Dissociable group

These are the definitions of the two characteristic dissociation constants normally expressed in terms of p K. When three dissociating groups are present in a molecule there are three piC values, ie, pfC, P 3- knowledge of these piC values is important in the separation or isolation of each amino acid by ion-exchange chromatography. 

The isoelectric species is the form of a molecule that has an equal number of positive and negative charges and thus is electrically neutral. The isoelectric pH, also called the pi, is the pH midway between pA values on either side of the isoelectric species. For an amino acid such as alanine that has only two dissociating groups, there is no ambiguity. The first pK (R—COOH) is 2.35 and the second pK (R—NH3+) is 9.69. The isoelectric pH (pi) of alanine thus is... 

The association between RNase A and 3 -UMP or 3 -dUMP has been studied by P n.m.r. and kinetic methods, respectively. In both cases the participation of two dissociable groups at the active site of the enzyme was demonstrated, in agreement with n.m.r. and A -ray - studies on the binding of 3 -CMP to RNase. In the binding of Tj RNase to purine nucleotide monophosphates, the phosphate group appears to have an important effect while the ribose ring is relatively unimportant. ... 

Polyelectrolytes (Table 6) exhibit a large amount of dissociable groups. These compounds are often used for ion-exchange chromatography. They can also be used to exchange their counterions with indicator ions (see section 4). 

The sulphosuccinamate surfactants CA540 (Allied Chemical, USA) and 845 (Cytec) both contain four dissociable groups, three carboxylate and one sulphonate. Both collectors contain impurities such as dodecyl amine, maleic acid derivative and residual alcohols. This may explain the quite strong frothing properties of these collectors. 

Alanine has two dissociable groups the carboxyl group with pA = 2.5 and the amino group with = 9.5. A buffering zone is evident near each group s pAi,... 

The denominators (which are the Michaelis pH functions given by the first three terms of Eq. 3-6) represent the fraction of enzyme or of ES complex in the monoprotonated state. The pH dependence of enzymatic action is often more complex than that shown in Fig. 9-8 and given by the foregoing equations. However, it is easy to write Michaelis pH functions (see Chapter 3) for enzymes with any number of dissociable groups in both E and ES and to write appropriate equations of the type of... 

A more elegant procedure is to use a photochemical reaction to dissociate groups from the polymer chains and form radicals capable of polymerization with an added monomer. 

Intrinsic pKl values No. of groups/ molecule Tentative assignment to dissociable groups... 

The enzymatic activity of RNase A shows a bellshaped dependence on pH, with an optimum near pH 7 (fig. 8.14). The operation of the enzyme appears to require that a dissociable group with a pKa of about 6.3 be in the proton-ated form, and that a group with a pKa of about 8 be unpro-tonated. These groups have been identified as His 12 and His 119 by measuring the NMR spectrum of the enzyme as a function of pH. Spectral peaks due to histidine residues undergo characteristic shifts as the imidazole ring is proton-ated. 

To calculate the surface potential we consider the simplest example of a surface with one dissociable group. Dissociation leads to a negatively charged group according to... 

The surface concentrations of the negatively charged dissociated groups and the neutral nondis-sociated groups are given in mol per surface area and not in mol per volume. 

Neutral salts have a pronounced effect on the solubility of proteins, especially if they are globular. At low concentration, salts increase the solubility of many proteins in a phenomenon known as salting-in. This solubilization is a function of the solvent ionic strength, which depends on the concentration and on the electrical charge of the cations and anions that constitute the salt. These effects are caused by changes in the ionization of dissociating groups of the protein. 

In the present simplified model, we neglect conformational degrees of freedom of the PE and assume a straight polymer consisting of N monomers with a bond length (i.e. distance between dissociable groups) a, located at a distance D from a dielectric interface, as is depicted in Fig. 24. This model is applicable to stiff PEs and for strongly adsorbed PEs, since they are indeed flat. The exact partition function reads... 

A large fraction of biological molecules has dissociable groups in aqueous solution, giving rise to H+(aq) or other cations or anions, the extent of ionization depending on pH. 

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