487 Acta Cryst. (1996). B52, 487-499 Hydrogen-Bond Patterns in 1,4-Dihydro-2,3-quinoxalinediones: Ligands for the Giycine Modulatory Site on the NMDA Receptor MACIEJ KUBICKI,t TERESAW. KINDOPP, MARIO V. CAPPARELLI~AND PENELOPEW. CODDING* Departments of Chemistry and of Pharmacology and Therapeutics, University of Calgary, Calgary, Alberta, Canada T2N 1N4 (Received 10 Januar 3' 1994; accepted 25 August 1995) Abstract The crystal structures of five 1,4-dihydro-2,3-quinoxali- nediones, antagonists of the NMDA modulatory glycine binding site on the excitary amino acid (EAA) receptor complex, have been determined: (I) 6,7-dinitro-l,4- dihydro-2,3-quinoxalinedione (DNQX); (II) 5,7-dinitro- 1,4-dihydro-2,3-quinoxalinedione (MNQX); (III) 6-nitro- 1,4-dihydro-2,3-quinoxalinedione hydrate; (IV) 6,7-di- chloro-l,4-dihydro-2,3-quinoxalinedione; (V) 5,7-di- chloro- 1,4-dihydro-2,3-quinoxalinedione dimethylform- amide. The crystal structure of the most active compound (II) contains a unique intramolecular N--H...O(NO2) hydrogen bond, which may be impor- tant for activity, as semiempirical calculations show that this bond is stable over a wide range of dihedral angles between the planes of the molecule and of the nitro group. In the other compounds the intermolecular hydrogen bonds connect molecules into three-dimen- sional networks. In compounds (I), (III) and (IV) head- to-tail :r-stacking is found between molecules connected by a center of symmetry. The geometries of the hydrogen-bonded --NH--C--O fragments show evidence of zr-cooperativity or resonance-assisted hydrogen bonding. Graph-set analysis of the hydrogen- bond patterns of quinoxalinedione derivatives shows a tendency to form two types of hydrogen-bonding motifs: a centrosymmetric dimeric ring and an infinite chain. Even though this pattern may be modified by the presence of additional hydrogen-bond acceptors and/or donors, as well as by solvent molecules, general similarities have been found. Comparison of all quinox- alinedione structures suggests that the hydrogen-bonding pattern necessary for the biological activity at the glycine binding site contains one donor and two acceptors. 1. Introduction Quinoxalinedione (QD) derivatives have been found to be highly potent antagonists in the excitary amino acid (EAA) neurotransmission system and to act by binding to tOn leave from Department of Chemistry, Adam Mickiewicz UniversiW, 60-780 Poznafi, Poland. ~:Present address: Escuela de Quimica, Universidad Central de Venezuela, Caracas 1051. Venezuela. © 1996 Intemational Union of Crystallography Printed in Great Britain - all rights reserved the glycine modulatory site of the NMDA (N-methyl-D- aspartic acid) receptor, as well as to other excitatory receptor subtypes (Honor6 et al., 1988; Randle et al., 1992). EAA receptors are part of a supramolecular complex that forms a ligand-gated ion channel. The NMDA receptor requires both glutamate and glycine for activity; agonists and antagonists for these two sites are potential therapies for epilepsy, learning disorders and ischemic damage (Kemp & Lesson, 1993; Reynolds & Miller, 1990). For this reason, extensive structural studies of the QD derivatives have been performed. Structural analysis identifies three important features for biological action: the hydrogen-bonding pattern, the distribution of charge throughout the molecule and steric factors (Kubicki & Codding, 1993; Kubicki, Kindopp, Capparelli & Codding, 1992). To obtain further insight into these features, we have extended the studies to three nitro and two chloro derivatives. The nitro derivatives are important, because a small structural change from 6,7-dinitro-QD (DNQX; hereinafter the basic molecule, 1,4-dihydro-2,3-quinoxalinedione, will be referred to as QD) to 5,7-dinitro-QD (MNQX) reduces the affinity of the compounds for the nonNMDA receptors fourfold, but increases the antagonistic activity at the glycine site 30- fold (Watkins, Krogsgaard-Larsen & Honorr, 1989; Randle et al., 1992). IC50 values measure the concentra- tion required to inhibit the binding of the natural ligand, glycine, by 50%. The IC50 value for (I) is ca 4.5 ~tM, for (II) is 0.14 I.tM, for (III) is ca 27 l.tM, and for (IV) and (V) is 4 l.tM (Pullan, 1991 ). Thus, (II) is the most active of the RI 15 i R2 ' ~ O3 R 3 7 g "~ N 0 2 8 I.l H (I) R2 = R 3 = NO 2, R I = H (II) R 1 =R 3 =NO,,,R 2=H (III) R 2 = NO 2, R t = R 3 = H (IV) R 2=R 3 =CI, R 1 =H (W) R t = R 3 = CI, R 2 = H Acta CrystallographicaSection B ISSN 0108-7681 © 1996