Investigation of Molecular Scale Surface Organization of Small Peptides Sorbed On Montmorillonite Using Synchrotron-Based Polarization-Dependent X-Ray Absorption near Edge Photoemission Electron Microscopy (X-PEEM).

Our lack of understanding of the micro-scale processes/mechanisms of interactions between soil minerals and soluble organic nitrogen (SON) compounds hinders our ability to predict N flux between different soil compartments at ecosystem-scale. It has been observed that SON-mineral interactions play an important role in regulating the bioavailability of SON to microorganisms and plants. Up to date, little is known about the nature of those interactions at molecular scale, especially regarding the molecular surface organization (molecular orientation and spatial distribution). Molecular surface organization may largely affect the reactivity, stability, and bioavailability of the adsorbed molecules, as well as modify the surface property of SON-minerals. Mono-layer montmorillonite was deposited on silicon wafer using the Langmuir–Blodgett (LB) method, followed by mono-layer sorption of target small peptide at three pH levels. The N (1s), C (1s), Al (1s), and K-edge NEXAFS image sequence scans were collected on the SM Beamline at the Canadian Light Source. The Al (1s) NEXAFS images provided locations of montmorillonite coated on the silicon wafer. Both N (1s) and C (1s) NEXAFS spectra and images provided information on spatial distribution of target peptide molecules on montmorillonite surfaces. The degree of surface orientation of sorbed peptide molecules was studied using polarization dependence experiment by applying the linear inclined polarized X-ray parallel and perpendicular to three specifically selected regions of mineral surface at different energies. This study demonstrated that hexa-glycine molecules are sorbed on montmorillonite surface at 70^o angle. The degree of surface orientation decreased with decreasing length of glycine peptides. This investigation provided, for the first time, direct molecular level information on how small peptides behave on mineral surface.