Stability of sub-surface oxygen at Rh(111)

Using density-functional theory (DFT) we investigate the incorporation of oxygen directly below the Rh(111) surface. We show that oxygen incorporation will only commence after nearly completion of a dense O adlayer (\theta_tot = 1.0 monolayer) with O in the fcc on-surface sites. The experimentally suggested octahedral sub-surface site occupancy, inducing a site-switch of the on-surface species from fcc to hcp sites, is indeed found to be a rather low energy structure. Our results indicate that at even higher coverages oxygen incorporation is followed by oxygen agglomeration in two-dimensional sub-surface islands directly below the first metal layer. Inside these islands, the metastable hcp/octahedral (on-surface/sub-surface) site combination will undergo a barrierless displacement, introducing a stacking fault of the first metal layer with respect to the underlying substrate and leading to a stable fcc/tetrahedral site occupation. We suggest that these elementary steps, namely, oxygen incorporation, aggregation into sub-surface islands and destabilization of the metal surface may be more general and precede the formation of a surface oxide at close-packed late transition metal surfaces.Comment: 9 pages including 9 figure files. Submitted to Phys. Rev. B. Related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm

Studying protein–protein affinity and immobilized ligand–protein affinity interactions using MS-based methods

This review discusses the most important current methods employing mass spectrometry (MS) analysis for the study of protein affinity interactions. The methods are discussed in depth with particular reference to MS-based approaches for analyzing protein–protein and protein–immobilized ligand interactions, analyzed either directly or indirectly. First, we introduce MS methods for the study of intact protein complexes in the gas phase. Next, pull-down methods for affinity-based analysis of protein–protein and protein–immobilized ligand interactions are discussed. Presently, this field of research is often called interactomics or interaction proteomics. A slightly different approach that will be discussed, chemical proteomics, allows one to analyze selectivity profiles of ligands for multiple drug targets and off-targets. Additionally, of particular interest is the use of surface plasmon resonance technologies coupled with MS for the study of protein interactions. The review addresses the principle of each of the methods with a focus on recent developments and the applicability to lead compound generation in drug discovery as well as the elucidation of protein interactions involved in cellular processes. The review focuses on the analysis of bioaffinity interactions of proteins with other proteins and with ligands, where the proteins are considered as the bioactives analyzed by MS

Coverage dependent desorption kinetics of CO from Rh(lll) using time-resolved specular helium scattering

Linearized measurements of the coverage dependent desorption rates of CO from Rh( 111) were made with a novel three molecular beam apparatus. To measure these isothermal and essentialy isosteric rates we introduce a new kinetic response ampl$er, namely time-resolved specular helium scattering, which makes use of the large attenuation cross section that CO has for specular helium scattering. The measurements are made by using one intense and continuous CO beam to establish a specific adsorbate coverage while another low intensity and chopped CO beam is used to weakly modulate the adsorbate density around the selected steady state. The transient helium reflectivity waveforms we measure during the modulated CO scattering contain the desired kinetic information, and are typically one to almost two orders of magnitude more sensitive to the desorption kinetics than are the signals arising from direct detection of desorbing CO. Desorption rates are reported for 0<0,, ~0.22 and 440 K <T, < 555 K. He diffraction measurements revealed that the CO overlayer was disordered for all conditions for which kinetics were measured, and that the sticking coefficient varied with coverage as S, ( 1 -38). At least a second order expansion of the chemical potential in terms of CO coverage was needed to explain these rates. The experimentally determined desorption rates can be fit equally well by placing the coverage dependence in either the pre-exponential factor or in the activation energy: k(O,T,) = 1.33X lOI exp [0.3448 + 48.88' -(32.3 s-'. The measured He diffraction data, sticking coefficient, specular He scattering attenuation vs CO coverage, and increase in CO desorption rate with increasing coverage imply nearest-neighbor repulsive interactions. The use of time-resolved specular He scattering for studying coverage dependent reactions is also discussed

Characterization of Grafting Density and Binding Efficiency of DNA and Proteins on Gold Surfaces

The surface grafting density of biomolecules is an important factor for quantitative assays using a wide range of biological sensors. We use a fluorescent measurement technique to characterize the immobilization density of thiolated probe DNA on gold and hybridization efficiency of target DNA as a function of oligonucleotide length and salt concentration. The results indicate the dominance of osmotic and hydration forces in different regimes of salt concentration, which was used to validate previous simulations and to optimize the performance of surface-stress based microcantilever biosensors. The difference in hybridization density between complementary and mismatched target sequences was also measured to understand the response of these sensors in base-pair mismatch detection experiments. Finally, two different techniques for immobilizing proteins on gold were considered and the surface densities obtained in both cases were compared