Electronic structure of bacterial surface protein layers

We report an approach for the calculation of the electronic density of states of the dried two-dimensional crystalline surface protein layer ($S$ layer) of the bacterium Bacillus sphaericus NCTC 9602. The proposed model is based on the consideration of individual amino acids in the corresponding conformation of the peptide chain which additively contribute to the electronic structure of the entire protein complex. The derived results agree well with the experimental data obtained by means of photoemission (PE), resonant PE, and near-edge x-ray absorption spectroscopy.

Figure 1

(Color online) Schematic viewgraphs of the structure of the peptide chain (left) and of the extended building-block model (right).

Figure 2

(Color online) Band structure of the $\beta$-sheet $(7\bullet \mathrm{GLY}+\mathrm{CYS})$ conformation (left) and total DOS (right) of a cysteine-doped polyglycine chain in three different conformations (see text) compared with the molecular levels and orbitals of the free methyl mercaptan $\left(\mathrm{C}{\mathrm{H}}_3\mathrm{S}\mathrm{H}\right)$ molecule shown in the middle panel.

Figure 3

(Color online) Experimentally determined electronic structure of the $S$ layer of $B$. sphaericus NCTC 9602 in comparison with the calculated total DOS and (a) its projections on different parts of the protein structure or (b) contributions of the charge densities located around the C and O atoms (“C-O bonds,” solid line in the inset) as well as around the C and N atoms (“C-N bonds,” dashed lines) of the backbone.

Figure 4

(Color online) Experimental spectra of the $S$ layer of $B$ sphaericus NCTC 9602. Main panel: $\mathrm{C}1s$ NEXAFS. Inset: resonant PE spectra taken with photon energies labeled in the NEXAFS plot (Ref. 3). The $\mathrm{C}1s$ core-level contribution excited with the second-order light is depicted by light color and marked additionally with a filled triangle.