Intramolecular Hydrogen Bonds in Amino Acids.

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(structure1) (structure2) Epsilon-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 delta-aminopentanoic acid, therefore is not valid for epsilon-aminohexanoic acid. This may be taken as another indication that the eight-membered ring, which is formed by the H-bonded conformers of delta-aminopentanoic acid, is a conformation with minimal sterical problems.

(structure3) (structure4) Epsilon-aminohexanoic acid, however, is a break-through in the homology series of the omega-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.

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