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

Phosphate sorption and desorption on pyrite in primitive aqueous scenarios: relevance of acidic --> alkaline transitions

Phosphate (P(i)) sorption assays onto pyrite in media simulating primeval aquatic scenarios affected by hydrothermal emissions, reveal that acidic conditions favour P(i) sorption whereas mild alkaline media--as well as those simulating sulfur oxidation to SO(2-) (4)--revert this capture process. Several mechanisms relevant to P(i) availability in prebiotic eras are implicated in the modulation of these processes. Those favouring sorption are: (a) hydrophobic coating of molecules, such as acetate that could be formed in the vicinity of hydrothermal vents; (b) water and Mg(2+) bridging in the interface mineral-aqueous media; (c) surface charge neutralization by monovalent cations (Na+ and K+). The increase of both the medium pH and the SO(2-) (4) trapping by the mineral interface would provoke the release of sorbed P(i) due to charge polarization. Moreover it is shown that P(i) self-modulates its sorption, a mechanism that depends on the abundance of SO(2-) (4) in the interface. The relevance of the proposed mechanisms of P(i) capture, release and trapping arises from the need of abundant presence of this molecule for primitive phosphorylations, since--similarly to contemporary aqueous media--inorganic phosphate concentrations in primitive seas should have been low. It is proposed that the presence of sulphide minerals with high affinity to P(i) could have trapped this molecule in an efficient manner, allowing its concentration in specific niches. In these niches, the conditions studied in the present work would have been relevant for its availability in soluble form, specially in primitive insulated systems with pH gradients across the wall.

Modulation of adenosine 5'-monophosphate adsorption onto aqueous resident pyrite: potential mechanisms for prebiotic reactions

The adsorption of adenosine 5'-monophosphate (5'-AMP) onto pyrite (FeS2) and its modulation by acetate, an organic precursor of complex metabolic pathways, was studied in aqueous media that simulate primitive environments. 5'-AMP adsorption requires divalent cations, indicating that a cationic bridge mediates its attachment to negatively charged sites of the mineral surface. The isotherm of 5'-AMP adsorption exhibits a strong cooperative effect at low nucleotide concentrations in acetate-rich medium, whereas high levels of adsorption were only found at high nucleotide concentrations in a model of primitive seawater (acetate free). The modulating role of acetate is also evidenced in the presence of high dipolar moment molecules: dimethyl sulfoxide (Me2SO) and dimethyl formamide (DMF) strongly inhibit 5'-AMP adsorption in acetate-rich media, whereas no effect of DMF was found in artificial seawater. The observation that exogenous divalent cations are not needed for acetate attachment onto FeS2 reveals that organic acids can interact with the Fe2+ atoms in the mineral surface. All considered, the results show that complex and flexible ironsulfide/biomonomers interactions can be modulated by molecules that accumulate in the interface layer.

Pyrite suspended in artificial sea water catalyzes hydrolysis of adsorbed ATP: enhancing effect of acetate

Minerals have been implicated in different catalytic processes during chemical evolution. It has been proposed that exergonic synthesis of pyrite (FeS2) could have served to promote the endergonic synthesis of biomonomers in early stages of life formation on Earth. The present study was aimed to investigate whether pyrite can adsorb nucleotides and oxo acids in the potentially mild prebiotic conditions found away from the hot hydrothermal vents. It is shown that pyrite strongly adsorbs adenosine 5'-triphosphate in an artificial medium that simulates primordial aqueous environments, and that adsorption is enhanced in the presence of acetate and in an oxygen-free atmosphere. Moreover, the mineral catalyzes the sequential hydrolysis of the gamma and beta phosphoanhydride bonds of the nucleotide.

Pyrophosphate and adenosine 5'-diphosphate synthesis from phospho(enol)pyruvate: catalysis by phosphate minerals and modulation by dimethyl sulfoxide

Phospho(enol)pyruvate (PEP) undergoes transphosphorylation to form pyrophosphate (PPi) and adenosine 5'-diphosphate (5'-ADP) with high yields in the presence of an adsorbent surface of calcium phosphate (Pi.Ca), which is considered to be an ancient mineral with catalytic properties. PPi formation is a result of the phosphorolytic cleavage of the enol phosphate group of PEP by precipitated Pi. The synthesis of PPi is dependent on the amount of the solid matrix; it increases with the amount of adsorbed PEP and upon addition of dimethyl sulfoxide (Me2SO), a molecule with high dipolar moment. Although it is saturated with PEP at neutral pH, the phosphorylating Pi.Ca surface becomes effective only in alkaline conditions. In a parallel reaction, PEP phosphorylates 5'-AMP to 5'-ADP with a yield that is sevenfold higher in the presence of the Pi.Ca surface than in its absence, indicating that the solid matrix promotes interaction between adsorbed molecules with a high potential for phosphoryl transfer. In contrast to phosphorolysis, this latter reaction is stimulated by Me2SO only in homogeneous solution. It is concluded that phosphate minerals may have coadjuvated in reactions involving different phosphorylated compounds and that molecules with high dipolar moment may have acted in mildly alkaline, primitive aqueous environments to modulate phosphoryl transfer reactions catalyzed by phosphate minerals.

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.