Physical Chemistry of Foods

theless, it is mostly not possible to predict the higher structures from the

primary structure. One reason is that the number of possible primary

structures is, for all purposes, infinite. As an example we take proteins of 100

residues (most proteins are far larger), of which 20^100 & 10130 different

primary structures can exist! Even if one assumes that many interchanges of

amino acids would not materially alter the properties of the protein, taking

the effective number of different residues to be as small as six, this leads to

6100 & 1078 different species. Assuming that of each of those one molecule

existed, their total mass would be the number times the average residue

molar massð& 0 :12 kg?mol
^1 Þover Avogadro’s number, i.e.,& 1053 kg,

which is greater than the presumed total mass of the universe.

The actual variation in primary structure really is very great. Some

proteins have a fairly regular structure, like collagen, where the greater part

of the amino acid sequence consists of repeats of Gly-Pro-Pro or Gly-Pro-

Lys, of which the third residue may be hydroxylated. By far most proteins

have much more intricate primary structures. Some of these are illustrated

in Figure 7.1; note the difference in structural heterogeneity.

Figure 7.2 shows the configuration and dimensions of apeptide unit.

Thepeptide bondhas some special features, since the electron distribution

over the O, C, and N of the bond is intermediate between that of the two

structures

This causes the peptide bond to be flat: rotation about the CO 22 NH axis is
not possible. The bond is in the trans configuration, which is far more stable
than the cis one. The peptide bond also has a significant dipole moment of
3.5 Debye units; this implies that it tends to be hydrated. The H of the 22 NH
group can act as a donor, the 55 O as a hydrogen acceptor in forming
hydrogen bonds.
Figure 7.2 gives an idea about theflexibility of the peptide unit;
rotation about thecandjangles is possible. This rotation is by no means
unlimited, because the side groups sterically prevent several conformations,
according to their size and shape. Even in the case of polyglycine (side group
22 H), some bond angles are clearly preferred. The peptide chain would have
about 4 degrees of (conformational) freedom per peptide unit, though this
may vary with the side groups involved. An unfolded peptide chain is much
more flexible than most polysaccharide chains.