Cell-transfer limiting dilution experiment

The cell transfer limiting dilution experiments were initiated when T and B cell cooperation was not yet discovered, but in most of the experiments care was taken to ensure that only one kind of cell was limiting (Brown et al., 1966 Shearer et al., 1968 Bosma and Weiler, 1970). In recent years the method has been adapted to estimate the frequencies separately for B cells... 

Examination of the behaviour of a dilute solution of the substrate at a small electrode is a preliminary step towards electrochemical transformation of an organic compound. The electrode potential is swept in a linear fashion and the current recorded. This experiment shows the potential range where the substrate is electroactive and information about the mechanism of the electrochemical process can be deduced from the shape of the voltammetric response curve [44]. Substrate concentrations of the order of 10 molar are used with electrodes of area 0.2 cm or less and a supporting electrolyte concentration around 0.1 molar. As the electrode potential is swept through the electroactive region, a current response of the order of microamperes is seen. The response rises and eventually reaches a maximum value. At such low substrate concentration, the rate of the surface electron transfer process eventually becomes limited by the rate of diffusion of substrate towards the electrode. The counter electrode is placed in the same reaction vessel. At these low concentrations, products formed at the counter electrode do not interfere with the working electrode process. The potential of the working electrode is controlled relative to a reference electrode. For most work, even in aprotic solvents, the reference electrode is the aqueous saturated calomel electrode. Quoted reaction potentials then include the liquid junction potential. A reference electrode, which uses the same solvent as the main electrochemical cell, is used when mechanistic conclusions are to be drawn from the experimental results. 

Purely hydrophobic compounds partition easily into bilayers. Because of their low water solubility, however, it is difficult to perform titration experiments. The only possible experimental approach is the titration of a lipid vesicle suspension into a very dilute aqueous solution of the hydrophobic compound in the cell. Many experiments have been perfonned with simple model compounds, such as indole derivatives using dialysis methods [115]. The enthalpy of incorporation was then detennined from the temperature dependence of the partition coefficient. This is inherently less precise than a direct calorimetric determination. On the other hand, when the concentrations of the hydrophobic molecules are small, then a detennination of the partition coefficients becomes difficult. The transfer enthalpies are then still accessible, though with limited precision. 

---