Intramolecular Hydrogen Bonds in Amino Acids.

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(structure1 with H-bond) (structure2 with H-bond) Delta-aminopentanoic acid 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 delta-hydroxypentanoic acid, but in contrast to many other systems, the various criteria for hydrogen bond strength that can be obtained from ab initio calculations (N···H distance; 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 H_3C-CH_2-COOH···NH_2-CH_2-CH_3, in which the COOH group is on cis-orientation too, shows that the eight-membered ring in delta-aminopentanoic acid is practically free of ring strain.

(structure3 with H-bond) (structure4 with H-bond) As in beta-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 omega-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            

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