The adsorption of methanethiol and n-propanethiol on the Au(111) surface has been studied by temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and low-temperature scanning tunneling microscopy (LT-STM). Methanethiol desorbs molecularly from the chemisorbed monolayer at temperatures below 220 K in three overlapping desorption processes. No evidence for S-H or C-S bond cleavage has been found on the basis of three types of observations: (1) A mixture of chemisorbed CH3SD and CD3SH does not yield CD3SD, (2) no sulfur remains after desorption, and (3) no residual surface species remain when the adsorbed layer is heated to 300 K as measured by STM. On the other hand, when defects are introduced on the surface by ion bombardment, the desorption temperature of CH3SH is extended to 300 K and a small amount of dimethyl disulfide is observed to desorb at 410 K, indicating that S-H bond scission occurs on defect sites on Au(111) followed by dimerization of CH3S(a) species. Propanethiol also adsorbs nondissociatively on the Au(111) surface and desorbs from the surface below 250 K.

The angular and velocity distributions of desorbing products N2 and CO2 were studied in a steady-state NO + CO reaction on Pd(110) and Rh(110) by cross-correlation time-of-flight techniques. The CO2 desorption sharply collimated along the surface normal on both surfaces. On the other hand, N2 desorption on Pd(110) sharply collimated along about 40° off the surface normal in the plane along the [001] direction below around 650 K, yielding a translational temperature of about 3600 K. At higher temperatures, the normally directed desorption was relatively enhanced. On Rh(110), desorbing N2 sharply collimated along the surface normal, yielding a translational temperature of about 2500 K. The inclined desorption was assigned to the decomposition of the intermediate, N2O(a) → N2(g) + O(a), and the normally directed component was proposed to be due to the associative desorption of adsorbed nitrogen atoms, 2N(a) → N2(g). The branching of these pathways was analyzed on Pd(110).

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The decomposition of N 2 O(a) was studied on Rh(110) at 95-200 K through the analysis of the angular distributions of desorbing N 2 by means of angle-resolved thermal desorption. N 2 O(a) was highly decomposed during the heating procedures, emitting N 2 (g) and releasing O(a). N 2 desorption showed four peaks, at 105-110 K (β 4 -N 2 ), 120-130 K (β 3 -N 2 ), 140-150 K (β 2 -N 2 ), and 160-165 K (β 1 -N 2 ). The appearance of each peak was sensitive to annealing after oxygen adsorption and also to the amount of N 2 O exposure. The β 1 -N 2 peak was major at low N 2 O exposures and showed a cosine distribution. On the other hand, β 2 -N 2 and β 3 -N 2 on an oxygen-modified surface revealed inclined and sharp collimation at around 30°off the surface normal in the plane along the [001] direction, whereas β 4 -N 2 on a clean surface collimated at around 70°off the surface normal, close to the [001] direction. An inclined or surface-parallel form of adsorbed N 2 O was proposed as the precursor for inclined N 2 desorption.

A growing number of studies suggest that the formation of toxic oligomers, precursors of amyloid fibrils, is initiated at the cell membrane and not in the cytosolic compartments of the cell. Studies of membrane-induced protein oligomerization are challenging due to the difficulties of probing small numbers of proteins present at membrane surfaces. Here, we employ surface-sensitive vibrational sum frequency generation (VSFG) to investigate the secondary structure of lysozyme at the surface of lipid monolayers. We investigate lysozyme aggregation at negatively charged 1,2-dipalmitoyl-sn-glycero-3-(phospho-rac-1-glycerol) (DPPG) lipid monolayers under different pH conditions. The changes in the molecular vibrations of lipids, proteins, and water as a function of pH and surface pressure allow us to simultaneously monitor details of the conformation state of lysozyme, the organization of lipids, and the state of lipid-bound water. At pH = 6 lysozyme induces significant disordering of the lipid layer, and it exists in two states: a monomeric state with a predominantly α-helix content and an oligomeric (za-mer) state. At pH ≤ 3, all membrane-bound lysozyme self-associates into oligomers characterized by an antiparallel β-sheet structure. This is different from the situation in bulk solution, for which circular dichroism (CD) shows that the protein maintains an α-helix conformation, under both neutral and acidic pH conditions. The transition from monomers to oligomers is also associated with a decreased hydration of the lipid monolayer resulting in an increase of the lipid acyl chains ordering. The results indicate that oligomerization requires cooperative action between lysozyme incorporated into the lipid membrane and peripherally adsorbed lysozyme and is associated with the membrane dehydration and lipid reorganization. Membrane-bound oligomers with antiparallel β-sheet structure are found to destabilize lipid membranes.

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Reviews of recent progress in angle-resolved measurements of desorbing surface reaction products are discussed. The angular and velocity distributions of desorbing products deliver information about the reaction site as well as the reaction mechanism when the products are repulsively desorbed. These distribution measurements can yield symmetry and orientation information of the reaction site for associative processes whereas, in dissociative desorption, the collimation of fragment desorption is related to the orientation of the intermediate species immediately before dissociation. These different collimations provide information on desorption steps whenever any step becomes rate determining.