Adsorption and discrimination of alanine and alanyl-alanine enantiomers by allophane

Abstract

We have investigated the adsorption of D- and L-alanine, and their respective dimers, by three allophanes with different Al/Si ratios. The Kanuma (Al/Si = 1.2) and Kitakami (Al/ Si = 1.5) allophanes were from Japan whereas the Te Kuiti (Al/Si = 1.6) sample came from New Zealand. The three allophanes differed in their capacity for adsorbing alanine but none of the samples showed a clear preference for either the D- or the L-form. In the case of alanyl-alanine both Kanuma and Kitakami allophanes gave a hint of preferring the L- to the D-enantiomer. On the other hand, the allophane sample from Te Kuiti showed a clear preference for L-alanyl-L-alanine. In keeping with this observation, circular dichroic spectrometry indicated that solutions of a racemic mixture of alanyl-alanine, after equilibration with Te Kuiti allophane, became relatively enriched in the Denantiomer. The size, intra-molecular charge separation, and surface orientation, of L-alanyl-Lalanine zwitterions apparently combine to confer ‘structural chirality’ to the complex (adduct) with Te Kuiti allophane. As a result, the mineral-organic complex develops a preference for the L-form of alanyl-alanine. This finding points to a possible role of certain allophanes in the origin of biochirality.