Isoelectric point , of protein

Module 1, Determination of Chemical and Structural Information on the Sample. The task of Module 1 is to provide non-chromato-graphic data for analytes prior to specification of the chromatographic method. Data bases have been developed for pK values of organic molecules, isoelectric points of proteins, and fluorescence spectral properties of organic molecules. 

Ferrozine 3-(2-pyridyl)-5,6-bis(4-phenylsulphonic acid)-1,2,4-triaz-ine (Fig. 4.3) reacts with iron in the iron(II) ferrous state to form an intense pink complex which can be measured spectrophotometrically (lmax = 562 nm). The assay can be performed at pH 1.6 to eliminate interference from turbidity at pH values close to the isoelectric points of proteins which may be present in biological samples under investigation, e.g., serum, and in some instances to decrease interference in the assay from copper (see below). In determinations in which such problems are not considerations, the assay may be carried out at pH 7.4. (This might be more relevant for the assessment of membrane-bound iron under in vivo conditions.)... 

At present neither of these two procedures is perfectly fitted for the determination of the isoelectric points of proteins, but they are quite suitable in the investigation of the microheterogeneity of proteins, as we shall see below. 

Thus, the equations of isotherms (30) and (31) can be used to determine the isoelectric points of proteins by the dependence of their sorption on pH. This approach is realized in Refs. [41,42]. However, it should be noted that this determination is only possible for a low affinity of protein molecules to surface sites. As Figs. 12 and 13 show, for a high affinity (a low Ka) a significant sorption is observed also at pH>pI for cation exchangers and at pH[Pg.721]

As natural milk proteins have acidic isoelectric points, they have low solubility at acid pH and this compromises many of their functional properties in acidic environments. Esterification of proteins increases the net negative charge and raises the isoelectric point of proteins, making them more functional at acidic pHs. 

As with emulsion formation, the maximal absorption of proteins has been reported to occur near the isoelectric point [6,18,47], The effects of pH on the properties of proteins at emulsion surfaces, noted previously, probably also apply to the behavior of proteins in films. While foams formed near the isoelectric point of proteins tend to be more stable than those formed at other pH values, it is often possible to obtain higher overruns away from the isoelectric point [48]. The largest effect of pH on foam stability can probably be explained by the more viscoelastic nature of the films formed in this pH region... 

Righetti P G (2004). Determination of the isoelectric point of proteins by capillary isoelectric focusing. J. Chromatog. A. 1037 491-499. 

Suggest applications that take advantage of the isoelectric point of proteins. 

Dissolution the creation of an aqueous solution containing two different polymers, generally a protein and a polysaccharide, usually at a temperature above the gelling point and pH that is above the isoelectric point of protein. 

Isoelectric focusing (lEF) is an electrophoretic technique facilitating the fractionation of molecular species differing only in net charge. lEF serves to determine the isoelectric point of proteins or, on the other hand, the proteins can be separated to their isoelectric points. 

Stretching the fiber to 4 times its original length A shift of the isoelectric point of protein fibers (keratin) was also observed... 

In Chapter 1 we saw how one can calculate pi value for individual amino acids. However, pi of polypeptide cannot be calculated by that method. To determine the isoelectric point of protein (poiypeptide), one has to (i) measure the mobility of the protein at several pH values, (ii) plot the mobility values so obtained against the pH values, and (Hi) take reading of the pH of that point where the point intersects zero mobility line the pH at this intersection is then taken to be the isoelectric point of the concerned protein (see Figure 1). 

Table 5 lists the names and isoelectric points of proteins suitable for learning the basic techniques of HPIEC. Several sources of these materials are listed in Appendix III at the end of this chapter. 

---