Table 2. Dipole Separation Distances of GABA Analogues^a.

ligand	potency pEC50	dipole separation (Å)	carboxylate oxygensb	ammonium hydrogens
muscimol	5.04	5.22	−0.26 (isoxazole N), −0.47	0.32, 0.32, 0.30
GABA	4.72	4.83	−0.60, −0.54	0.31, 0.31, 0.29
TACA	4.44	4.88	−0.57, −0.53	0.31, 0.31, 0.29
isoguvacine	4.19	4.86	−0.55, −0.53	0.29, 0.31, 0.29
THIP	3.65	5.14	−0.25 (isoxazole N), −0.46	0.29, 0.31, 0.31
β-alanine	3.09	4.37	−0.58, −0.51	0.29, 0.29, 0.29
5-AV	3.06	6.47	−0.61, −0.56	0.31, 0.31, 0.30
taurine	<2.0	4.46	−1.11, −1.12	0.30, 0.30, 0.28
GHB	NRd	5.10	−0.61, −0.61	−0.38 (hydroxyl O) c
3-APP	NR	4.50	−1.23, −1.23 (phosphate O)	0.29, 0.31, 0.31
glycine	NR	2.35	−0.47, −0.57	0.32, 0.32, 0.27
4-AB	NR	6.20	−0.38 (hydroxyl O)	0.19, 0.20, 0.24
PABA	NR	6.39	−0.54, −0.54	0.30, 0.30, 0.30
tyramine	NR	7.94	−0.47 (hydroxyl O)	0.31, 0.30, 0.30

^aMulliken charges of GABA analogues calculated using the GAMESS interface. Ammonium hydrogens and carboxylate oxygens, or their equivalent substituents for the dipole, are listed.

^bLigands without carboxyl groups were assessed by their equivalent groups: isoxazole nitrogens in THIP and muscimol, phosphate oxygen in 3-APP, and hydroxyl oxygen in 4-AB and tyramine.

^cGHB does not have an ammonium group. The charge on the hydroxyl oxygen is negative.

^dNR = no response.