 2447 reliably measured log P values.  KOWWIN’s "reductionist" fragment constant methodology (i.e. derivation via multiple regression) differs from the "constructionist" fragment constant methodology of Hansch and Leo (1979) that is available in the CLOGP Program (Daylight, 1995).  See the Meylan and Howard (1995) journal article for a more complete description of KOWWIN’s methodology.
- To estimate log P, KOWWIN initially separates a molecule into distinct atom/fragments.  In general, each non-hydrogen atom (e.g. carbon, nitrogen, oxygen, sulphur, etc.) in a structure is a "core" for a fragment; the exact fragment is determined by what is connected to the atom.  Several functional groups are treated as core "atoms"; these include carbonyl (C=O), thiocarbonyl (C=S), nitro (-NO2), nitrate (ONO2), cyano (-C/N), and isothiocyanate (-N=C=S).  Connections to each core "atom" are either general or specific; specific connections take precedence over general connections.  For example, aromatic carbon, aromatic oxygen and aromatic sulphur atoms have nothing but general connections; i.e., the fragment is the same no matter what is connected to the atom.  In contrast, there are 5 aromatic nitrogen fragments: (a) in a five-member ring, (b) in a six-member ring, (c) if the nitrogen is an oxide-type {i.e. pyridine oxide}, (d) if the nitrogen has a fused ring location {i.e. indolizine}, and (e) if the nitrogen has a +5 valence {i.e. N-methyl pyridinium iodide}; since the oxide-type is most specific, it takes precedence over the other four.  The aliphatic carbon atom is another example; it does not matter what is connected to -CH3, -CH2-, or -CH< , the  fragment is the same; however, an aliphatic carbon with no hydrogens has two possible fragments: (a) if there are four single bonds with 3 or more carbon connections and (b) any other not meeting the first criteria.
- It became apparent, for various types of structures, that log P estimates made from atom/fragment values alone could or needed to be improved by inclusion of  substructures larger or more complex than "atoms"; hence, correction factors were added to the AFC method.  The term "correction factor" is appropriate because their values are derived from the differences between the log P estimates from atoms alone and the measured log P values.  The correction factors have two main groupings: first, factors involving aromatic ring substituent positions and second,  miscellaneous factors.  In general, the correction factors are values for various steric interactions, hydrogen-bondings, and effects from polar functional substructures.  Individual correction factors were selected through a tedious process of correlating the differences (between log P estimates from atom/fragments alone and measured log P values) with common substructures.
- Two separate regression analyses were performed.  The first regression related log P to atom/fragments of compounds that do not require correction factors (i.e., compounds estimated adequately by fragments alone).  The general regression equation has the following form:  log P  = Σ(fini ) +  b     (Equation 1)
where Σ(fini )  is the summation of fi (the coefficient for each atom/fragment) times ni (the number of times the atom/fragment occurs in the structure) and b  is the linear equation constant.  This initial regression used 1120 compounds of the 2447 compounds in the total training dataset.
- The correction factors were then derived from a multiple linear regression that correlated differences between the experimental (expl) log P and the log P estimated by Equation 1 above with the correction factor descriptors.  This regression did not utilise an additional equation constant.  The equation for the second regression is: lop P (expl)  -  log P (eq 1)  = Σ(cjnj )       (Equation 1)
where Σ(cjnj )  is the summation of cj (the coefficient for each correction factor) times nj  (the number of times the correction factor occurs (or is applied) in the molecule).
- Regression Results
Results of the two successive multiple regressions (first for atom/fragments and second for correction factors) yield the following general equation for estimating log P of any organic compound: log P  = Σ(fini ) + Σ(cjnj ) + 0.229     (Equation 3)
(num = 2447,   r² = 0.982,  std dev = 0.217,  mean error = 0.159)

5. APPLICABILITY DOMAIN
- To be effective an estimation method must be capable of making accurate predictions for chemicals not included in the training set. Currently, KOWWIN has been tested on an external validation dataset of 10,946 compounds (compounds not included in the training set). The validation set includes a diverse selection of chemical structures that rigorously test the predictive accuracy of any model. It contains many chemicals that are similar in structure to chemicals in the training set, but also many chemicals that are different from and structurally more complex than chemicals in the training set. The average molecular weight of compounds in the validation set is 258.98 versus 199.98 for the training set. Minimum MW: 18.02, Maximum MW: 719.92
- Currently there is no universally accepted definition of model domain.  However, users may wish to consider the possibility that log P estimates are less accurate for compounds outside the MW range of the training set compounds, and/or that have more instances of a given fragment than the maximum for all training set compounds.  It is also possible that a compound may have a functional group(s) or other structural features not represented in the training set, and for which no fragment coefficient was developed.  These points should be taken into consideration when interpreting model results.

6. ADEQUACY OF THE RESULT
A value was estimated for this substance using KOWWIN v1.68, given that the substance is an organic molecule the prediction is considered to be acceptable, however the molecular weight of the substance components are slightly greater than the molecular weights in the training set and as a result the partition coefficient estimate is less accurate.
Qualifier:
according to guideline
Guideline:
other: REACH Guidance on QSARs R.6
Version / remarks:
May/July 2008
Deviations:
no
GLP compliance:
no
Type of method:
other: calculation
Specific details on test material used for the study:
- Molecular weight
CAS: 73246-85-2: 864.02
CAS: 79330-84-0: 920.13
Key result
Type:
log Pow
Partition coefficient:
21.394
Remarks on result:
other: CAS 79330-84-0
Key result
Type:
log Pow
Partition coefficient:
19.43
Remarks on result:
other: CAS 73246-85-2

CAS 79330-84-0

Type

Num

LogKow Fragment Description

Coefficient

Value

Frag

4

-CH3 [aliphatic carbon]

0.5473

2.1892

Frag

42

-CH2- [aliphatic carbon]

0.4911

20.6262

Frag

2

-C(=O)O [ester, aliphatic attach]

-0.9505

-1.9010

Frag

2

-S- [aliphatic attach]

-0.4045

-0.8090

Frag

1

Tin [Sn]

1.0600

1.0600

Const

 

Equation Constant

 

0.2290

CAS 73246-85-2

Type

Num

LogKow Fragment Description

Coefficient

Value

Frag

4

-CH3 [aliphatic carbon]

0.5473

2.1892

Frag

38

-CH2- [aliphatic carbon]

0.4911

18.6618

Frag

2

-C(=O)O [ester, aliphatic attach]

-0.9505

-1.9010

Frag

2

-S- [aliphatic attach]

-0.4045

-0.8090

Frag

1

Tin [Sn]

1.0600

1.0600

Const

 

Equation Constant

 

0.2290

Conclusions:
The Log Kow of the test material components were calculated to be 21.3944 and 19.4300.
Executive summary:

The partition coefficient of the test material was calculated using KOWWIN v1.68. Given that the substance is an organic molecule the prediction is considered to be acceptable, however the molecular weight of the substance components are slightly greater than the molecular weights in the training set and as a result the partition coefficient estimate is less accurate.

The Log Kow of the test material components were calculated to be 21.3944 and 19.4300.

Description of key information

The Log Kow of the test material components were calculated to be 21.3944 and 19.4300.

Key value for chemical safety assessment

Additional information

The partition coefficient of the test material was calculated using KOWWIN v1.68. Given that the substance is an organic molecule the prediction is considered to be acceptable, however the molecular weight of the substance components are slightly greater than the molecular weights in the training set and as a result the partition coefficient estimate is less accurate.

The Log Kow of the test material components were calculated to be 21.3944 and 19.4300.

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