Stabilization of Zwitterions in Solution: GABA Analogues

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The solution-phase structures of a number of conformationally restricted gamma-aminobutyric acid (GABA) analogues are investigated at the MP2/6-31+G* level of theory, using both explicit water molecules and the conductor-like screening solvation model (COSMO) to model solvation. GABA analogues constrained in a cis conformation by either a double bond or cyclopropane ring have the potential to attain either folded, intramolecularly hydrogen-bonded, or partially folded conformations in solution. Systems constrained in a cis conformation by a cyclopentane or cyclopentene ring are more conformationally restricted and exist only in a folded, intramolecularly hydrogen-bonded form. GABA analogues constrained in a trans conformation by either a double bond or cyclopropane ring have the potential to adopt either partially folded or fully extended conformations in solution. Due to a lack of conformational flexibility, analogues that are constrained in a trans conformation by a cyclopentane or cyclopentene ring attain only partially folded conformations. Like GABA, conformationally flexible GABA analogues possess a large number of stable rotamers, and may exist in any or all of these conformations in aqueous solution. The structures of these analogues provide an essential foundation for subsequent structure-activity analysis of ligand binding at GABA receptors and transporters. This work is therefore expected to facilitate the design and development of new biologically active GABA analogues to treat GABA-related neurological disorders.