Nanosilica Formation at Lipid Membranes Induced by Silaffin Peptides
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Nanosilica Formation at Lipid Membranes Induced by Silaffin Peptides M. S. Kent;1 J. K. Murton;1 S. Satija;2 I. Kuzmenko;3 and B. A. Simmons4 1 Sandia National Labs, Albuquerque, NM, USA. 2 National Institute of Standards and Technology, Gaithersburg, MD, USA. 3 Argonne National Labs, Argonne, IL, USA. 4 Sandia National Labs, Livermore, CA, USA. ABSTRACT Diatoms are unicellular eukaryotic algae found in fresh and marine water. Each cell is surrounded by an outer shell called a frustule that is composed of highly structured amorphous silica. Diatoms are able to transform silicic acid into these sturdy intricate structures at ambient temperatures and pressures, whereas the chemical synthesis of silica-based materials typically requires extremes of temperature and pH. Cationic polypeptides, termed silica affinity proteins (or silaffins) recently identified from dissolved frustules of specific species of diatoms are clearly involved and have been shown to initiate the formation of silica in solution. The relationship between the local environment of catalytic sites on these peptides, which can be influenced by the amino acid sequence and the extent of aggregation, and the observed structure of the silica is not understood. Moreover, the activity of these peptides in promoting silicification at lipid membranes has not yet been clarified. In this work we developed a model system to address some of these questions. We studied peptide adsorption to Langmuir monolayers and subsequent silicification using X-ray reflectivity and grazing incidence X-ray diffraction. The results demonstrate the lipid affinity of the parent sequences of several silaffin peptides. Further, the results show that the membrane-bound peptides promote the formation of interfacial nanoscale layers of amorphous silica at the lipid-water interface that vary in structure according to the peptide sequence. INTRODUCTION Diatoms are one of the most important biological systems on the planet, accounting for up to 25% of global carbon fixation.1, 2 These eukaryotic organisms typically possess a highly silicified structure known as the frustule.3-7 Evidence indicates that diatoms utilize several organic components, including silica affinity peptides or silaffins, to promote silica growth during cell division.8-12 The local silica-precipitating environment is influenced by the amino acid sequence and the nature of peptide aggregation, among other factors.13 Interactions between the peptides and lipid membranes may also affect the local environment of the silica-initiating sites. The present work reports the adsorption of the parent sequences of several silaffin peptides to lipid monolayers and the subsequent peptide-induced formation of nanosilica layers upon introduction of silicic acid. This study extends our prior work involving a single peptide that demonstrated that fine details of the silicification process can be revealed using planar lipid membranes combined with analysis by X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GI
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