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Figure 5<br />

Comparison of 8-amino and 9-deaza substitutions on guanine. Details are extensively<br />

discussed in the text. Cross-hatching schematically indicates the enzyme. Ball and<br />

stick diagrams show the inhibitor and key side-chain residues. Nitrogens are dark<br />

gray spheres, oxygens are light gray. Arrows indicate hydrogen bonding, with the<br />

arrow size showing relative strength.<br />

amino group, generating a hydrophobic environment for the group and decreasing binding affinity.<br />

Page 162<br />

The carbon-for-nitrogen switch in the 9-deaza variant favors association with PNP <strong>by</strong> substituting a<br />

strong hydrogen bond for the relatively weak one occurring between Asn 243 and guanine. Formation of<br />

a simple 8-aminoguanine variant leads to tight binding <strong>by</strong> giving rise to an extra hydrogen bond between<br />

the purine derivative and Thr 242.<br />

The combination of the two “improvements”—the carbon-for-nitrogen substitution and the addition of<br />

the amino group to position eight—was counterproductive because the carbon in position nine prevented<br />

the amino group at<br />

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