Amino Acids in Iron Oxide Mineralization: (Incomplete) Crystal Phase Selection Is Achieved Even with Single Amino Acids

Abstract

Iron oxides are important minerals in biology and materials science. Using biomimetic synthesis, a variety of iron oxides have been fabricated. However, it is still not clear how growth modifiers like amino acids and peptides select different crystal phases of a complex material like iron oxide. The current paper shows that already with single amino acids, (incomplete) crystal phase selection is achieved in vitro. In particular, l-histidine, l-threonine, and l-cysteine favor the formation of unstable crystal phases like ferrihydrite or lepidocrocite, although sometimes only at high amino acid concentrations. Other amino acids like l-valine have only minor effects when compared to control samples grown in the absence of amino acids. The effects of the amino acids can be rationalized via kinetic trapping and different interaction strengths of the amino acids with the growing iron oxide particles. The effects of the amino acids on the particle morphologies are less significant. The paper therefore shows that single amino acids can be a valuable tool for the materials chemist to fabricate and stabilize even unstable iron oxide crystal phases.