Sandwich-type packings

The three-dimensional structure of bovine pancreatic DNase I was first determined st 3,5 A, rssolstics by x- ay tEfTisction from single crystals [23], DNase I proved to be a compact a,fJ-pn tin with two sixstranded sheets packed against each other forming the cove of a sandwich type structure. Tbe... 

The binding assays that are performed in this flow injection system may be either competitive or sandwich type. An assay for the serum protein transferrin using a competitive ELISA (129) illustrates the principle (Fig. 5). An immunoafiinity purified polyclonal antiserum raised in rabbits against human transferrin is the binder. The immobilized antibody is packed in a small column (100-200 pL), which is placed in a continuous flow of buffer. The experimental setup is shown in Fig. 6. 

In principle, there are no major differences between solid-phase extraction and liquid-liquid extraction, but SPE can avoid or reduce some of the disadvantages of liquid-liquid extraction. Thus, SPE can handle small samples and very dilute solutions, it overcomes the formation of emulsions, and can be easily automated. Furthermore, the sorbents that are commonly used are commercially available as cartridges. These sorbents are alumina, silica gel, reversed-phase silica gel, and various ion-exchange resins. It is also possible to pack different adsorbents in layers inside the same cartridge to give a sandwich-type extraction column. 

Figure 8.52 A packing diagram of 25 viewed down the a axis shows the sandwich-type structure. Two independent [(n-C4Hg)4N] cations are accommodated between the corrugated layers, each being represented by large shaded circles...

The TCNQ molecule in [TR(bzim)]2-TCNQ is sandwiched between two units of [ J,-N, C -bzimAu]3 in a face-to-face manner so that it is best represented by the formula (7t-[ J,-N, C -bzimAu]3)( j,-TCNQ)(7t-[p-N, C -bzimAu]3). The cyanide groups clearly are not coordinated to the gold atoms. The distance between the centroid of TCNQ to the centroid of the AU3 unit is 3.964 A. The packing of [TR(bzim)]2-TCNQ shows a stacked linear-chain structure with a repeat pattern of-(Au3)(Au3)(p-TCNQ) (Au3)(Au3)(p-TCNQ)- an ABBABB repeat The complex [TR(bzim)]2-TCNQ contains two very short intermolecular Au Au distances of 3.152 A (identical for the two aurophilic bonds). The intermolecular Au Au distance is even shorter than the intramolecular distances in the starting compound, which are 3.475, 3.471, and 3.534 A. The adjacent AU3 units in [TR(bzim)]2-TCNQ form a chair-type structure rather than the face-to-face (nearly eclipsed) pattern reported in Balch s studies of the nitro-9-fluorenones adducts with the trinuclear Au(I) alkyl-substituted carbeniate complexes. 

Cartridge columns consist of 0.5-1 g of 40- m particles sandwiched between 30-jUm frits. The column s body may be a tube shrunk around a sandwich of bed support frits and packing. In another type, the frits and packing may be pushed into a small syringe barrel. The 0.5-g SFE has a sample capacity of about 25 mg and a void volume of about 1.5 mL. 

Another type of disk, SPEC made by Ansys, Inc., uses a glass-fiber matrix rather than Teflon to hold the sorbent particles. This disk has a somewhat more rapid flow rate and is more rigid and thicker than the Teflon disk. There is another disk called the Speedisk made by J. T. Baker that consists of lO-pm packing material that is sandwiched between two glass-fiber filters without any type of Teflon binder. The various types of disks are discussed in detail in Chapter 11. 

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