The X-ray structure of previously studied dipeptidomimetic inhibitors bound in the active site of neuronal nitric oxide synthase (nNOS) presented a possibility for optimizing the strength of enzyme-inhibitor interactions as well as for enhancing bioavailability. These desirable properties may be attainable by replacement of the terminal amino group of the parent compounds (1-6) with a hydroxyl group (11-13, and 18-20). The hypothesized effect would be twofold: first, a change from a positively charged amino group to a neutral hydroxyl group might afford more drug-like character and blood-brain barrier permeability to the inhibitors; second, as suggested by docking studies, the incorporated hydroxyl group might displace an active site water molecule with which the terminal amino group of the original compounds indirectly hydrogen bonds. In vitro activity assays of the hydroxyl-terminated analogs (11-13 and 18-20) showed greater than an order of magnitude increase in K(i) values (decreased potency) relative to the amino-terminated compounds. These experimental data support the importance to enzyme binding of a potential electrostatic interaction relative to a hydrogen bonding interaction.