Intramolecular Hydrogen Bonds in Amino Acids.
All -amino acids
are capable of forming
a N···H-O hydrogen bond between the two terminal groups
-COOH and -NH. Up to -aminopentanoic acid,
this hydrogen bond can only be formed if the COOH group is in
trans-orientation, which is energetically unfavourable.
For this reason none of the hydrogen bonded conformers is the
global minimum of the ab initio energy surface.
The strength of the hydrogen bonds shows a steady increase
up to -aminopentanoic acid, in
which the hydrogen bond closes an
eight-membered ring. In -aminohexanoic acid the hydrogen bond
is weaker again, due to steric hinderings, but several hydrogen
bonded conformers exist in which the COOH group is in a distorted
cis-orientation. It can be anticipated that
zeta-aminoheptanoic acid will allow a conformation with a
ten-membered ring that is closed by a hydrogen bond, in which the COOH group
is in a pure, undistorted cis-orientation.
The finding that it requires a
to close a hydrogen
bond, without the disadvantage of unfavourable orientations in
another part of the molecule, gives a deep insight into peptide and
protein chemistry. Peptides and proteins are built up by
and are able to
form intramolecular C=O···H-N bonds that are similar to
the hydrogen bonds in -amino acids.
These intramolecular hydrogen bonds are responsible for the over-all structure,
and thus to the function of proteins and peptides. Due to the
building principle of the peptide chain, the size of the rings, which are
formed by these H-bonds, is limited to a few different values: