Isoelectric region

In the isoelectric region, octamerization occurs in addition to the monomer-dimer association (Gilbert and Gilbert 1973). Between pH 3.5 and 5.2, the dimers of both the A and B variants associate to form... 

The g-lactoglobulin normally exists in a monomer or dimer form although one variant is known to exist in an octamer form at pH 4.65. Little or no association of a-lactalbumin has been observed on the alkaline side of the isoelectric region. On the acid side (pH 2-4) a-lactalbumin associates easily and the association has previously been described as a "denaturation-like" process (3). 

The presence of salt seems to stabilize the protein structure against denaturation. The effect of salt on the denaturation temperature is more pronounced at pH values outside the isoelectric region. Table II gives an example of the effect of NaCl concentration on the denaturation temperature. It can be seen that the increase in Tj in the direction from 0 to 1.0 M NaCl is >20°C. This drastic effect of NaCl on the denaturation tempe-... 

From both the DSC-thermograms of whey and soy proteins it can be concluded that the native structure is most stable against denaturation in the isoelectric region where the net charge is low. 

Speakman and Hirst (1933) and Speakman and Stott (1934) consider that an isoelectric region rather than an isoelectric point should be recognized similar results are observed with silk (Howitt, 1946). Lemin and Vickerstaff (1946) claim to have determined the isoionic point of wool by measurement of the pH at which addition of salt does not affect the pH. They obtained a value of 6.2 which is approximately the midpoint of the isoelectric region. It is not possible to determine the isoionic point of a protein by measurements made, as in the present instance, at constant salt concentration. The true value could be obtained by carrjdng out the... 

The effects of salt concentration on the acid titration curve for wool are illustrated in Fig. 8. The broad isoelectric region observed in the absence of salt becomes less apparent with increasing salt concentration,... 

Usually, the plateau-value, F, of the isotherm corresponds roughly to a close-packed monolayer of native molecules in a side-on or end-on orientation. It indicates that, at least at solution concentrations that are not excessively high, multilayers are not formed. In various systems it is observed that Fp(pH) is at a maximum in the isoelectric region of the protein molecule (e.g., 7,12,13,14). For example, Fp(pH) curves for HPA at several sorbents are shown in Figure 1. The occurrence of a maximum at the i.e.p., independent of the nature of the sorbent, suggests that the charge of the protein molecule greatly influences Fp. In particular, for HPA on polystyrene surfaces ample evidence has been collected to... 

As the titration curve (Fig. 28) indicates the polymer exhibits a broad isoelectric region ranging from pH 6.0 to pH 7.5. Apparently the interactions with the solvent molecules are only weak in this region, very likely due to the ability of the zwitterionic structure II to form an intramolecular salt IV... 

Other polypeptide networks, for example based on 1-glutamic acid and dl-cystine residues contracted in their isoelectric region and extended in both acidic and basic media. 

H. Yuki and his coworkers synthesized three-dimensional polypeptide networks composed of 1-glutamic acid and dl-cystine residues, and their reversible contractility upon changes in pH and ionic strength was studied. A foil made of this polypeptide contracted in its isoelectric region and extended in both acidic and basic media. 

The constancy of ri dity over a considerable pH range in the isoelectric region recalls the meager dependence of melting point on pH in this range (Section 2, b, page 23). However, the possibility of anomalous behavior in a very narrow pH zone at the isoelectric point in the absence of salt is not precluded (see Section e, page 36). 

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