GEOCHEMICAL CONSTRAINTS ON HISTIDINE ADSORPTION BY LAYERED DOUBLE HYDROXIDES

Abstract

To unravel the mystery of how life may have emerged on the early Earth, the last century has, increasingly, seen concerted attempts to investigate the environmental conditions and potential chemistry present. As more hypotheses have been tested, many researchers now believe that it is the hydrothermal vents under the ancient ocean that would have provided the environment and the starting materials ready for the reactions to create life.
Layered clay minerals would also most likely have played a significant role in these prebiotic reactions. These minerals would have concentrated the starting materials for life's chemistry through intercalation, catalysed the reactions that formed biomolecules, and protected the resulting products from decomposition, in a sense the same processes as life carries out today, but in different ways.
In this study, the layered hydroxide mineral Mg2Al(OH)6Cl·zH2O was used to mimic the green rust, FeII2FeIII(OH)6An−1/n·yH2O, a mineral that could have been one of the most-common components of early ocean sediments but is also highly redox reactive, in the intercalation reaction of histidine, one of the amino acids essential in modern day biochemistry. A co-precipitation method was used in the preparation of the Mg2Al-Cl LDH, which was later used in the ion-exchange reactions with d, l-histidine solution at different pH (7 and 10), temperature (30 °C and 60 °C), and pressure (ambient and ~ 50 bar). Among all the potential environmental conditions tested, the reaction at pH 10, 30 °C showed the highest ion-exchange rate. There was also a small (less than 2 % in exchanging rates) chiral selectivity observed in all reactions, but no correlation in trends between different conditions could be drawn.