Efficient radiopharmaceutical design demands an understanding of factors that lead to one isomeric species in one ionization state at physiological pH. Thus, all pK(a) values must be outside the range of 6-9 for the typical M(V)O(N2S2) (M = Tc-99m, Re-186/188) agents. The pendant carboxyl group needed for rapid clearance of renal agents in particular must be either only syn or only anti to the oxo ligand with respect to the N2S2 ligand plane. Monoamide-monoamine-dithiol (monoamide-monoamine = MAMA) ligands useful in preparing radiopharmaceuticals typically form M(V)O(N2S2) complexes with one core ligand pK(a) of similar to 6-7 (secondary amine) and with both syn and anti isomers. We designed a new MAMA ligand, mercaptoacetamide-ethylene-cysteine (MAECH(5)), with the electron-withdrawing carboxyl group separated by only two bonds from the NH group. Only syn-ReO(MAECH(2)) was isolated. The structure of the monoanion syn-[ReO(MAECH)](-) in the crystal of a [AsPh4](+) salt reveals lattice H-bonding between the CO2H of a tautomer (t(2)) with a CO2H and an amine N- and the C=O of a neighboring t(2) anion; this interaction results in preferential crystallization of t(2). However, in aqueous solutions of syn-[ReO(MAECH)](-), the predominant monoanionic tautomer (t(1)) has a CO2- and an amine NH, as indicated by H-1 NMR and resonance Raman spectra, The endo-NH configuration favored in M(V)O(N2S2) complexes places the NH and CO2- groups in t(1) spatially close. The NH is less acidic due to the cancellation of the electron-withdrawing and electrostatic effects of the negative CO2-; as a result, syn-[ReO(MAECH)](-) has a pK(a) value (6.0 +/- 0.1) similar to that of the regioisomer syn-[ReO(CACAH)](-) in which the carboxyl group and the NH are not close (CACAH(5) = cysteine-acetyl-cysteamine). Our results suggest that the carboxyl group position also influences the syn/anti equilibrium. Attachment of the carboxyl group to a puckered ring in syn-[ReO(MAECH)](-) appears both to favor the syn isomer and to increase the rate of syn/anti isomerization. ReO(CACAH(2)), with a carboxyl group attached to a less puckered chelate ring anchored by the amido donor, formed as a noninterconverting roughly equal mixture of syn/anti isomers. Thus, for a MAMA ligand to form a syn isomer with a pK(a) < 6, it must be designed with a nonionizable electron-withdrawing group near the NH group and a pendant carboxyl on a puckered ring.