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the hydration of the side-chain; proteins are surrounded by a hydration layer, equivalent to about 0.3 g H2O per gram of protein, which represents about two water molecules per amino acid residue. Disturbing this layer has serious consequences.

The phase behaviour of protein solutions is affected by pH, ionic strength and temperature.

Protein-water solutions sometimes exhibit critical solution temperatures (see Chapter 5 for a discussion of phase separation). Phase transitions are important not only in manufacture and formulation, but also because they have some pathophysiological implications. Because of the involvement of phase separation in the opacification of the lens of the eye in certain cataracts, the phase separation of the y-crystallins has been studied. Figure 11.5 shows the phase diagram obtained.

In section 5.2.3 we discussed the effect of salts on the solubility of organic electrolytes. The parabolic effects of salts on protein solubility (Fig. 11.6) might, at first sight, seem unexpected. Data, produced over 70 years ago, on haemoglobin solubility (Fig. 11.6b) shows a general increase in solubility with increasing ionic strength of salts such as NaCl, KCl and

Table 11.4 Values of pK and isoelectric point (IP) of common L-amino acids

Amino acid

PK, (COOH)

pK2 (NHS)

0 0

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