Experimental and Theoretical Charge Density Studies of 8-Hydroxyquinoline Cocrystallized with Salicylic Acid

The experimental electron density distribution (EDD) in 8-hydroxyquinoline cocrystallized with salicylic acid, <b>1</b>, has been determined from a multipole refinement of high-resolution X-ray diffraction data collected at 100 K. The experimental EDD is compared with theoretical densities resulting from high-level ab initio and BHandH calculations using Atoms in Molecules theory. <b>1</b> crystallizes in the triclinic crystal system, and the asymmetric unit consists of a neutral salicylic acid molecule, a salicylate anion, and an 8-hydroxyquinolinium cation exhibiting a number of inter- and intramolecular hydrogen bonds and π–π interactions. Topological analysis reveals that π–π interactions are of the “closed-shell” type, characterized by rather low and flat charge density. In general, the agreement of the topological values (ρ_bcp and ∇²ρ_bcp) between experiment and theory is good, with mean differences of 0.010 e Å^–3 and 0.036 e Å^–5, respectively. The energetics of the π–π interactions have been estimated, and excellent agreement is observed between the relative energy and the strength of π-stacking derived from the Espinosa approach, with an average difference of only 4.4 kJ mol^–1

Effect of conformational restriction on the efficacy of ligands at GABA receptors.

<p>Concentration response curves of GABA and the unsaturated analogues, TACA and CACA at (A) ρ1 WT and (B) ρ1 T244S receptors, (Data = Mean ± SEM, n = 5). (C) Sample traces of the maximal responses of GABA and TACA at ρ1 WT and ρ1 T244S receptors.</p

Enhancement of the GABA EC₅₀ response by CACA at ρ1 WT and ρ1 T244S receptors.

<p>(A) Concentration response curve of the co-application of increasing concentrations of CACA in the presence of GABA EC₅₀ at ρ1 WT and ρ1 T244S receptors, (Data = Mean ± SEM, n = 5). Sample traces of CACA co-applied with GABA EC₅₀ at ρ1GABA_C (B) WT and (C) ρ1 T244S receptors.</p

Docking studies of isoguvacine in the orthosteric binding site of ρ1 GABA_C homology model.

<p>(A) H-bonds and (B) hydrophobic interactions predicted to be formed by isoguvacine and the ρ1 GABA_C receptor.</p

Effect of GABA at mutant receptors.

<p>A. GABA and surrounding residues in the orthosteric binding site of the ρ1 GABA_C homology model. B. Concentration response curves of GABA at ρ1 GABA_C WT, ρ1 T244S, ρ1 T244A and ρ1 T244C receptors, (Data = Mean ± SEM, n = 5). Note: GABA elicited sub-maximal efficacy compared to β-alanine and MTSEA at ρ1 T244A and ρ1 T244C mutant receptors, respectively.</p

MTSEA produces a reversible response at ρ1 T244C receptors.

<p>(A) Concentration response curves of MTSEA and GABA at ρ1 T244C mutant receptors, (Data = Mean ± SEM, n = 15). (B) Sample response traces of MTSEA and GABA at ρ1 WT receptors. (C) Sample response traces of MTSEA at ρ1 T244C receptors.</p

Docking studies showing interactions between ligands and ρ1 receptors.

<p>(A) GABA and β-alanine docking studies in the orthosteric binding site of GABA_C homology model based on GluCl in open conformation. (Left) hydrogen bonds between the side chains of GABA and β-alanine with side chains of Thr244 and Glu196 residues. (Right) various hydrophobic interactions between the side chains of GABA and β-alanine with side chains of Thr244 and Glu196. (B) GABA and β-alanine docking studies in the orthosteric binding site of GABA_C homology model based on GluCl in open conformation. (Left) hydrophobic interactions between GABA side chain and alanine residues at the site of Thr244. (Right) hydrophobic interactions between β-alanine side chain and alanine residue at the site of Thr244. (C) TPMPA docking studies in the orthosteric binding site of GABA_C homology model based on GluCl in <i>apo</i> conformation. (Left) hydrophobic interactions between the side chain of TPMPA and the side chain of Thr244. (Middle) hydrophobic interactions between the side chain of TPMPA and the side chain of serine residue at the site of Thr244. (Right) hydrophobic interactions between the side chain of TPMPA and the side chain of alanine residue at the site of Thr244.</p

5-Aminovaleric acid fits in the ρ1 GABA_C orthosteric binding site in a folded conformation.

<p>GABA (white) and 5-aminovaleric acid (red) docked in the orthosteric binding site of ρ1 GABA_C homology model based on GluCl in <i>apo</i> state.</p

Activity of GABA, glycine, β-alanine and 5-aminovaleric acid at ρ1 WT and ρ1 T244S receptors.

<p>Concentration response curves of GABA, glycine, β-alanine and 5-aminovaleric acid at ρ1 WT (A) and ρ1 T244S (B) receptors, (Data = Mean ± SEM, n = 5).</p

Characterisation of response and binding of GABA and TACA at ρ1 WT and ρ1 T244S receptors.

<p>(A) GABA, TACA and CACA docked in the orthosteric binding site of ρ1 GABA_C homology model based on GluCl in open conformation. (B) Poses of GABA and TACA in their binding conformations showing distances between the two poles. (C) Various contacts of the side chain of GABA with the side chain of serine at Thr244 site. (D) Various contacts of the side chain of TACA with the side chain of serine at Thr244 site. The rotamer of serine shown in D and E has similar Chi1 and Chi2 (i.e. first and second dihedral angles of the side chain) values to the predicted conformation of Thr244 at this site (<i>i</i>.<i>e</i>. Chi = 91 and Chi2 = –179).</p