-aminohexanoic
acid forms 14 symmetry unique conformers
with O-H···N hydrogen bonds, according to
ab initio calculations. (The structures of these conformers
are available for interactive viewing).
These 14 conformers and their enantiomers are interconnected by
hydrogen bond preserving reactions in a very complicated way.
The straightforward relation between the carbon
chain length and the number of hydrogen bonded conformers as well as
the number of H-bond preserving reaction paths, which could be
observed up to -aminopentanoic acid,
therefore is not valid for -aminohexanoic acid. This may be taken as another indication
that the eight-membered ring, which is formed by the H-bonded
conformers of -aminopentanoic acid, is a conformation with
minimal sterical problems.
-aminohexanoic acid, however,
is a break-through in the homology series of the
-amino acids in a different aspect:
among the 14 symmetry unique conformers with
O-H···N hydrogen bonds
formed by this compound there are four in which the COOH group is
not restricted to the energetically unfavourable trans-orientation,
but in a distorted cis-orientation. These four conformers,
which are shown in the graphic inserts on this page, are not the most
stable H-bonded conformers though, because of the distorted orientation
of the COOH group.
-aminohexanoic
acid forms 14 symmetry unique conformers
with O-H···N hydrogen bonds, according to
ab initio calculations. (The structures of these conformers
are available for interactive viewing).
These 14 conformers and their enantiomers are interconnected by
hydrogen bond preserving reactions in a very complicated way.
The straightforward relation between the carbon
chain length and the number of hydrogen bonded conformers as well as
the number of H-bond preserving reaction paths, which could be
observed up to 
-aminopentanoic acid,
therefore is not valid for 
-aminohexanoic acid. This may be taken as another indication
that the eight-membered ring, which is formed by the H-bonded
conformers of 
-amino acids in a different aspect:
among the 14 symmetry unique conformers with
O-H···N hydrogen bonds
formed by this compound there are four in which the COOH group is
not restricted to the energetically unfavourable trans-orientation,
but in a distorted cis-orientation. These four conformers,
which are shown in the graphic inserts on this page, are not the most
stable H-bonded conformers though, because of the distorted orientation
of the COOH group.
conclusions.
Quantum Chemistry Group.