Kinetics SAMs

A system has been constructed which allows combined studies of reaction kinetics and catalyst surface properties. Key elements of the system are a computer-controlled pilot plant with a plug flow reactor coupled In series to a minireactor which Is connected, via a high vacuum sample transfer system, to a surface analysis Instrument equipped with XFS, AES, SAM, and SIMS. When Interesting kinetic data are observed, the reaction Is stopped and the test sample Is transferred from the mlnlreactor to the surface analysis chamber. Unique features and problem areas of this new approach will be discussed. The power of the system will be Illustrated with a study of surface chemical changes of a Cu0/Zn0/Al203 catalyst during activation and methanol synthesis. Metallic Cu was Identified by XFS as the only Cu surface site during methanol synthesis. 

One of the most compelling targets in the polyamine biosynthesis pathway has been S-adenosylmethionine decarboxylase (SAM-DC). This target was chemically validated with the discovery of trypanocidal activity of MDL-73811 nearly two decades ago. Work to understand the unique kinetics for inhibition of this enzyme in T. brucei has shown that a catalytically... 

The advantages of SAMs are that they are sturdily anchored at a fixed distance from the metal substrate, may be more robust than Langmuir films, and can be convenient to prepare. A disadvantage is that uniform monolayer coverage, so easily achieved kinetically for LB films, is more difficult to obtain in SAMs. This is because SAMs are created by random attack on the electrode surface, in contrast to Langmuir films, which are transferred when they are close-packed. 

In their study, Park et al.100 investigated the frictional properties of fluorine-terminated alkanethiol SAMs grafted to gold surfaces. The frictional properties of the system were investigated by sliding two SAMs past one another at velocities in the stick-slip regime under various external loads. The simulations yield the shear stress as and the kinetic friction coefficient pk can be estimated from the slope of a plot of as versus load, using the relationships contained in Eqs. [4] and [5]. 

Addressing the kinetics of UPD it is obvious from Figure 5.16, which compares CVs of Cu UPD of a bare gold electrode with SAM-coated ones, that the typical UPD features are completely quenched by SAMs [183]. However, even when a CV lacks any features indicating UPD this is by no means a sign that it does not occur. It is rather a question of time scale [40, 43, 204]. Therefore, caution is required when comparing results, in particular, if different techniques working on different time scales such a... 

The process of formation of SAMs on the various substrates strongly depends on the nature of all three moieties of the surface active molecule. If a suitable mesogen is chosen, the affinity of the head group toward the substrate determines the kinetics of the physisorption and chemisorption as well as the stability of the resulting layer. 

Auer et al. [134] presented an example for multilayer formation and controlled deposition of functionalized nanoparticles on SAM of mercaptohexadecanoic acid (MHA) using electrostatic interactions. As a pH-sensitive switchable linker between the SAM of MHA and negatively charged gold nanoparticles, bis-benzami-dine bolaamphiphiles having different alkyl spacers were used [135]. This strategy resulted in a potentially tunable and switchable property of the entire assembly. For example, the kinetics of adsorption as well as the final particle layer thickness can be controlled by the kind of bis-benzamidine used as the linker (Fig. 9.16). 

In this pull-down assay, the enzymatic reaction is carried out completely in solution. Samples taken from the reaction mixture are then transferred to a SAM-modified MALDI target, on which the remaining substrate and the reaction product are selectively immobilized. Subsequent to the extraction of the analytes, the target is rinsed, treated with matrix, and MALDI-MS analysis is carried out. A major advantage of this assay scheme is that the inherent danger of negative influences on the reaction kinetics, which may be caused by immobilization of the substrate as in standard SAMDI-MS-based assay formats, is circumvented. Additionally, by selective extraction of the analytes of interest and removal of the other... 

Since high kinetic energies of the impinging metal atoms or clusters can have deleterious effects, including damage to the SAMs, accelerating undesired... 

Thus, the quality of the SAM does play a role in the overall growth kinetics and resulting film thickness. The role of chain transfer has not been quantified but it clearly does not prevent photo-SlP. 

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