50 1 Amino Acids, Peptides, Proteins
Table 1.21.Regular structural elements (secondary structures) in polypeptides
Structure Φ ψ na db rc Comments
(◦)(◦)(Å)(Å)
β-Pleated sheet, parallel − 119 +113 2. 03. 21 .1 Occurs occasionally in neighbouring
chain sectors of globular proteins
β-Pleated sheet, antiparallel − 139 +135 2. 03. 40 .9 Common in proteins and synthetic
polypeptides
310 -Helix − 49 −26 2. 32. 01 .9 Observed at the ends ofα-helixes
α-Helix, left-handed coiling + 57 +47 3. 61. 52 .3 Common in globular proteins, asα
“coiled coil” in fibrous proteins
α-Helix, right-handed coiling − 57 −47 3. 61. 52 .3 Poly-D-amino acids poly-(β-benzyl)-
L-aspartate
π-Helix − 57 −70 4. 41 .15 2.8 Hypothetical
Polyglycine II − 80 +150 3. 03. 1 Similar to antiparallelβ-pleated-
sheet formation
Polyglycine II, + 80 −150 3. 03. 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. 31. 9 Synthetic poly-L-proline, only
cis-peptide bonds
Poly-L-proline II − 78 +149 3. 03. 1 As left-handed polyglycine II, as
triple helix in collagen
aAmino acid residues per turn.
bThe rise along the axis direction, per residue.
cThe 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
Fig. 1.18.φ,ψ-Diagram for observed values of 13 dif-
ferent proteins containing a total of 2500 amino acids.
(according toSchulzandSchirmer, 1979)
2500 amino acid residues have been shown
empirically to have values ofψ,φ-pairs within
the permissible range. When a multitude of
equalψ,φ-pairs occurs consecutively in a peptide
chain, the chain acquires regular repeating struc-
tural elements. The types of structural elements
are compiled in Table 1.21.1.4.2.2.1 β-Sheet................................................
Three regular structural elements (pleated-
sheet structures) have values in the range of
φ=− 120 ◦Candψ=+ 120 ◦C. The peptide
chain is always lightly folded on the Cαatom
(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 alter-
nately from+zto−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 adja-
cent chains run in the same direction, the peptide
chains are parallel. This provides a stabilized,