Biology Reference
In-Depth Information
132
Figure 5-10.
The Ramanchandran
plot.
Such calculations will depend on the chemical structure of the amino acid
residue.
ALLOWED CONFIGURATIONS OF AMINO ACID RESIDUES
For the 20 different amino acid residues, they can be divided into three
groups. Glycine residue is different from the others, with its R-group
attached to -atom being a hydrogen atom. Proline residue has a ring
structure for its side chain joining to the backbone N-atom. Sometimes, it
can be in the cis-configuration. All the other 18 amino acid residues have a
-atom covalently bonded to the -atom. They are usually considered
together for steric hindrance calculations, and represented by the alanine
residue. Ramakrishnan and Ramachandran (1965) have calculated the
allowed regions for alanine and glycine residues as shown in Fig. 5-11 and
Fig. 5-12 respectively. The extreme minimal limits are used for these plots.
In the case of proline residue, the ring structure of its side chain restricts its
angle to about -60
0
in the new convention (Fig. 5-13). Proline residues
may sometimes be in the
cis
-configuration, and have
around +60
0
instead.
