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netLibrary - eBook Summary Structure-based Drug Design by ...

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Figure 14<br />

The energy profile (as calculated at the 3-21G//PM3 level [24]) for the proton<br />

transfer step A rarrow.gif B and the methyl transfer step B rarrow.gif C for 3,4-dinitrocatechol as a<br />

substrate.<br />

in a rat COMT assay, that the rate of nitecapone methylation is equivalent to about 1% of the rate of<br />

dopamine methylation [47].<br />

Page 357<br />

The energy profile for the hypothetical methylation of 3,5-dinitrocatechol is shown in Figure 14 [24].<br />

The electronegative nitro groups strongly stabilize the ionized catechol–COMT complex, and the energy<br />

barrier for the methylation step is high (see Figure 9 for comparison). This can be understood as<br />

decreased nucleophilicity of the hydroxyl oxygen, due to the electron-with- drawing properties of the<br />

nitro groups.<br />

The electronic effect of the substituents of the catechol ring to the nucleophilicity of the hydroxyl group<br />

at the active site can be readily seen from the molecular electrostatic potential (MEP) surfaces of the<br />

system. The MEP surfaces were calculated at the PM3 level and plotted at –20 kcal/mol for catechol and<br />

3,5-dinitrocatechol at the active site of COMT [24]. The results are summarized in Figure 15. In the case<br />

of catechol the effect of the proton transfer form OH to Lys144 is seen as a remarkable increase in the<br />

negative potential between AdoMet and the substrate. 3,5-Dinitro substitution of the catechol ring<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_357.html (1 of 2) [4/5/2004 5:28:58 PM]

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