Adsorption of amino acids at clay surfaces and implication for biochemical reactions: Role and impact of surface charges.

Almost all clay minerals carry an abundance of surface charges. The role and impacts of surface charges during adsorption of amino acids and biochemical reactions are of great importance while currently remain elusive, which are to be tackled in this study by first-principles density functional calculations. A wide range of surface charges (-0.42˜ + 0.42 C m) have been considered. Distribution of different amino acid isomers and their interaction with clay minerals rely strongly on the sign and amount of surface charges. Zwitterionic structures remain stable for all negative surface charges and become dominant when negative surface charges are abundant (σ ≤ -0.28 C m), whereas only very high positive surface charges (σ ≥ +0.35 C m) can stabilize zwitterionic glycine. Increase of surface charges pronouncedly enhances the interactions of amino acids with clay minerals, which favors their gathering at clay surfaces and condensation to protein fragments. The superior binding of amino acids by negatively rather than positively charged clay minerals is due to stronger H bonding and electrostatic interactions. The biochemical reactions are greatly accelerated at higher surface charges and zwitterion formation becomes almost barrierless; however, the reverse reactions of forming canonical isomers have so moderate activation barriers that can occur facilely and get ready for the condensation to protein fragments. Accordingly, clay minerals, even in the anhydrous state, should be the suitable birthplace for life, where surface charges play a central role.