Radio-labelled molecules

In a number of experimental studies of polymer diffusion, molar mass exponents close to 2 have been found, though always with some deviations. For example, using radio-labelled molecules, the diffusion coefficient of polystyrene in dibutyl phthalate was found to follow the relationship... 

Occasionally, a laboratory will need an in-line detector of radio-labeled molecules. These detectors take the flow from the column or from an initial detector, mix it with fluorescing compound, and measure the fluorescence due to radioactive breakdown. A different system uses beads in the flow cell with an immobilized fluorescing compound, but these systems suffer from ghosting and cannot be used with very hot labeled compounds because of secondary radiation problems. These systems are very useful with tritiated samples and less so with carbon14 labeled compounds. Some success has been reported with sulfur32 label detection. 

Nevertheless, desorption can occur over fairly short time scales, e.g., 15 min. It has been shown, for instance by using radio-labeled molecules, that flexible large polymer molecules do exchange between bulk and interface. Another indication is that two molecules of about equal surface... 

One of the older techniques for measuring directly the adsorbed amoimt of surfactant molecules or polymers at liquid interfaces is the radiotracer technique. Its idea is the measurement of the radiation emitted by radio-labelled molecules, adsorbed at an interface (Sally et al. 1950, Flengas Rideal 1959). Because of the background radiation the method yields relative data only. Using equilibrium adsorption isotherms, the dynamics of adsorption can also be followed by the radiotracer method. Experiments were performed with various surfactant systems (Matuura et al. 1958, 1959, 1961, 1962, Tajima 1970, Konya et al. 1973, Muramatsu et al. 1973, Okumura et al. 1974) and adsorbed polymers (Frommer Miller 1966, Adams et al. 1971, Graham Phillips 1979b). Due to the development of more efficient methods the use of this technique has been reduced. 

The emitted gamma photons produced by positron-electron annihilation can be detected using scintillation crystal detectors such as sodiiun iodide (Nal), bismuth germaniiun oxide (BGO) and cerium doped lutetium oxyorthosili-cate (LSO). The short half-life of most positron emitters leads to high specific activity. Only a very small quantity of radio-labeled molecules is thus required, making positron annihilation detection techniques very non-invasive. In fact, practical catalyst studies can be carried out using less than 37 kBq... 

Before the sequencing begins it is necessary to prepare a short primer that is complementary to a sequence at one end of the DNA strand to be sequenced. This may be prepared enzymatically,622 623 or by non-enzymatic synthesis. The short primer is annealed to the end of the DNA and the resulting molecule is incubated with a DNA polymerase and a mixture of the four mononucleotide triphosphates, one of which is radio-labeled in this position. Four reaction mixtures are prepared. Each mixture contains all four nucleoside triphosphates and also one of four different chain-terminating inhibitors, the most popular of which are the 2, 3 -dideoxyribonucleoside triphosphates ... 

For small molecule analytes (see Note 6) for which a radiotracer form is available, sequentially load a known quantity of tracer dissolved in buffer and determine the amount of analyte in the eluant. When the radioactivity not retained by the immunoaffinity column plateaus, the column binding sites are saturated. Wash the column, and elute the retained radioactivity. The mass of analyte in the eluted volume is the apparent column capacity. In many instances a radio-labeled analyte may not be available. In such cases, high-performance liquid chromatography, UV spectroscopy, or any other analytical tool capable of selectively quantifying the analyte may be used to determine column capacity. 

Adsorption can be measured by direct or indirect methods. Direct methods include surface microtome method [46], foam generation method [47] and radio-labelled surfactant adsorption method [48]. These direct methods have several disadvantages. Hence, the amount of surfactant adsorbed per unit area of interface (T) at surface saturation is mostly determined by indirect methods namely surface and interfacial tension measurements along with the application of Gibbs adsorption equations (see Section 2.2.3 and Figure 2.1). Surfactant structure, presence of electrolyte, nature of non-polar liquid and temperature significantly affect the T value. The T values and the area occupied per surfactant molecule at water-air and water-hydrocarbon interfaces for several anionic, cationic, non-ionic and amphoteric surfactants can be found in Chapter 2 of [2]. 

Together with details of sample preparation and storage, an appropriate analytical method of known accuracy, precision, and sensitivity must be available for the quantification of the substance in the test solution and in the biological material. If these are lacking it is impossible to determine a true BCF. The use of radiolabelled test substance can facilitate the analysis of water and fish samples. However, unless combined with a specific analytical method, the total radioactivity measurements potentially reflect the presence of parent substance, possible metabolite(s), and possible metabolized carbon, which have been incorporated in the fish tissue in organic molecules. For the determination of a true BCF it is essential to clearly discriminate the parent substance from possible metabolites. If radiolabelled materials are used in the test, it is possible to analyse for total radio label (i.e. parent and metabolites) or the samples may be purified so that the parent compound can be analysed separately. 

In order to determine the number of histidine residues involved in zinc coordination, the L chains of TeTx and BoNT/A, B and E were modified with diethyl pyrocarbonate (DEPC), a reagent that specifically modifies histidine residues. In each case, two additional histidines were modified in the apo-toxin that were not affected in the holo-neurotoxin (Schiavo etal., 1992 b, c). These results indicate that the zinc atom of CNTs is coordinated via two histidines and a Glu-bound water molecule, as in thermolysin. Mutations at the two histidines of the motif inactivate TeTx and suppress its ability to bind radio-labeled Zn " (Yamasaki etal., 1994 b). In addition, mutations of the conserved Glu-271 and Glu-272 of TeTx, predicted to be in an a-helical segment (Lebeda and Olson, 1994), result in decreased zinc binding and loss of activity. Based on these experimental results, it has... 

---