O

O"

Aspartic acid (Asp) D Glutamic acid (Glu) E Lysine (Lys) K

Arginine (Arg) R

Histidine (His) H

O"

Figure 11.1 (continued) (b) An idealised a-helix, drawn as a ribbon showing typical stabilising intrahelical interactions. Specific hydrogen-bonded interactions are said to 'cap' the ends of the chain, known specifically as the N-terminal and C-terminal ends of the helix. Free energies (A G) of the interactions, compared to the free energy of hydrophobic bonding involving an isobutyl side-chain (-4.18 kJ mol-1) are: N-cap 4.2-8.4 kJ mol-1 C-cap ~2 kJ mol-1

Side-chain-side-chain electrostatic interactions ~2 kJ mol-1

(Reproduced from J. W. Bryson, S. F. Betz and H. S. Lu etal., Science, 270, 935 (1995).]

(a)

chain B

chain A

chain B

chain A

Figure 11.2 (a) A representation of ,8-sheet formation by four antiparallel polypeptide chains (to be visualised as being in and out of the plane on the paper). The interactions which determine ,8-sheet stability are more variable than those determining a-helix stability. Different amino acids have different propensities for forming ,8-sheets. (Reproduced from reference 3, with permission.) (b) A representation of the concertina shape of a ,8-sheet showing two antiparallel polypeptide chains, A and B, associated by hydrogen bonding as in (a). Exposed amides at the edge of a ,6-sheet can hydrogen-bond to other sheets, leading to the formation of insoluble aggregates. (Reproduced from J. W. Bryson, S. F. Betz, H. S. Lu, et al., Science, 270, 935 (1995).)

Figure 11.2 (a) A representation of ,8-sheet formation by four antiparallel polypeptide chains (to be visualised as being in and out of the plane on the paper). The interactions which determine ,8-sheet stability are more variable than those determining a-helix stability. Different amino acids have different propensities for forming ,8-sheets. (Reproduced from reference 3, with permission.) (b) A representation of the concertina shape of a ,8-sheet showing two antiparallel polypeptide chains, A and B, associated by hydrogen bonding as in (a). Exposed amides at the edge of a ,6-sheet can hydrogen-bond to other sheets, leading to the formation of insoluble aggregates. (Reproduced from J. W. Bryson, S. F. Betz, H. S. Lu, et al., Science, 270, 935 (1995).)

protein will have an amphiphilic nature. Table 11.3 lists the relative hydrophobic character of a range of amino acids, where Gly is considered to have a value of zero. The amino acids range from very hydrophobic to very hydrophilic. The side-chains of selected amino acids are shown in the table and demonstrate clearly their order in the table. Some values of log P (octanol/water) are given, demonstrating the trend in hydrophobic parameters.

An idea of the overall hydrophobicity of a peptide or protein may be gained from the use of indices of the hydrophobicity of the individual amino acids. Secondary and tertiary structures are important in determining the actual hydrophobic nature of the polypeptide, however, and this complicates the prediction of their physicochemical properties such as solubility and adsorption.

If alternating hydrophilic and hydrophobic amino acid sequences in synthetic peptides are at the right distances in space, the molecule coils with the hydrophobic amino acids on the inside of each coil and the hydrophilic ones to the outside. There are still, however, many structural mysteries: the interior of many protein structures with myriads of side-chains, and the way in which metal ions can stabilize three-dimensional structures, have been likened to a terra incognita.

11.1.3 Solubility of peptides and proteins

In physiological conditions the aqueous solubilities of proteins vary enormously from the very soluble to the virtually insoluble. In section 5.2.4 we discussed the solubility profiles of zwitterions, including tryptophan, which all have biphasic solubility-pH profiles. One would expect proteins with terminal —NH2 and —COOH groups to behave similarly, although the effect will be complicated by the behaviour of the multitude of the intermediate amino acids, as can be seen from the pH-solubility profile of insulin in Fig. 9.20. Figure 11.4 shows the general solubility behaviour of a protein as a function of pH at two ionic strengths.

The solubility of globular proteins increases as the pH of the solution moves away from the isoelectric point (IP), which is the pH at which the molecule has a net zero charge and does not migrate in an electric field. Some examples of the IPs of amino acids are shown in Table 11.4. At its IP a protein has no net charge and, therefore, has a greater tendency to self-associate. As the net charge increases, the affinity of the protein for the aqueous environment increases and the protein molecules also exert a greater electrostatic repulsion. However, extremes of pH can cause protein unfolding which, not infrequently, exposes further nonpolar groups.

The relative hydrophilicities of the side-chains of the amino acids correlate well with

Table 11.3 Relative hydrophobic character of amino

acid side-chains (Gly = 0)

0 0

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