J. C. Dobrowolsk et al.
138
it would provide new valuable results for poorly soluble amino acids. Some other
innovations can be found in Ref. [56, 229, 230].
Water is a natural solvent for Raman measurements at different ph levels.
moreover, the array of Raman spectroscopy methods is much larger than those
available for absorption IR spectroscopy. therefore, we expect much more intensive studies on amino acids in water media by RoA than by vCd techniques.
In the late 1990s, Nafie proposed the single electronic-state limit of the theory of
Resonance RoA (RRoA) effect and explained why the sign of all RRoA bands is
the same [231]. Subsequently, the RRoA was measured [232–234] and the theory
was extended and improved [235–237]. Quite recently, RRoA involving more
than one resonant electronic state has been reported [238]. Also very recently,
induced resonance Raman optical activity (IRRoA) has been detected in the presence of a europium complex, achieving a 10
4
-fold increase in sensitivity in comparison to non-resonant RoA [235]. Registration of RRoA of amino acids may be
important for aromatic phenylalanine and tryptophan, and, especially, for tyrosine
which, because of its extremely poor solubility in water, have not yet been studied
by vCd and RoA methods.
using a uv frequency laser for RoA measurements was considered, as changing the beam frequency from 488 to 244 nm would increase the RoA signals by
a factor of 32 [55]. Also, the uv incident beam makes the near resonance Raman
conditions additionally probable by increasing the intensity. Recently, Kapitán,
Barron, and hecht experimentally devised an RoA spectrometer with uv excitation wavelength of 244 nm [239]. this technique is promising for a significant
extension of the capabilities of RoA spectroscopy. this would be especially interesting for amino acids and peptides, since indeed high-quality uv RoA spectra of zwitterionic cyclo(L-Ala-L-Ala) and cyclo(d-Ala-d-Ala) and other alanine
peptides have been demonstrated [239]. on the other hand, excitation in NIR decreases the fluorescence of the sample. In 2010, it was reported that a near IR RoA
spectrometer was constructed and applied for measurements of S-(-)-α-pinene and
L-alanyl-L-alanine in water [240]. Also in 2010, NIR RoA measurements were
performed on proteins using a 785 nm NIR diode laser [241]. despite the fact that,
with the NIR beam frequency, RoA signals are decreased by significant factor,
such spectrometers seem to present high potential for measuring biomolecules
in situ and, in particular, for measurements of free amino acids in their native
surrounding.
In 2008, Abdali and Blanch surveyed the theoretical and experimental work
undertaken to develop surface-enhanced Raman optical activity (SERoA), which
is complementary to both SERS and RoA [242]. the application of SERoA spectra to studies of amino acids was done in 2011 for L-phenylalanine and L- and
d-cysteine adsorbed on silver [243]. however, for phenylalanine, the presence of
SERoA signals was questionable, while for L- and d-cysteine on a silver electrode
mirror image surface, the RoA spectra were registered in a broad wavenumber
region, confirming the observation of the SERoA effect. theoretical models of
the SERoA are continuously being developed [244–246]. moreover, observations
138
it would provide new valuable results for poorly soluble amino acids. Some other
innovations can be found in Ref. [56, 229, 230].
Water is a natural solvent for Raman measurements at different ph levels.
moreover, the array of Raman spectroscopy methods is much larger than those
available for absorption IR spectroscopy. therefore, we expect much more intensive studies on amino acids in water media by RoA than by vCd techniques.
In the late 1990s, Nafie proposed the single electronic-state limit of the theory of
Resonance RoA (RRoA) effect and explained why the sign of all RRoA bands is
the same [231]. Subsequently, the RRoA was measured [232–234] and the theory
was extended and improved [235–237]. Quite recently, RRoA involving more
than one resonant electronic state has been reported [238]. Also very recently,
induced resonance Raman optical activity (IRRoA) has been detected in the presence of a europium complex, achieving a 10
4
-fold increase in sensitivity in comparison to non-resonant RoA [235]. Registration of RRoA of amino acids may be
important for aromatic phenylalanine and tryptophan, and, especially, for tyrosine
which, because of its extremely poor solubility in water, have not yet been studied
by vCd and RoA methods.
using a uv frequency laser for RoA measurements was considered, as changing the beam frequency from 488 to 244 nm would increase the RoA signals by
a factor of 32 [55]. Also, the uv incident beam makes the near resonance Raman
conditions additionally probable by increasing the intensity. Recently, Kapitán,
Barron, and hecht experimentally devised an RoA spectrometer with uv excitation wavelength of 244 nm [239]. this technique is promising for a significant
extension of the capabilities of RoA spectroscopy. this would be especially interesting for amino acids and peptides, since indeed high-quality uv RoA spectra of zwitterionic cyclo(L-Ala-L-Ala) and cyclo(d-Ala-d-Ala) and other alanine
peptides have been demonstrated [239]. on the other hand, excitation in NIR decreases the fluorescence of the sample. In 2010, it was reported that a near IR RoA
spectrometer was constructed and applied for measurements of S-(-)-α-pinene and
L-alanyl-L-alanine in water [240]. Also in 2010, NIR RoA measurements were
performed on proteins using a 785 nm NIR diode laser [241]. despite the fact that,
with the NIR beam frequency, RoA signals are decreased by significant factor,
such spectrometers seem to present high potential for measuring biomolecules
in situ and, in particular, for measurements of free amino acids in their native
surrounding.
In 2008, Abdali and Blanch surveyed the theoretical and experimental work
undertaken to develop surface-enhanced Raman optical activity (SERoA), which
is complementary to both SERS and RoA [242]. the application of SERoA spectra to studies of amino acids was done in 2011 for L-phenylalanine and L- and
d-cysteine adsorbed on silver [243]. however, for phenylalanine, the presence of
SERoA signals was questionable, while for L- and d-cysteine on a silver electrode
mirror image surface, the RoA spectra were registered in a broad wavenumber
region, confirming the observation of the SERoA effect. theoretical models of
the SERoA are continuously being developed [244–246]. moreover, observations