Binding sodium chloride

A receptor for solvent-separated ion pairs is receptor 10, a combination of a dibenzo-18-crown-6 and abridging 1,3-phenyldicarboxamide. In the presence of 1 mol equivalent of metal cation, chloride affinities are enhanced in the following order K+ (ninefold enhancement), Na+ (eightfold enhancement), and Cs" " (no enhancement). An X-ray crystal structure shows that the receptor binds sodium chloride as a solvent-shared ion pair (Figure 8). 

The theory and application of this fluorescence method have been discussed in detail by LePecq and others (3,8). The assay requires that there is sufficient ionic strength to minimize ionic binding (e.g., O.IM sodium chloride), that the pH is 4-10, that no heavy metals are present, that the fluorescence is not enhanced on binding to other excipients (e.g., proteins) and that at least portions of the nucleic acids are not complexed. These requirements can usually he met when dealing with recombinant products in some cases the samples must he manipulated to create the appropriate conditions. In the intercalative method of dye binding, proteins rarely interfere with the assay, and procedures have been developed to remove the few interferences they may cause (e.g., the use of heparin or enzymatic digestion of the protein 9). 

The number of binding sites can be determined in this model by a plot of d Ink /dlnm at constant temperature, pH, and ion valency. To do that, it may be assumed that dlny /dlnm is approximately zero. The actual value is -0.04 for 0.1 to 0.5 M sodium chloride and less at lower concentrations. To a first approximation, the stoichiometry of water molecules released by binding protein could be determined from the slope of the plot of dink /dlnm vs. m. However, especially at low salt concentration and near the isoelectric point, the slope of such plots is nonlinear. The nonlinearity may be due to hydrophobic interaction between stationary phase and protein or a large change of ionic hydration on binding.34... 

Urokinase utilized medically is generally purified directly from human urine. It binds to a range of adsorbents, such as silica gel and, especially, kaolin (hydrated aluminium silicate), which can be used initially to concentrate and partially purify the product. It may also be concentrated and partially purified by precipitation using sodium chloride, ammonium sulfate or ethanol as precipi-tants. 

Sodium chloride and other soluble ionic solids dissolve in polar solvents such as water because of ion-dipole forces. An ion-dipole force is the force of attraction between an ion and a polar molecule (a dipole). For example, NaCl dissolves in water because the attractions between the Na and Cl ions and the water molecules provide enough energy to overcome the forces that bind the ions together. Figure 4.14 shows how ion-dipole forces dissolve any type of soluble ionic compound. 

Sometimes the easiest way to destroy drug-protein binding is to dilute the sample with a physiological saline solution. For instance, for isoxazolyl penicillins with a binding percentage of over 95%, dilution with 9 volumes of 0.9% sodium chloride solution can be sufficient (9). Alternative widely used techniques are based on either protein denaturation or enzymatic/chemical hydrolysis of the drug-protein complexes (10-12). 

The increase in Tc depended on metal ions, as shown in Fig. 5. A large temperature increase was observed for potassium chloride, while the increase was only 1.5 °C for sodium chloride. The temperature increase was not observed by the addition of lithium and cesium chlorides. The binding affinity of a crown ether with metal ions depends on the cavity size [16]. When the ion diameter fits in the cavity size, the ion is captured by the crown ether. The cavity size of benzo[18]crown-6 is known to accommodate the diameter of K+ [17]. The relative Tc increase correlates well with the binding affinity of benzo[18]crown-6 to metal ions. K+ which efficiently binds to pendant benzo[18]crown-6 most pronouncedly increased Tc, while Li+ and Cs+ which are hardly captured by the crown ether groups could not increase Tc. 

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