Divalent cations modify adsorption of 5'-AMP onto precipitated calcium phosphate: a model for cation modulation of adsorptive processes in primitive aqueous environments

The adsorption of 5'-AMP onto precipitated calcium phosphate (CaPi) requires the presence of soluble calcium and this dependence exhibits a Michaelian-like behavior. This result suggests that the formation of a complex between 5'-AMP and free Ca2+ (CaAMP) is a prelude to the adsorption of the nucleotide in the solid matrix. At concentrations one order of magnitude higher, Mn2+ and Mg2+ can substitute for soluble Ca2+ in the adsorption of 5'-AMP onto solid CaPi. However, when added simultaneously with 5'-AMP to a heterogeneous mixture that contains CaPi and soluble Ca2+, Mn2+ and Mg2+ inhibit the adsorption of 5'-AMP in a concentration-dependent manner. This suggests the formation of complexes that are much less effective for 5'-AMP adsorption than the CaAMP complex. On the other hand, Mn2+ and Mg2+ cannot promote desorption of the nucleotide attached to the precipitate in the presence of soluble Ca2+ if they are added after adsorption has attained equilibrium. Although desorption of 5'-AMP can be obtained by a sequential dilution of the soluble phase with buffer and no nucleotide in a process that obeys a Langmuir equation, the lack of effect of Mn2+ or Mg2+ when adsorption has attained its maximal value suggests strong interactions between the CaAMP complex and the solid matrix when adsorption equilibrium is reached. The divalent cations present in the matrix also participate with different selectivity in the attachment of the CaAMP complex, indicating that a cation-exchange mechanism could have acted in the modulation of adsorptive/desorptive processes involving biomonomers and phosphate surfaces in primitive aqueous environments.