Chapter 21
Effect of Tautomeric Equilibria on Hydrophobicity as Measured by Partition Coefficients Albert J. Leo
Downloaded by TUFTS UNIV on June 5, 2018 | https://pubs.acs.org Publication Date: March 31, 1995 | doi: 10.1021/bk-1995-0589.ch021
Department of Chemistry, Pomona College, Claremont, CA 91711
The tautomeric form of a solute which is preferred by any given nonpolar solvent depends largely on it's hydrogen bond donor/acceptor strength and it's polarity—the same properties which determine its partition coefficient, P, between water and that solvent (1). Octanol/water and chloroform/water log Ρ values can give useful predictions of tautomeric ratios, and help predict transport rates and bioaccumulation of pesticides and drugs that fall into this class. Alkane/water and ester/water log Ρ values complete the "critical quartet" (2) and can describe more completely solute-solvent behavior. It is to be expected that the tautomeric forms of a given drug or pesticide molecule will react differently at the receptor site and thus they may evoke different biological responses. Some effort has been made to determine the effect of the solvent upon the tautomeric equilibrium constant of some important bioactive molecules, such as the purine bases (3). See Figure 1A. It is not surprising that a non-polar solvent can drive the equilibrium away from the form 'normally' expected in an aqueous medium. It can thus greatly influence the tendency of the solute to self-associate. Both tautomers of the 6-dimethylamino analog can dimerize, but a higher degree of association is not favored. As is noted for almost all enol/keto tautomers, the enol is favored by non-polar solvents and the keto by polar ones. The parent isoguanine can associate beyond the dimer stage. It forms helical gels in aqueous solution, and it surely is in the keto form as it does this (Figure IB). The capability of existing in two tautomeric forms can affect the rate of the 'random walk' process which is needed to reach the active site. Depicted in a very simplified fashion in Figure 2, a tautomeric solute might traverse the serum or cytosol largely as the keto form, but be more effective in crossing a non-polar membrane as the enol. U V and IR absorption spectra were employed to obtain the results shown in Figure 1, and, of course, these have been the preferred techniques (along with NMR) to determine tautomeric ratios. However, interpretation of these spectra are not always as straightforward as one might hope for, as will become apparent later in this paper, and so results from an entirely different methodology can be a useful addition to the picture.
0097-6156/95/0589-0292$12.00A) © 1995 American Chemical Society Reynolds et al.; Computer-Aided Molecular Design ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
LEO
Effect of Tautomeric Equilibria on Hydrophobicity
)
NMe
/
N
j
2
NMe
Lk
N
^ O H
A l k
2
/
A | k
H N
Downloaded by TUFTS UNIV on June 5, 2018 | https://pubs.acs.org Publication Date: March 31, 1995 | doi: 10.1021/bk-1995-0589.ch021
NMe?
Enol favored by Non-Polar Solvents 90% in Ethyl Acetate
v
^ k
|
f
I
K e t o P o l a r
favored by Solvents
90% in Water 4
Dimerization Negligible at 1 0 M . Other Solvents Studied: ethyl acetate acetonitrile formamide
chloroform ethanol
DMSO methanol
Figure 1. Tautomerism and Association in Biologically Important Solutes
Reynolds et al.; Computer-Aided Molecular Design ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
294
COMPUTER-AIDED M O L E C U L A R DESIGN
Tautomers Stabilized by Internal Hydrogen Bonds: β-Dicarbonyl Solutes There are well-established methods of assigning fragmentai values to 'isolated' carbonyl, hydroxyl, amino and imino groups so that their sum contributes to an estimate of the octanol/water log Ρ (log P ) of the solute (4,5). When these groups are present in a tautomeric solute, the observed log P ct will most generally lie somewhere in between the values calculated for the two structural forms, with the enol form calculating higher due to its lesser hydrogen-bond-acceptor (HBA) strength, b. See Figure 3. In the case of acetylacetone, where spectral evidence indicates that about 83% of the solute is in the keto form in water (6), the calculated (5) log Ps bracket the observed value but seem to indicate that the wet octanol may favor the enol form. Even at relatively high concentrations the octanol/water partition coefficient is constant over a ten-fold concentration range, indicating that intramolecular hydrogen bonding must prevail over self-association via /ntermolecular bonds. A reliable fragmentai procedure is not available for the calculation of chloroform/water log Ρ values. Spectral evidence indicates that the tautomeric ratio for acetylacetone in chloroform is just about the reverse of what it is in water; i.e., 86% is in the enol form. In the case of ethylacetoacetate, spectral evidence (6) points to a very small enol content in water. It must not be much higher in wet octanol, because the CLOGP value (5) for the keto form is even a bit higher than the measured log P t and the CLOGP value for the enol form is much higher still. Mills and Beak (6) use N M R data to calculate an 8% enol content in chloroform. This seems a bit strange, since the chloroform/water partitioning data does show a concentration dependence, indicating that some self-association must be taking place. This could hardly occur without the enol form being present at significant levels. The measured log P ct for diethylmalonate is also very close to the calculated value for the di-keto form and far from the enol form. This indicates that in wet octanol diethylmalonate is mostly in the keto form that water also favors. There is no concentration dependence in the partition coefficients in either octanol/water or chloroform/water indicating an absence of association. A later section will deal with the calculation of effective hydrogen bond donor strength (HBD) from octanol and chloroform log Ρ values. At this point one can note that for all three compounds in Figure 3 the effective HBD calculates as zero or slightly negative. However, this is not a true indicator of possible enol level, because intramolecular Η-bonding would effectively negate any tendency to donate to the solvent. oct
Downloaded by TUFTS UNIV on June 5, 2018 | https://pubs.acs.org Publication Date: March 31, 1995 | doi: 10.1021/bk-1995-0589.ch021
0
oc
0
Tautomerism Where No Internal Η-Bond is Possible (Figure 4) 4-Hydroxypyridine is known to be predominantly in the pyridone form in the vapor state and in most polar solvents. If any solvent promotes association, it must be of the head-to-tail variety. The octanol/water log Ρ of the keto form is calculated satisfactorily, while the enol form would be expected to be over two log units more lipophilic. This is strong evidence for the dominance of pyridone in wet octanol as well as water. No data is available to evaluate the enol/keto ratio in chloroform. Roughly the same results are seen for the 4-quinolone where the octanol/water partitioning data indicate that wet octanol supports as little as 10% as the enol form. However, based on estimation of the effective H B D strength (ea, as explained below), it would seem that the enol form dominates in chloroform. This is not unexpected, since it has been proposed that benzene ring annelation should shift the equilibrium toward the enol form (7). Wheland (8) reports that the 9-anthrone is the more stable of the two tautomers, even though each can be separately isolated. It is difficult to measure the ratio in
Reynolds et al.; Computer-Aided Molecular Design ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
Downloaded by TUFTS UNIV on June 5, 2018 | https://pubs.acs.org Publication Date: March 31, 1995 | doi: 10.1021/bk-1995-0589.ch021
21.
LEO
Effect of Tautomeric Equilibria on Hydrophobicity
A . Acetylacetone
YY
Log Poet Meas. CLOGP In: Water Chloroform Octanol
=
YV HO
+0.40 +0.66 Association 17%* No Dimer 86%* No Dimer «70% No dimer
-0.46 14%* «30%
B. Ethylacetoacetate OEt
YV
YY Ο
Log Ρ oct Meas. CLOGP In: Water Chloroform Octanol C. Diethylmalonate
+0.24 +0.33 93%* 92%* >90%
OEt
HO
+0.75 Association 7%* No dimer 8%* Dimer
