Au-Interaction of Slp1 Polymers and Monolayer from Lysinibacillus sphaericus JG-B53 - QCM-D, ICP-MS and AFM as Tools for Biomolecule-metal Studies.

In this publication the gold sorption behavior of surface layer (S-layer) proteins (Slp1) of Lysinibacillus sphaericus JG-B53 is described. These biomolecules arrange in paracrystalline two-dimensional arrays on surfaces, bind metals, and are thus interesting for several biotechnical applications, such as biosorptive materials for the removal or recovery of different elements from the environment and industrial processes. The deposition of Au(0) nanoparticles on S-layers, either by S-layer directed synthesis or adsorption of nanoparticles, opens new possibilities for diverse sensory applications. Although numerous studies have described the biosorptive properties of S-layers, a deeper understanding of protein-protein and protein-metal interaction still remains challenging. In the following study, inductively coupled mass spectrometry (ICP-MS) was used for the detection of metal sorption by suspended S-layers. This was correlated to measurements of quartz crystal microbalance with dissipation monitoring (QCM-D), which allows the online detection of proteinaceous monolayer formation and metal deposition, and thus, a more detailed understanding on metal binding. The ICP-MS results indicated that the binding of Au(III) to the suspended S-layer polymers is pH dependent. The maximum binding of Au(III) was obtained at pH 4.0. The QCM-D investigations enabled the detection of Au(III) sorption as well as the deposition of Au(0)-NPs in real-time during the in situ experiments. Further, this method allowed studying the influence of metal binding on the protein lattice stability of Slp1. Structural properties and protein layer stability could be visualized directly after QCM-D experiment using atomic force microscopy (AFM). In conclusion, the combination of these different methods provides a deeper understanding of metal binding by bacterial S-layer proteins in suspension or as monolayers on either bacterial cells or recrystallized surfaces.