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-2) 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-2), whereas only very high positive surface charges (σ ≥ +0.35 C m-2) 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.

Catalytic conversions of atmospheric sulfur dioxide and formation of acid rain over mineral dusts: Molecular oxygen as the oxygen source.

Sulfur dioxide (SO2) ranks as a major air pollutant and is likely to generate acid rain. When molecular oxygen is the oxygen source, the regular surfaces of gibbsite (one of the most abundant mineral dusts) show no reactivity for SO2 conversions to H2SO4, while the partially dehydrated (100) surface with coordination-unsaturated Al sites becomes catalytically effective. Because of the easy availability of molecular oxygen, results manifest that acid rain can form under all atmospheric conditions and may account for the high conversion ratio of atmospheric SO2. The (100) and (001) surfaces show divergent catalytic effects, and hydrolysis is always the rate-limiting step. Path A (hydrolysis and then oxidation) is preferred for (100) surface, whereas a third path with obviously lower activation barriers is presented for (001) surface, which is non-existent for (100) surface. Atomic oxygen originating from the dissociation of molecular oxygen is catalytically active for (100) surface, while the active site of (001) surface fails to be recovered, suggesting that SO2 conversions over gibbsite surfaces are facet-controlled. This work also offers an environmentally friendly route for production of H2SO4 (one of the essential compounds in chemical industry), directly using molecular oxygen as the oxygen source.

Non-covalent interactions for carbonaceous materials: impacts of doping, curving and their combination.

Non-covalent interactions play a critical role during the application of carbonaceous materials. DFT calculations are presently employed to study the non-covalent interactions of graphene flakes (GFs) with ion pairs, considering the impacts of doping (electron-deficient and electron-rich) and curving (direction, curvature and surface: inner and outer) as well as their combined effects. The results are relevant to carbon nanotubes, from which curved graphene sheets can be facilely produced. Doping changes the predominant binding configurations and fundamentally affects non-covalent interactions, and all dopants enhance the binding strength, especially the electron-deficient ones that alter frontier orbitals. Curving will not alter the binding configurations but despite the lower impact compared to doping, larger curvatures may result in structural collapse. The changing trends of non-covalent interactions are opposite for inner and outer surfaces. Combined effects during non-covalent interactions are then tackled, producing four influencing factors that decrease as identity of dopant > curvature > curving direction and identity of dopant > surface. The sign of the combined effects (Ω > 0: counteractive while Ω < 0: synergetic) relies strongly on the identity of the dopants, and the other factors contribute less as elaborated in the text. Meanwhile, insightful clues about utilizing different computational methods to handle non-covalent interactions are offered. The results obtained thus far greatly further the understanding of non-covalent interactions regarding carbonaceous materials.