Studies on the adsorption of peptides of glycine/alanine on montmorillonite clay with or without co-ordinated divalent cations

The adsorption of simple peptides of glycine/alanine from their aqueous solutions onto montmorillonite, Ca2+ and Mg2+ exchanged montmorillonite clay studied UV spectrophotometrically at constant pH 7.02 and temperature 23 °C. The percent binding of Gly2, Gly3, Gly4 and Gly-ala is calculated in terms of their optical density. The adsorption parameters, i.e. Xm and KL have been calculated from Langmuir adsorption isotherm. Similar adsorption behaviour was observed with or without divalent cation exchanged adsorbent, but the percent binding and monolayer capacity appear to depend on the molecular weight, i.e. number of aliphatic carbon atoms of the adsorbates. The adsorption was significantly affected by the concentration of peptide, pH and temperature of the system. Equilibrium constant (K) and the free energies of adsorption (−ΔG) were determined from the isotherm measured under static conditions. Tetra glycine (Gly4) has positive −ΔG and K>1 showing greater adsorptibility, whereas for other peptides, −ΔG values were negative and K<1, thus showing very weak adsorption. A linear dependence of −ΔG on the number of aliphatic carbon atoms (nc) from Gly2 to Gly4 in adsorbate molecule was found. Thermodynamic data strongly support the quantitative data obtained from Langmuir adsorption isotherm. Ca2+ montmorillonite exhibited relatively better adsorption as compared to Mg2+ exchanged form or montmorillonite without Ca2+ or Mg2+. Results have shown that clay minerals might have played a significant role in prebiotic formation of proteins via adsorption of simple bio-oligomers on their surface.