Is NH stronger than Oh

The ab initio Results presented on this tour can be summarized as follows:

  1. The presence of a C = O group has more influence on the structure than any intramolecular hydrogen bond.

  2. The strongest intramolecular hydrogen bond in the compounds investigated is OC-O-H ··· N in the -Amino acids; this bond is

  3. The CO-O-H ··· N hydrogen bond has in the -Amino acids no competition worth mentioning: the other interactions between the two functional groups, N-H ··· O = C and N-H ··· O-C = O, are weak and in most cases no hydrogen bonds.
    In contrast, the corresponding interactions are O-H ··· O = C and O-H ··· O-C = O in -Hydroxy acids stronger and in many cases forms real hydrogen bonds.

  4. In 3-aminopropionamide the elongated forms are remarkable because of the rotation of the CONH-Group does not result in any other energy minima. The comparison with -Alanine shows that steric factors can be ruled out as the cause of this rather rare energy profile.

The first three points of this summary impressively show the uniqueness of amino acids. This applies not only to the interactions in the gas phase structures, but also in solution: are in the living organism -Amino acids are the building blocks for proteins and peptides; For example, there are no polyesters of comparable biological importance. (Some polyester analogues of peptides are even biodegradable.) Also, only amino acids form zwitterions; the stable hydrogen bond N ··· H-O-CO, which has no competition in the gas phase, is a starting point for the formation of these zwitterions, which can be formed from them without any external proton donors.

The last point of the above summary also has an interesting connection to peptide and protein chemistry, because it can only be explained by the fact that the elongated form of the fragment -CH-CO-NH- is of extraordinary stability. A repeated arrangement of substituted analogues of precisely this fragment in precisely this arrangement forms the "sheet" structure of the proteins and peptides. In the model tripeptide For-L-Ala-L-Ala-NH For example, the elongated conformer is almost as stable as the global minimum, although the latter is stabilized by two intramolecular hydrogen bonds.

compared with -Alanine.
© Publications (ours and selected others).
Starting point of the hydrogen bridge tour.

Quantum Chemistry Department.

Disclosure according to §25 MedienG: Dr. Michael Ramek, Graz.