Intramolecular Hydrogen Bonds in Amino Acids.
forms a total of eight conformers with O-H···N
hydrogen bonds, i.e., four pairs of enantiomers.
The strength of the hydrogen bond
is quite different among these conformations.
Similar to 5-aminopentanol and
-hydroxypentanoic acid, but
in contrast to many other systems, the various criteria
for hydrogen bond strength
that can be obtained from ab initio calculations
increase of O-H distance;
decrease of O-H vibration frequency;
deviation of the N···H-O angle from linearity;
electron density of the H-bond;
Hartree-Fock energies, with or without vibrational zero-point
correction) do not give the same ranking in all cases, but lead to
three different rankings. Comparison with the corresponding values
of adducts between propionic acid and ethylamine
in which the COOH group is on cis-orientation too,
shows that the eight-membered ring in -aminopentanoic acid is practically free of ring strain.
As in -alanine
and GABA, there
are reaction paths between these conformers that preserve the hydrogen
bond. It turns out that each of the H-bonded conformers may choose
among three different of these reaction paths.
This gives an interesting progression for the number of H-bonded conformers
and the number of H-bond conserving reactions in
the series of the -amino acids,
as outlined in the following table.
compound conformers non-equivalent reac- tions per conformer glycine 1 - beta-alanine 2 1 gamma-aminobutyric acid 4 2 delta-aminopentanoic acid 8 3