510
AMINO ACIDS, PEPTIDES AND PROTEINS
N
N
N
N
N
O
O
O
O
R
R
R
R
H
H
H
H
H
R
O
N
N
N
N
N
O
O
O
O
R
R
R
R
H
H
H
H
H
R
O
N
N
N
N
N
O
O
O
O
R
R
R
R
H
H
H
H
H
R
O
N
N
N
N
N
O
O
O
O
R
R
R
R
H
H
H
H
H
R
O
N
N
N
N
N
O
O
O
O
R
R
R
R
H
H
H
H
H
R
O
N
N
N
N
N
O
O
O
O
R
R
R
R
H
H
H
H
H
R
O
anti-parallel β-sheet:
carbonyl to amino peptide
linkages in opposite directions
minimizing interactions between R
groups leads to a pleated array:
R
C N
N
C
R
C N
R
N
C
C
R
N
R
O
H
O
H
O
H
H
O
O
H
parallel β-sheet:
carbonyl to amino peptide
linkages in same direction
N
H
O
N
H
O
N
H
O
N
H
O
N
H
O
N
H
O
side view showing pleats
Figure 13.1 Secondary structure of proteins: hydrogen bonding in β-sheets
both are possible since there is little difference in
energy between them. Further, there is no N–H in
proline, so hydrogen bonding involving this residue
is not possible.
N
N
O
O
R
H
O
H
N
N
O
O
R
H
O
H
R
N
H
N
R
H
peptide bond involving proline:
trans configuration
peptide bond involving proline:
cis configuration