Surface SH groups

Since hydrotreating catalysts arc usually used in the presence of H2 and H2S, it is important to understand the influence of phosphorus on the reduction and sulfidation of the supported metal-oxo-specics. It is also important to know whether the phosphates arc sensitive to such treatments. In this section, activation of the catalysts is discussed on the basis of XPS, TPR, and temperature-programmed sulfidation results. Note that the bulk of the alumina support is not chemically modified by the reduction-sulfidation treatments. However, some hydrogen-reactive species and surface SH groups have already been detected on it (31, 70). 

The relative number of CUS and surface SH groups was evaluated from the radioisotope testing of the different catalysts. As a result, the number and the ratio of the two active site types (if any) and their productivity during one thiophene pulse of 7 conversion ratio was determined by expression... 

Data discussed in Sec. 3.1 seem to indicate that HDS activity is associated with sulfur vacancies, as exchange requires sulfur attachment to the surface. Exchange mechanism via surface SH groups was also assumed by Massoth and Zeuthen.I 1... 

Of the 20 residues that react with A-ethylmaleimide in the non-reduced denatured Ca -ATPase at least 15 are available for reaction with various SH reagents in the native enzyme [75,239,310]. These residues are all exposed on the cytoplasmic surface. After reaction of these SH groups with Hg-phenyl azoferritin, tightly packed ferritin particles can be seen by electron microscopy only on the outer surface of the sarcoplasmic reticulum vesicles [143,311-314]. Even after the vesicles were ruptured by sonication, aging, or exposure to distilled water, alkaline solutions or oleate, the asymmetric localization of the ferritin particles on the outer surface was preserved [311,313,314]. 

Monoclonal antibodies to sulfhydryl-terminated PAMAM dendrimers [9,10] could be bound to sulfhydryl-terminated PAMAM dendrimers under stoichiometric conditions favoring complexes of one antibody to one dendrimer. After binding, the exposed sulfhydryl surface may be derivatized to eliminate SH groups. Antibody and dendrimer complexes are then dissociated and the patterned dendrimers are isolated. Only areas masked by the antibody retain sulfhydryl groups, and can therefore be differentially derivatized (to a gold adduct, or to maleimide-linked macromolecules, for instance). 

Abstracting from the complexity of the real systems, there is one common property of all natural particles. Their surfaces contain functional groups which can interact with H+, OH and metal ions and - if Lewis acid sites, e.g., =AI and =Fe, are available on the surface - with ligands. Many inorganic solids (oxidesQand silicates) contain hydroxo groups carbonates and sulfides expose -C-oh,-c oh, MeOH and -SH groups, respectively. While the interaction of alkaline and earth-alkaline ions... 

Prior to about 1981, when the unique species Co(Ni)-Mo-S or SBMS was first identified, mechanisms proposed for HDS envisioned that reactions occurred through interactions between the organosulfur molecule and Mo orbital vacancies on the external surface of MoS2 crystallites or between the organosulfur molecule and peripheral Mo-SH groups, as illustrated in Fig. 23. The olefinic or acetylenic byproducts were assumed to be hydrogenated much faster than the desulfurization reactions occurred thus they may or may not be observed in reaction products. 

Prostatic acid phosphatase is reversibly inactivated by p-mercuri-benzoate and by Cu2+ and Fe3+ (59). In contrast to red cell acid phosphatase, prostatic acid phosphatase is only partially inactivated even after prolonged periods of incubation at high concentrations of p-mercuri-benzoate. Addition of cysteine to the p-mercuribenzoate-treated enzyme produces complete reactivation. Binding of SH groups by p-mercuri-benzoate renders the enzyme more labile to thermal denaturation, but no difference is obtained with surface inactivation (23). Similar partial inactivation with Cu2+ is also subject to reactivation. 

In their natural habitat, proteins are usually surrounded by other proteins and organic factors. When these are removed or diluted as during purification, the protein becomes surrounded by water on all sides. Proteins react differently to a pure aqueous environment many are destabilized and rapidly denatured. A common remedial measure is to add 5%-20% glycerol to the purification buffer. The organic surface of the glycerol is believed to simulate the environment of the protein in the intact cell. Two other ingredients that are most frequently added to purification buffers are mercaptoethanol and ethylenedia-minetetraacetate (EDTA). The mercaptoethanol inhibits the oxidation of protein —SH groups, and the EDTA chelates... 

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