Iso-electric point

Enzyme UE/mg Optimum pH Iso-electric point Molar mass... 

Negatively charged species such as carboxylic acid group in acid-treated CNTs can attract positively charged enzymes from solution as long as the pH value of the enzyme solution is controlled to be lower than the iso-electric point of the enzyme thus, multilayer films of the enzyme can be formed by the layer-by-layer technique. For example, five layers of GOx can be immobilized on the electrode surface by alternatively dipping a poly(diallyldimethylammonium chloride (PDDA))-functionalized GC into a CNT solution and a GOx solution (pH 3.8). Figure 15.15 illustrates the preparation process for the formation of a multilayer film of GOx on the electrode. 

There is considerable evidence that whenever two immiscible substances are brought together a double layer at the interface results, but recent work has made it doubtful whether, in every case, the iso-electric point corresponds with the condition of maximum surface tension. 

Greater purification could be achieved, usually on a smaller scale, if the desired protein had easily measurable properties (e.g. was an enzyme), so that the specific activity of the product and extent of purification could be estimated. Well into the 1950s any enzyme required in a laboratory had first to be isolated by those needing it. With increased knowledge of enzyme properties two further methods of purification were commonly tried heat denaturation of contaminating proteins (with the incidental discovery of some remarkably heat-resistant enzymes), and protein precipitation at the iso-electric point. 

Fig. 6-(K2. Potential Mut, across a compact layer at the flat band potential as a function of solution pH pHiq, = iso-electric point pH ...

Figure 5-54 shows the flat band potential observed for an intrinsic semiconductor electrode of germanium in aqueous solutions as a function of pH. The flat band potential, shifts in the cathodic (negative) direction with increasing pH at the rate of 59 mV / pH in agreement with Eqn. 5-87 at room temperature. The iso-electric point of germanium electrode is found to be about pH 2.5, which is close to the iso-electric point of germanium oxide. 

The ion that determines the p>otential of the compact layer is called the potential-determining ion. In cases in which the potentied of compact layer is determined by the dissociation reaction of adsorbed hydroxyl groups, the potential -determining ions are hydrated protons or hydroxide ions. For cadmium sulfide electrodes, the potential-determining ions are not hydrated protons but hydrated sulfide ions the iso-electric point is at the sulfide ion concentration of 4 x 10 M [Ginley-Butler, 1978]. 

TABLE 5- Tbe flat band potential , the iso-electric point pHi, the potential of the conduction band edge aiep) at pH q> for metal oxide semiconductor electrodes in aqueous solutions t, = band gap of metal oxides pH= solution pH at which the flat band potential is measured. [From Morrison, 1980.]... 

Table 5-6 shows for semiconductor metal oxides the flat band potential in aqueous solutions at various pH values and the conduction band edge potential E c at the iso-electric point pH,. ... 

Fig. 9-22. Unitary proton levels of hydrated and adsorbed hydronium ions (acidic proton) and of hydrated and adsorbed water molecules (basic proton) the left side is the occupied proton level (the real potential of acidic protons), and the right side is the vacant proton level. Hi/HjO) = unitary occupied proton level of adsorbed hydronium ions (acidic proton level) H20.d = unitary vacant proton level of adsorbed hydronium ions (acidic proton level) and unitary occupied proton level of adsorbed water molecules (basic proton level) OH = unitary vacant proton level of adsorbed water molecules (basic proton level) (pHi, ) = hydrated proton level at iso-electric point pR...

The pH at which the concentration of acidic occupied proton levels of adsorbed h3dronium ions equals the concentration of basic vacant proton levels of adsorbed water molecules is called the iso-electric point pHi, here, the net interfacial charge of adsorbed ions at the interface is zero. The iso-electric point pH,, is expressed in Eqn. 9-73 ... 

For metal oxide electrodes, the iso-electric point pH p is also located midway between the unitary acidic proton level and the unitary basic proton level of adsorbed water. Table 9-1 shows the iso-electric point pHi, of several metal oxides in aqueous solutions. 

TABLE 9-1. Iso-electric point pHi, of metal oxides in aqueous solution. [From Aikawa, 1992.]... 

Classical characterization procedures of non-purified receptors were based on behavior of the radioactive label of the steroid bound to the receptor. Sucrose gradient centrifugation and gel filtration have been used for estimation of size and molecular mass (Table I, Section 2.2). Iso-electric focussing under non-denaturing conditions revealed iso-electric points between 4.5 and 6.0 for the monomeric forms. 

Colloidal metal-oxide particles, with hydroxyl groups at their surface, may undergo proton association or dissociation depending on the pH of the solution. At low pH, a metal-oxide particle will be charged positively and at high pH negatively. The pH at which the net charge is zero, is the iso-electric point. 

Theoretically, the precipitation of a protein, which is built up of amino-acids and functions as a typical amphoteric electrolyte, should best be accomplished at the isoelectric point, that is, at the hydrogen ion concentration at which the acidic and basic functions of the protein are equal and at a minimum. According to Soerensen, the iso-electric point of egg albumin is at pH 4.8,10 and while precipitation would undoubtedly be most complete at this point, it is not necessarily the most favorable for crystallization, as the egg albumin crystals are not isoelectric protein, but a compound of this with the sulfate ion, and perhaps the ammonium ion as well.11... 

Dipolar structure of amino acids—The dipole moment data support the dipolar, zwitterionic, structure of the amino acids, viz HgRGHCOO-. If the molecule at its iso-electric point possesses this structure, it will have a very large dipole moment of the order of 13 9 D since the distance between the charges is approximately 2-9 A. Unfortunately such compounds are only soluble in polar solvents and it is therefore impossible to measure the dipole moments directly. The dielectric constant of aqueous solutions of amino acids is, however, greater than that of water and furthermore increases in direct proportion to the concentration. The value of A el Ac... 

Introduction. Proteins are substances produced by living matter which on enzymatic or acid hydrolysis yield amino acids. They are non-volatile and of high molecular weight, and form colloidal dispersions from which they may be precipitated by-heat, alcohol, various salts, and acids (tannic, picric, and phosphotungstic). Each protein has a minimum solubility at a characteristic pH which is called the iso-electric point. At this point the protein molecules exist as a regates and the solution has the maximum turbidity. The nature of the union of various amino acids in the protein molecule is not known. One theory assumes that the carboxyl of one amino acid unites with the amino group of another, thus ... 

The synthesis conditions which lead to weakly branched systems involve the use of an acid catalyst where pH < 2.2 (iso-electric point of silica) and the use of low to moderate water content (r < 10). Hydrolysis (see reactions in Section 7.4) then takes place via a fast protonation of the alkoxide, followed by attack of water, resulting in the substitution of the alkoxy group with an hydroxyl group. Protonation becomes slower when more hydroxyls are present. The hydrolysis rate will therefore decrease with the extent of OH substitution. Acid catalysed condensation reactions proceed analogously where a protonated silanol species is attacked by water. The condensation reaction rate decreases with the number of condensed Si-O-Si groups. 

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