Probing the molecular structure of antimicrobial peptide-mediated silica condensation using X-ray photoelectron spectroscopy† D. Matthew Eby, * ad Kateryna Artyushkova, * b Anant K. Paravastu c and Glenn R. Johnson d Received 10th February 2012, Accepted 2nd March 2012 DOI: 10.1039/c2jm30837a The antimicrobial peptide KSL (KKVVFKVKFK) mediates the rapid condensation of tetramethyl orthosilicate to form silica nanoparticles. X-ray photoelectron spectroscopy (XPS) was employed to identify the molecular interactions between protein and silica on the surface of nanoparticles containing antimicrobial peptide and silica. Comparative high resolution spectral analysis between KSL peptide and KSL-catalyzed silica nanoparticles revealed that imidates are present in the KSL peptide backbone after silica formation. Supporting evidence for the presence of an imidate is provided by FTIR spectroscopic analysis of the amide I and V bands. XPS analysis also shows that side-chain amines of lysine do not interact with the silica product and the lack of association is supported further by 15 N- 29 Si REDOR NMR. Quantitative analysis of XPS elemental spectra determined the silica O:Si ratio is 3.6 : 1, suggesting that the nuclei (sol) particles are not highly condensed structures. Results were supported using 29 Si CPMAS NMR to show that the majority of the silica is Q 2 groups. A proposed mechanism of rapid silicification with involvement of a peptide imidate is presented. Introduction Controlled synthesis of complex structures at the molecular level is a key objective in nanotechnology. One field of study demonstrating great potential in this area is protein-mediated synthesis of biomineralized nanostructured composites. 1–3 Much to our advantage, Nature has already perfected this method of assembly, where formation of highly ordered structures made of silica, calcium, and other inorganic molecules are a common phenomenon. One prime example is the intricately structured diatom frustules made through the controlled deposition of silica. Hence, the organism’s precise control over shell design is the envy of the nanofabrication scientist. Ultimately, our objective is to understand and exploit this mechanism for prac- tical applications in nanotechnology. Spurred by the discovery of silica-precipitating peptides (silaffins) from the diatom Cylindrotheca fusiformis, 4 many natural and synthetic proteins and peptides have been discovered that mediate rapid silica formation in vitro from silicic acid and silicate alkoxides, such as tetramethyl orthosilicate (TMOS). 5–11 These silicifying peptides vary in length and sequence and many studies have shown that the amino acid side chains affect condensation kinetics, aggregation rates, and silica morphology. 12 It has been shown in many studies that peptide functional groups can affect all aspects of silica formation: initial condensation, polymer aggregation and agglomeration, and the gelation or flocculation of sol particles. Yet, we do not have a complete understanding of the molecular interactions that control these processes. In this work, we utilized X-ray photoelectron spectroscopy (XPS) to probe molecular interactions that occur at the bio- inorganic interface in peptide-mediated silica formation. XPS is a powerful technique that characterizes the elemental and chemical composition of the upper 10 nm of a surface and is an effective tool to characterize protein within composite materials or adsorbed onto surfaces. 13–15 The focus of our study was the synthetic antimicrobial decapeptide KSL (KKVVFKVKFK), 16 which induces precipitation of amorphous silica within seconds from pre-hydrolyzed TMOS in phosphate buffer at pH 8. 6 Total elemental and high-resolution XPS spectra of the peptide were acquired and curve-fitting of each element’s photoemission peak was completed to obtain a quantitative measurement of the peptide’s chemical composition. With a sufficient understanding of KSL structure derived from its XPS spectra, analysis was then completed on KSL-precipitated silica nanoparticles to identify a Universal Technology Corporation, 139 Barnes Dr , Bldg. 1117, Tyndall AFB, FL 32403, USA. E-mail: donald.eby.ctr@us.af.mil; Fax: +850 283 6090; Tel: +850 283 6026 b Chemical and Nuclear Engineering Department, University of New Mexico, Albuquerque, NM, 87131, USA. E-mail: kartyush@unm.edu; Fax: +505 227 5433; Tel: +505 277 0750 c Department of Chemical and Biomedical Engineering, FAMU/FSU College of Engineering, and the National High Magnetic Field Laboratory, Tallahassee, FL, USA. E-mail: paravastu@eng.fsu.edu; Fax: +850 410 6150; Tel: +850 410 6578 d Microbiology and Applied Biochemistry, Materials and Manufacturing Directorate, Air Force Research Laboratory, 139 Barnes Dr, Bldg. 1117, Tyndall AFB, FL 32403, USA. E-mail: glenn.johnson.8@us.af.mil; Fax: +850 283 6090; Tel: +850 283 6223 † Electronic Supplementary Information (ESI) available: Total elemental XPS; AFM images of KSL-Si nanoparticles. See DOI: 10.1039/c2jm30837a This journal is ª The Royal Society of Chemistry 2012 J. Mater. Chem., 2012, 22, 9875–9883 | 9875 Dynamic Article Links C < Journal of Materials Chemistry Cite this: J. Mater. Chem., 2012, 22, 9875 www.rsc.org/materials PAPER Downloaded by University of New Mexico on 21 May 2012 Published on 05 March 2012 on http://pubs.rsc.org | doi:10.1039/C2JM30837A View Online / Journal Homepage / Table of Contents for this issue