Intramolecular Hydrogen Bonds in Amino Acids.

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(H-bonded form of GABA) All omega-amino acids are capable of forming a N···H-O hydrogen bond between the two terminal groups -COOH and -NH_2. Up to delta-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.

(H-bonded form of epsilon-aminohexanoic acid) The strength of the hydrogen bonds shows a steady increase up to delta-aminopentanoic acid, in which the hydrogen bond closes an eight-membered ring. In epsilon-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.

(For-L-Ala-L-Ala-NH2 with gamma turn) The finding that it requires a ten-membered ring 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 alpha-amino acids and are able to form intramolecular C=O···H-N bonds that are similar to the hydrogen bonds in omega-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:

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