Food Chemistry

1.4 Proteins 51

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.
Theβstructures can also be regarded as special
helix with a continuation of 2 residues per turn.
With proline, the formation of aβstructure is not
possible.

Fig. 1.20.Diagrammatic presentation of antiparallel (a)
and parallel (b) peptide chain arrangements

Fig. 1.21.Diagrammatic presentation of a twisted sheet

structure of parallel peptide chains (according toSchulz

andSchirmer, 1979)

1.4.2.2.2 HelicalStructures

There are three regular structural elements in the

range ofφ=− 60 ◦andψ=− 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 theα-helix and for

polypeptides fromL-amino acids, exclusively the

right-handedα-helix (Fig. 1.22). The left-handed

α-helix is energetically unfavourable forL-amino

acids, since the side chains here are in close

contact with the backbone. Noα-helix is possible

with proline. The 3 10 -helix was observed only at

the ends ofα-helices but not as an independent

regular structure. The π-helix is hypotheti-

cal. 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 aβ-structure, while polyglcine II

corresponds largely to the polyproline II-helix.

A helix is characterized by the anglesφandψ,

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 aφ,ψ

plot in Fig. 1.23.