Ab initio study of the nonequivalence of adsorption of D- and L-peptides on clay mineral surfaces

Ab initio geometry optimizations were performed of the structures of the L and D enantiomers of the model dipeptide, N-formyl alanine amide (L-Ala and D-Ala, respectively) adsorbed on the mineral surface in the interlayer spa ce of the 2:1 clay, nontronite. Density functional procedures were used as implemented in the ab initio program CASTEP, in fully periodic calculations in which the properties of a model unit cell are determined by including t he effects of its neighboring cells in an infinite crystal. Geometry optimi zation included the atomic positions of the mineral and the adsorbates with in the unit cell, in addition to unit cell lengths and angles, achieving fu ll optimization of the entire crystal system. In agreement with previous st udies of the adsorption of organic compounds on clay mineral surfaces, in t he most stable arrangement Of L-Ala and D-Ala found here, the two molecules reside flat on the mineral basal plane, forming a parallel monolayer. The resulting L-Ala/mineral cocrystal is more stable, by 6 kcal/mol, than D-Ala . A characteristic structural difference exists for the enantiomers in that , in the optimized structure Of L-Ala, the C(alpha)-C(beta) bond is directe d away from one of the mineral basal planes toward the dioctahedral cavity of the other, while the side group of D-Ala is parallel to the mineral basa l plane and pointing to the nearest neighbor in an adjacent unit cell. As e xpected, the reverse results are obtained for the adsorption Of L-Ala and D -Ala on a nontronite surface that is the enantiomer of the one used above. The geometry optimizations illustrate the structural compatibility of clay mineral lattices with peptide structures. That is, balance of adsorption en ergies and peptide interaction energies, and mineral lattice structure and periodicity allow for adsorption structures, which involve the entire backb one of a single peptide molecule and can, at the same time, immobilize the adsorbates in such a way that stabilizing intermolecular interactions occur across unit cell boundaries. Compatibility of the repeat distances on the clay surface with repeat distances of peptides should be an important prope rty when clay minerals serve as templates for protein synthesis.