Equilibrium constants for association of guanidinium and ammonium ions with oxyanions: The effect of changing basicity of the oxyanion

Using guanidinium and n-butylammonium cations (C⁺) as models for the positively charged side chains in arginine and lysine, we have determined the association constants with various oxyanions by potentiometric titration. For a dibasic acid, H₂A, three association complexes may exist: $\text{K}{}_{\mathrm{<mtext>1M</mtext>}}\text{=}\text{CHA]}\text{C}{}^{\mathrm{+}}\text{]}\text{HA}{}^{\mathrm{?}}\text{]}$; $\text{K}{}_{\mathrm{<mtext>1D</mtext>}}\text{=}\text{CA}{}^{\mathrm{?}}\text{]}\text{C}{}^{\mathrm{+}}\text{]}\text{A}{}^{\mathrm{2?}}\text{]}$; $\text{K}{}_{\mathrm{<mtext>2D</mtext>}}\text{=}\text{C}{}_2\text{A]}\text{C}{}^{\mathrm{+}}\text{]}\text{CA}{}^{\mathrm{?}}\text{]}$. For guanidinium ion and phosphate, K_1M = 1.4, K_1D = 2.6, and K_2D = 5.1. The data for carboxylates indicate that the basicity of the oxyanion does not affect the association constant: acetate, pK_a = 4.8, K_1M = 0.37; formate, pK_a = 3.8, K_1M = 0.32; and chloroacetate, pK_a = 2.9, K_1M = 0.43, all with guanidinium ion. Association constants are also reported for carbonate, dimethylphosphinate, benzylphosphonate, and adenylate anions.