2009_Book_FoodChemistry

1.4 Proteins 51
Fig. 1.21. Diagrammatic presentation of a twisted sheet
structure of parallel peptide chains (according to Schulz
and Schirmer, 1979)
1.4.2.2.2 Helical Structures
Fig. 1.19. A pleated sheet structure of a peptide chain
planar, parallel sheet structure. When the chains
run in opposite directions, a planar, antiparallel
sheet structure is stabilized (Fig. 1.20). The
lower free energy, twisted sheet structures, in
which the main axes of the neighboring chains
are arranged at an angle of 25 ◦ C (Fig. 1.21), are
more common than planar sheet structures.
The β structures can also be regarded as special
helix with a continuation of 2 residues per turn.
With proline, the formation of a β structure is not
possible.
Fig. 1.20. Diagrammatic presentation of antiparallel (a)
and parallel (b) peptide chain arrangements
There are three regular structural elements in the
range of φ = −60 ◦ and ψ = −60 ◦ (cf. Fig. 1.17)
in which the peptide chains are coiled like
a threaded screw. These structures are stabilized
by intrachain hydrogen bridges which extend
almost parallel to the helix axis, crosslinking
the CO and NH groups, i. e., the CO group
of amino acid residue i with the NH group of
residue i + 3(3 10 -helix), 1 + 4(α-helix) or i + 5
(π-helix).
The most common structure is the α-helix and for
polypeptides from L-amino acids, exclusively the
right-handed α-helix (Fig. 1.22). The left-handed
α-helix is energetically unfavourable for L-amino
acids, since the side chains here are in close
contact with the backbone. No α-helix is possible
with proline. The 3 10 -helix was observed only at
the ends of α-helices but not as an independent
regular structure. The π-helix is hypothetical.
Two helical conformations are known of
polyproline (I and II). Polyproline I contains
only cispeptide bonds and is right-handed, while
polyproline II contains trans-peptide bonds
and is left-handed. The stability of the two
conformations depends on the solvent and other
factors. In water, polyproline II predominates.
Polyglycine can also occur in two conformations.
Polyglycine I is a β-structure, while polyglcine II
corresponds largely to the polyproline II-helix.
A helix is characterized by the angles φ and ψ,
or by the parameters derived from these angles:
n, the number of amino acid residues per turn;
d, the rise along the main axis per amino acid
residue; and r, the radius of the helix. Thus,
the equation for the pitch, p, is p = n · d. The
parameters n and d are presented within a φ,ψ
plot in Fig. 1.23.