The interest to amino acid amides arises from their biological
important role. Some C-nGBP\- amidated amino acids Ile, Val, Thr,
Ser, Met, Trp, Gln and Arg have been studied by single crystal
X-ray diffraction and their bioactivity have been compared with the
corresponding amino acids due to most of mammalian peptide hormones
as calcitonin, gastrin, neurokinins or neuropeptides possess a
C-GBP\-terminal-amides. The most C-GBP\- amides are much more
biologically active, comparing with the corresponding C-GBP\-
terminal free acids. For example the "potency ratio" of peptide
amide towards the corresponding peptide free acid in neurocinin is
more than 40 000. Since the protonated forms of amino acid amides
and C-GBP\-amidated peptides exists in the living cell their
investigation could provide an understanding of their biological
role. The choice of the acidity agent for the in vitro
investigations are based manly of its own biological activity as
for example squaric acid (H2Sq). Its application for synthesis of
optically active amino acid derivatives with potential non-linear
optical and electro-optical properties is well known, but its
important biological role is intensively studied in last five
years. A large number of medications based on H2Sq derivatives are
effective inhibitors of protein tyrosine phosphatases or DNA
polymerases from several viruses. H2Sq diamides replaced a
phosphate diester linkage in oligodeoxynucleotide. Selective
antagonist of ionotropic glutamate receptors is obtained by
replacing of GBP^-carboxylic acid of a glutamate residue within a
polyamine toxin with squaric acid derivatives. Some H2Sq-based
peptides are inhibitors of matrix metalloprotease V1. These facts
provoked the systematic investigations of hydrochlorides,
hydrogensquarates and ester amides of squaric acid of amino acid
amides of Ala, Arg, Tyr, Ser, Met, Ile, Lys, Tyr, Val, Leu, Pro and
Phe. Some of them have been structurally characterized by single
crystal X-ray diffraction. Their spectroscopic properties have been
obtained using solid-state conventional and linearpolarised IR- and
Raman spectroscopy and 1H- and 13C-NMR. However, the complicated
spectroscopic data difficult in significant level their
interpretation. Moreover, in the cases of hydrogensquarates and
ester amides of squaric acid various intermolecular hydrogen bonds
in solid-state with participation of H2Sq have been established.
Having in mind that physical and chemical properties of above
mentioned compounds can be precise calculated by means of ab initio
and DFT methods at Hartee-Fock, MP2 and B3LYP level of theory,
varying basis sets (6-31G*, 6-31G**, 6-31++G, 6-31++G*, 6-31++G**,
6-311G, 6-311G*, 6-311G** and 6-31++G**) have been employed. The
results obtained allow a precise assignment of many vibrational
bands to the corresponding normal modes as well as the electronic
structure and conformational analysis have been carried out.
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