Small‐Angle Scattering of S‐Layer Metallization

Abstract

Template-directed materials synthesis is an effective tool to meet the challenge of controlling the structure of advanced materials at the nanometer-scale. Inspired by the process of biomineralization, preformed organic matrices may be employed to direct the nucleation and growth of inorganic nanoparticles.[1] In this way, synthetic colloidal self-assemblies [2] as well as natural (biomolecular) structures such as DNA [3][4][5] or viruses [6,7] can be used as templates for the formation of nanoparticles. Crystalline bacterial cell surface layers (S layers), which represent one of the most common cell-envelope components of bacteria and archaea, [8] have been proven to be a suitable biomolecular template for the formation of regular two-dimensional semiconducting [9,10] and metal [11][12][13] nanoparticle arrays. Recent investigations on the electronic structure of S layers revealed a semiconductor-like behavior with an energy gap value of ca. 3.0 eV and a Fermi energy close to the bottom of the lowest unoccupied molecular orbital, [14] which will open up new possibilities for the electric addressability of biotemplated nanoparticle arrays. The present communication provides a new insight into the metallization of S-layer proteins with regard to the interplay between the metal particles and the protein template. Small-angle scattering of X-rays (SAXS) [15] enables the direct study of the metallization process in vitro without any further sample preparation steps such as staining, adsorption on a surface, or drying as is usually necessary for TEM (transmission electron microscopy) and SFM (scanning force microscopy). Hence, it is possible to characterize an intermediate state during the nucleation of the particles, the so-called activation, that is the binding of metal salt complexes onto the S layers prior to their chemical reduction. Moreover, small-angle neutron scattering (SANS) with contrast variation [16] allows for the simultaneous investigation of the metal particles and the underlying protein template, enlightening the changes in the biological template due to the metallization process. Isolated and recrystallized S layers of Bacillus sphaericus NCTC 9602 are used here as templates for the wet chemical formation of Pd clusters. Figure 1 shows a transmission electron micrograph of the negatively stained protein with the pores appearing dark. The inset, obtained from image analysis with FFT (fast Fourier transform), reflects the p4 symmetry of the two-dimensional protein crystal with a lattice constant of 12.5 ± 0.8 nm. By combining this most commonly used technique for the structural characterization of S layers [17][18][19] with SAXS, the characterization of the unstained protein in solution is possible. The lowermost curve (filled circles) in Figure 2a shows the SAXS signal of the native S layer in aqueous solution. The indicated diffraction peaks correspond to a square lattice structure. For the lattice constant, a value of 12.9 ± 0.2 nm is observed, which is in fair agreement with the TEM result of 12.5...

Biophysical Methods for the Elucidation of the S-Layer Proteins/Metal Interaction

“…A 2-layer sandwich architecture has been observed for the N-terminal domain of the C. difficile SLP [96], three triple-helix bundles for the N-terminal domain of the SbsC SLP from G. stearothermophilus [97], and a β-propeller fold for a putative SLP of the archaeal Methanosarcina acetivorans [98] and the Gram-negative bacteria Bacteroides uniformis [99]. This technique has also been used to analyze the formation of Pd nanoparticles on crystalline bacterial S-layers, showing the response of the S-layer lattice to the loading with Pd complexes and metallic Pd particles as well as the effects of two different reducing agents (H 2 and dimethylaminoborane) on the nanoparticle arrays [100].…”

Catalytic Carbon Monoxide Oxidation Using Bio-Templated Platinum Clusters

“…The possibility to reconstitute isolated S-layer monomers in vitro into two-dimensional (2D) arrays with perfect uniformity [5,6] makes them an almost ideal biological template for supra-molecular engineering. Taking advantage of their spatially well-defined physical and chemical surface properties, S layers have extensively been used for the template-directed chemical synthesis of regular 2D semiconducting [7,8] and metallic [9][10][11][12][13][14] nanoparticle arrays. The specific property that their regular 2D structure is preserved under dry conditions [14] has recently allowed using S layers as templates for the organization of gasphase deposited FePt nanoparticles into 2D arrays [15][16][17].…”

“…Taking advantage of their spatially well-defined physical and chemical surface properties, S layers have extensively been used for the template-directed chemical synthesis of regular 2D semiconducting [7,8] and metallic [9][10][11][12][13][14] nanoparticle arrays. The specific property that their regular 2D structure is preserved under dry conditions [14] has recently allowed using S layers as templates for the organization of gasphase deposited FePt nanoparticles into 2D arrays [15][16][17]. In addition, the electronic structure of S layers has been investigated by photoemission and NEXAFS spectroscopy [18][19][20][21][22].…”

Small-Angle X-Ray Scattering for Imaging of Surface Layers on Intact Bacteria in the Native Environment

“…In the case of interaction of the proteins, such as in lattices of noncovalently linked S-layer proteins, the structure factor value differs from 1, which considerably complicates procedures to gain information on the system. With regard to S-layers, SAXS has been used to describe the wet chemical metallization of S-layers under different chemical conditions (21). There, an approach was chosen to model the scattering intensity at low values of the scattering vector q by a mixture of mono-and bilayers and the scattering intensity at high q values by a square lattice as concluded from reconstruction of TEM images (21).…”

“…With regard to S-layers, SAXS has been used to describe the wet chemical metallization of S-layers under different chemical conditions (21). There, an approach was chosen to model the scattering intensity at low values of the scattering vector q by a mixture of mono-and bilayers and the scattering intensity at high q values by a square lattice as concluded from reconstruction of TEM images (21). Furthermore, protein-protein interaction within the S-layer was taken into account by combining molecular dynamics simulations and refining the model with experimental SAXS data (22), where the subunit also was modeled on the amino acid level (23).…”