A series of N-substituted alpha-amino acids containing terminal phosphonic acid groups has been synthesized as potential N-methyl-D-aspartate (NMDA) receptor antagonists. NMDA receptor affinity was determined by displacement of a known ligand ([3H]CPP) from crude rat brain synaptic membranes; an antagonist action was demonstrated by the inhibition of glutamate-induced accumulation of [45Ca2+] in cultured rat cortical neurons. Receptor affinity was significantly correlated with antagonist activity (Figure 1). Moderate affinity (IC50 = 1-2 microM) was retained for analogues (31 and 32, Table I; and 59 and 66, Table II) with reduced flexibility in their phosphonate side chains and is consistent with entropy playing a role in determining receptor affinity. Modeling studies suggest a folded conformation that brings the distal phosphonic acid group into close proximity with the alpha-carboxylate is required for binding. Each of the active analogues possess entropy-limiting features (double bonds, phenyl rings) in their side chains that allows the superposition of their key NH2, alpha-COOH, and distal PO3H2 groups with those of known competitive antagonists. Affinity decreased for analogues with alpha-carbon substitution, presumably because the alpha-substituent inhibits the folding of these structures into a bioactive conformation and occupies receptor-excluded volume. A complete description of the NMDA antagonist pharmacophore model is provided in a companion paper.