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R. Fausto and N. Kuş
7.3 Amino Acids
In the crystalline state, amino acids exist as zwitterions. upon sublimation,
however, these forms convert into the non-ionic forms. very interestingly, fast
deposition of amino acid vapors onto a cold (~ 10 K) infrared-transparent substrate
allowed spectroscopic observation of the non-ionic forms of glycine, sarcosine
and N,N-dimethylglycine in their neat amorphous solid phases [21]. the spectra
obtained immediately after deposition revealed the presence in the initially produced amorphous film of both the non-ionic and zwitterionic forms of the studied
amino acids. the identification of the non-ionic forms and their vibrational signatures could be unequivocally established by taking into account data for the matrixisolated non-ionic monomeric and aggregated amino acids [22–24] as well as solid
state data for acid and alkaline salts of glycine [25]. upon temperature increase, the
non-ionic forms were found to convert non-reversibly into the zwitterionic forms
(Fig. 7.3). observation of aggregates of non-ionic forms of the amino acids in the
solid phase, together with the fact that the amount of these forms diminished with
osition, enabled to conclude that the observed (non-ionic form)→zwitterion conversion occurs in the solid state, where the zwitterionic species is most stable, and that
intermolecular interactions in this phase play an important role in the mechanism of
proton transfer [21].
detailed structural and spectroscopic characterizations of the monomeric nonionic forms of many simple amino acids have been achieved by combining matrix
isolation IR spectroscopy with contemporary methods of quantum chemistry.
the simultaneous experimental observation of the three lower energy conformers of the simplest amino acid, glycine, by matrix isolation infrared spectroscopy
[22], has been a milestone in the structural characterization of this compound. Previous microwave spectroscopy studies [26–30] had already provided experimental evidence of existence of two conformers of glycine in the gas phase (I and II;
Fig. 7.4), though the significantly lower dipole moment of the most stable conformer I (bearing an intramolecular hydrogen-bond of the Nh 2 ···o = C type), compared
to conformer II (possessing an intramolecular o–h···N hydrogen-bond), initially
made detection of the first one difficult by that technique. Conformer III (with an
Nh 2 ···oh h-bond; Fig. 7.4) has also a low dipole moment, and its expected small
relative population makes its experimental detection by microwave spectroscopy
very hard. on the other hand, a gas phase electron diffraction study [31], had suggested that, at 219 °C, about 24 % of the compound should correspond to a mixture
of minor conformers of glycine, which were supposed to be conformers II and III.
however, these conformers are different from each other only in the position of
the hydrogens, and they are indistinguishable by their electron scattering intensities. thus, the question of the conformational structure of glycine in the gas phase
could only be effectively solved after its study by matrix isolation infrared spectroscopy. In the matrix isolation study [22], the compound and some of its deuterated
isotopologues were isolated in low-temperature argon matrices, and the infrared
R. Fausto and N. Kuş
7.3 Amino Acids
In the crystalline state, amino acids exist as zwitterions. upon sublimation,
however, these forms convert into the non-ionic forms. very interestingly, fast
deposition of amino acid vapors onto a cold (~ 10 K) infrared-transparent substrate
allowed spectroscopic observation of the non-ionic forms of glycine, sarcosine
and N,N-dimethylglycine in their neat amorphous solid phases [21]. the spectra
obtained immediately after deposition revealed the presence in the initially produced amorphous film of both the non-ionic and zwitterionic forms of the studied
amino acids. the identification of the non-ionic forms and their vibrational signatures could be unequivocally established by taking into account data for the matrixisolated non-ionic monomeric and aggregated amino acids [22–24] as well as solid
state data for acid and alkaline salts of glycine [25]. upon temperature increase, the
non-ionic forms were found to convert non-reversibly into the zwitterionic forms
(Fig. 7.3). observation of aggregates of non-ionic forms of the amino acids in the
solid phase, together with the fact that the amount of these forms diminished with
osition, enabled to conclude that the observed (non-ionic form)→zwitterion conversion occurs in the solid state, where the zwitterionic species is most stable, and that
intermolecular interactions in this phase play an important role in the mechanism of
proton transfer [21].
detailed structural and spectroscopic characterizations of the monomeric nonionic forms of many simple amino acids have been achieved by combining matrix
isolation IR spectroscopy with contemporary methods of quantum chemistry.
the simultaneous experimental observation of the three lower energy conformers of the simplest amino acid, glycine, by matrix isolation infrared spectroscopy
[22], has been a milestone in the structural characterization of this compound. Previous microwave spectroscopy studies [26–30] had already provided experimental evidence of existence of two conformers of glycine in the gas phase (I and II;
Fig. 7.4), though the significantly lower dipole moment of the most stable conformer I (bearing an intramolecular hydrogen-bond of the Nh 2 ···o = C type), compared
to conformer II (possessing an intramolecular o–h···N hydrogen-bond), initially
made detection of the first one difficult by that technique. Conformer III (with an
Nh 2 ···oh h-bond; Fig. 7.4) has also a low dipole moment, and its expected small
relative population makes its experimental detection by microwave spectroscopy
very hard. on the other hand, a gas phase electron diffraction study [31], had suggested that, at 219 °C, about 24 % of the compound should correspond to a mixture
of minor conformers of glycine, which were supposed to be conformers II and III.
however, these conformers are different from each other only in the position of
the hydrogens, and they are indistinguishable by their electron scattering intensities. thus, the question of the conformational structure of glycine in the gas phase
could only be effectively solved after its study by matrix isolation infrared spectroscopy. In the matrix isolation study [22], the compound and some of its deuterated
isotopologues were isolated in low-temperature argon matrices, and the infrared