Capping/decapping

When the number of metal atoms in a cluster increases, the geometries of the clusters become more complex, and some are often structurally better described in terms of capped or decapped polyhedra and condensed polyhedra. For example, the first and second clusters listed in Table 19.4.3 are a capped octahedron and a bicapped octahedron, respectively. Consequently, capping or decapping with a transition-metal fragment to a deltapolyhedral cluster leads to an increase or decrease in the cluster valence electron count of 12. When a transition-metal atom caps a triangular face of the cluster, it forms three M-M bonds with the vertex atoms, so according to the 18-electron rule, the cluster needs an additional 18 - 6 = 12 electrons. The parent octahedron of [Os6(CO)is]2- has g = 86, the monocapped octahedron Osy(CO)2i has g = 98, and the bicapped octahedron [Oss(CO)22]2- hasg = 110. 

Generally, capped or decapped deltapolyhedral clusters are characterized by the number of skeletal valence electrons g... 

A classical example of correlation of structure with valence electron count of transition-metal clusters is shown in Fig. 19.4.2. There the structures of a series of osmium clusters are systematized by applying the capping and decapping procedures. 

Decapping station An area or device in the automated system in which specimen caps or stoppers are automatically removed and discarded into a waste container. 

A FIGURE 12-29 Pathways for degradation of eukaryotic mRNAs. In the deadenylation-dependent middle) pathways, the poly(A) tail is progressively shortened by a deadenylase (orange) until it reaches a length of 20 or fewer A residues at which the interaction with PABPI is destabilized, leading to weakened interactions between the 5 cap and translation initiation factors. The deadenylated mRNA then may either (1) be decapped and... 

As shown in Figure 12-29, some mRNAs are degraded in pathways that do not involve significant deadenylation. In one of these, mRNAs are decapped before the poly(A) tail is shortened extensively. It appears that certain mRNA sequences make the cap sensitive to the decapping enzyme, but the precise mechanism is unclear. In the other alternative pathway, mRNAs first are cleaved internally by endonucleases. The RNA-induced silencing complex (RISC) discussed earlier is an example of such an endonuclease (see Figure... 

In a review of pyramidal carbon species [18], Rasmussen and Radom approach these from the viewpoint of their potentially planar carbon molecirles [35,36] from which one cap has been removed (Fig. 2.11). The resulting molecides are called hemialkaplanes and hemispiroalkaplanes. The former have a neopentane moiety capped by a cycloalkane, the latter a spiropentane moiety capped by a cycloalkarre. Molecules representing decapped alkaplanes do not have truly pyrarrridal carbon, as pointed out above for bowlane. However, decapping spiroalkaplanes can lead to genuine pyramidal-carbon stractures in particular, the iconic pyranridane. 

To prepare TFAA derivatives, 100 pL of TFAA should be added to the dried sample in a 2-mL vial. The vial should be immediately capped, vortexed for 2 min, and heated for 20 min in a heating block at bO C. After that time, the vial should be removed from the heating block and cooled, then decapped, and TFAA should be removed very carefully (to avoid the derivatized toxins losses) under a delicate stream of nitrogen. When evaporated, 1 mL of isooctane should be added, vortexed, and analyzed by GC/MS. 

Fig. 2M. Application of the decapping and capping principles to osmium carbonyl clusters, pn with permission from Mingos DMP, May AS (1990) In Shriver DF, Kaesz HD, Adams...

Note that the 5 -end labeling of most eukaryotic mRNAs requires prior removal of the cap structure. T4 PNK is reported to have the capacity to decap, generating the m G(5 )pp and an mRNA with a 5 -P terminus (33). Alternatively, the cap can be removed by treatment with tobacco acid pyrophosphatase (34). 

Figure 13.16 Illustration of a cap-ping/decapping procedure. After growth of a compound substrate, the surface is protected by an amorphous capping layer, for example, of arsenic, which allows contamination-free...

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