Scheme 1 . Fluorinated CA Inhibitors
Figure 1. LFERs between log Kd and log P for hydrophobic (A)
and fluorinated (B) inhibitors of CA. The slope of the dependence
of binding affinity on hydrophobicity is -0.36 (r2 ) 0.775) for
(A) and -0.83 (r2 ) 0.962) for (B). Note that the binding energies
of several of the inhibitors are not resolved on this plot (see Table
1).
nance of each inhibitor relative to the methyl resonances from
DMF.4
To elucidate the origin of tight binding of our library of
inhibitors, we used CAChe5 to calculate the octanol/water
partition coefficient (log P) for each compound. A linear
free energy relationship (LFER) between log Kd for each
inhibitor6 and its calculated value of log P is shown in Figure
1b.7 The log Kds for each extent of fluorination (non-, mono-,
di-, tri-, tetra-, and penta-) have also been averaged, and these
data, as a function of log P, have been fitted to a line with
slope 0.83 (r2 ) 0.962). A similar plot of data from a
comparable library of known nonfluorinated hydrophobic
inhibitors1 is shown in Figure 1a, and the slope of the best
fit line to these data is 0.36 (r2 ) 0.775). These results are
consistent with a model where hydrophobicity generally
increases the affinity of inhibitors and where fluorine (Figure
1b) seems to be more hydrophobic than hydrocarbons (Figure
1a).8 The large variability in the individual data points in
Figure 1b, however, suggests that the pattern of fluorine
substitution also affects the affinity of fluoroaromatic deriva-
tives. Notably, in the case of mono- and difluorinated
compounds, para substitution by fluorine seems to have little
effect on binding affinity (Table 1).
On the basis of these results, on a crystal structure of the
nonfluorinated benzyl amide derivative bound to CA,1 and
on preliminary data from ab initio calculations on a model
system,9 we propose the following two conformations for
the interaction of fluoroaromatic inhibitors with the active
site of carbonic anhydrase. In conformation I (Figure 2a),
the ortho and meta hydrogens of Phe131 in the active site of
CA interact with fluorines at the 2 and/or 3 position of our
inhibitors via electrostatic contact(s) (F‚‚‚H hydrogen
bonds10,11). In conformation II (Figure 2b), the electron-rich
aromatic ring of Phe131 interacts in a stacked manner with
the electron-deficient ring of inhibitors bearing three or more
fluorine atoms.12,13 This conformation allows the molecular
quadrupoles of the two aromatic rings to be aligned in their
most favorable orientation.14 These conformations are con-
(4) Jain, A.; Huang, S.-G.; Whitesides, G. M. J. Am. Chem. Soc. 1994,
116, 5057-5062.
(5) Version 4.02, Oxford Molecular Group, 1998.
(6) Kds were determined by the procedure described in ref 5.
(7) Hansch, C. W.; McClarin, J.; Klein, T.; Langridge, R. Mol. Phar-
macol. 1985, 27, 493-498.
(8) An alternate interpretation would be that addition of fluorines to the
benzyl amide ring lowers the pKa of the sulfonamide, which should also
increase the affinity of inhibitors, resulting in a more steep LFER.
(9) Manuscript in preparation. Preliminary data from calculations at MP2/
6-31G* suggest that an F‚‚‚H bond provides about 4 kJ/mol.
(10) Dunitz, J. D.; Taylor, R. Chem. Eur. J. 1997, 3, 89-98.
(11) Thalladi, V. R.; Weiss, H.-C.; Blaser, D.; Boese, R.; Nangia, A.;
Desiraju, G. R. J. Am. Chem. Soc. 1998, 120, 8702-8710.
(12) West, A. P.; Mecozzi, S.; Dougherty, D. A. J. Phys. Org. Chem.
1997, 10, 347-350.
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Org. Lett., Vol. 1, No. 2, 1999