Intramolecular Hydrogen Bonds in Amino Amides.

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The ab initio results, which are presented along this tour, can be summarized as follows:

  1. (3-aminopropanal with repulsive interaction) The presence of a C=O group is more influential on the structures than any intramolecular hydrogen bond.

  2. (H-bonded beta-alanine) The strongest intramolecular hydrogen bond, which is formed in these compounds, is the OC-O-H···N hydrogen bond in omega-amino acids; it is

  3. (global minimum of beta-alanine) The CO-O-H···N hydrogen bond in omega-amino acids has no significant competition: the other interactions, which can occur between the two functional groups, namely N-H···O=C and N-H···O-C=O, are weak and in most cases no real hydrogen bonds.
    (2nd lowest energy minimum of beta-hydroxypropionic acid) In contrast, the corresponding O-H···O=C and O-H···O-C=O interactions in omega-hydroxy acids are stronger and form real hydrogen bonds in many cases.

  4. (extended structure of 3-aminopropionamide) In 3-aminopropionamide, the extended structures are remarkable because the internal rotation of the CONH_2-group does not lead to other energy minima. Comparison with beta-alanine shows that sterical factors can be ruled out as an explanation for this rather unique energy profile.

The first three items in this summary show impressively that amino acids are a unique class of compounds. This is true for the interactions in the gas-phase structures as well as for the solvated species: in the living organism, alpha-amino acids are the building blocks of proteins and peptides; there are, e. g., no polyesters of comparable importance. (In fact, some polyester analogues of peptides are biodegradable.) Also, hydroxy acids do not form zwitterions in polar media, whereas amino acids do. The stable N···H-O-CO hydrogen bond that is formed without competition in the gas phase, is the precursor of this zwitterion formation, which can occur without any proton donating solvent.

The last item of the above summary also has an interesting link to peptide and protein chemistry, since it can only be explained by assuming that the extended structure implies a special stability for the fragment -CH_2-CO-NH-. It is a repetitive arrangement (extended structure of For-L-Ala-L-Ala-NH2) of a substituted version of this very fragment in this very orientation, which generates the sheet structure of peptides and proteins. In the model tripeptide For-L-Ala-L-Ala-NH_2, e. g., the extended structure conformer is almost as stable as the global minimum, although the latter is stabilized by two intramolecular hydrogen bonds.

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