2009_Book_FoodChemistry

50 1 Amino Acids, Peptides, Proteins
Table 1.21. Regular structural elements (secondary structures) in polypeptides
Structure Φ ψ n a d b r c Comments
( ◦ ) ( ◦ ) (Å) (Å)
β-Pleated sheet, parallel −119 +113 2.0 3.2 1.1 Occurs occasionally in neighbouring
chain sectors of globular proteins
β-Pleated sheet, antiparallel −139 +135 2.0 3.4 0.9 Common in proteins and synthetic
polypeptides
3 10 -Helix −49 −26 2.3 2.0 1.9 Observed at the ends of α-helixes
α-Helix, left-handed coiling +57 +47 3.6 1.5 2.3 Common in globular proteins, as α
“coiled coil” in fibrous proteins
α-Helix, right-handed coiling −57 −47 3.6 1.5 2.3 Poly-D-amino acids poly-(β-benzyl)-
L-aspartate
π-Helix −57 −70 4.4 1.15 2.8 Hypothetical
Polyglycine II −80 +150 3.0 3.1 Similar to antiparallel β-pleatedsheet
formation
Polyglycine II, +80 −150 3.0 3.1 Synthetic polyglycine is a mixture of
left-handed coiling
right- and left-handed helices; in
some silk fibroins, the left-handed
helix occurs
Poly-L-proline I −83 +158 3.3 1.9 Synthetic poly-L-proline, only
cis-peptide bonds
Poly-L-proline II −78 +149 3.0 3.1 As left-handed polyglycine II, as
triple helix in collagen
a Amino acid residues per turn.
b The rise along the axis direction, per residue.
c The radius of the helix.
permissible ranges for amino acids other than
glycine (R ≠ H). The range is broader for glycine
(R = H). Figure 1.18 demonstrates that most
of 13 different proteins with a total of about
2500 amino acid residues have been shown
empirically to have values of ψ,φ-pairs within
the permissible range. When a multitude of
equal ψ,φ-pairs occurs consecutively in a peptide
chain, the chain acquires regular repeating structural
elements. The types of structural elements
are compiled in Table 1.21.
1.4.2.2.1 β-Sheet
Fig. 1.18. φ,ψ-Diagram for observed values of 13 different
proteins containing a total of 2500 amino acids.
(according to Schulz and Schirmer, 1979)
Three regular structural elements (pleatedsheet
structures) have values in the range of
φ = −120 ◦ C and ψ =+120 ◦ C. The peptide
chain is always lightly folded on the C α atom
(cf. Fig. 1.19), thus the R side chains extend
perpendicularly to the extension axis of the chain,
i. e. the side chains change their projections alternately
from +z to−z. Such a pleated structure is
stabilized when more chains are present.
Subsequently, adjacent chains interact along the
x-axis by hydrogen bonding, thus providing the
cross-linking required for stability. When adjacent
chains run in the same direction, the peptide
chains are parallel. This provides a stabilized,