Conformational Flexibility of Buspirone Analogues from X-ray in the Solids and Nmr in Solution

Bioactive molecular conformation of a drug is one of the key points for understanding the ligand-receptor interactions. N-substituted arylpiperazines are important class of 5-HT receptor ligands (particularly 5-HT1A, 5-HT2A and 5-HT7) widely used in the treatment of anxiety, depression, schizophrenia, sleep and memory disorders. Majority of compounds of this group possess hydrocarbon linker of different length and type, which connects the arylpiperazine moiety with the second terminal group, e. g. cyclic imide. According to SAR (structure-activity relationship) analyses, the presence of an aryl group at N4 atom of the piperazine ring is critical in respect to the biological activity of these compounds. The protonated nitrogen atom forms the hydrogen bond with the carboxyl oxygen of the Asp 3.32 side chain, that is crucial for the interaction with the 5-HT receptors. In addition, the aromatic ring stabilizes the ligand binding by a CH...π interaction with the Phe 6.52 residue. Nowak et al. claimed that o-methoxy-phenyl moieties of a ligand form a hydrogen bond with the hydroxyl group of Ser 5.42 [1]. Some authors have underlined also a role of the terminal hydrophobic imide part in a stabilization of the ligand-receptor complex by π...π and/or dipole...dipole interactions [2].