Adsorption of 5'-AMP and catalytic synthesis of 5'-ADP onto phosphate surfaces: correlation to solid matrix structures.

A non-enzymatic formation of 5'-ADP starting from phosphorylation of 5'-AMP in the presence of either calcium phosphate or calcium pyrophosphate precipitates is reported. This reaction is taken as a model for the study of heterogeneous catalysis of transphosphorylation in prebiotic conditions. Experiments were performed in completely aqueous media and in media containing dimethyl sulfoxide (Me2SO), to simulate periods of dehydration in primitive aquatic environments. It has been observed that the nucleotide is adsorbed onto both calcium phosphate and calcium pyrophosphate in accordance with Langmuir isotherms. Adsorptive capacity and affinity of the precipitates for nucleotide are changed by the presence of Me2SO, suggesting that the interaction between biomonomers and surfaces can be modulated by the degree of hydration of the anionic components of these compounds. In completely aqueous environments, formation of 5'-ADP from 5'-AMP adsorbed on precipitates of calcium phosphate and calcium pyrophosphate is very small. However, in the presence of 60% Me2SO this synthesis increases by factors of 3 and 6 for surfaces of calcium phosphate and calcium pyrophosphate, respectively, and follows first-order kinetics. Determinations of free energy changes show that phosphorylation of 5'-AMP adsorbed to these precipitates is thermodynamically favorable. Depending on the precipitation time of the samples and the composition of the medium, structural analysis of these precipitates by electron and X-ray diffraction shows changes in their cristallinity grade. It is proposed that these changes are responsible for the modulation of the quantity of adsorbed nucleotides to the surface of solid matrices as well as the catalytic activity of the precipitates.