5 α-Amino Acids In Water: A Review of VCD and ROA Spectra
137
configurational landscape and its dynamics is currently a challenging aspect of
computational work.
5.6.3 Promising Experimental Techniques
measuring vibrational chiroptical spectra meets two serious problems of an objective nature: (1) the sensitivity of the vCd and RoA spectra and (2) the very low solubility of several amino acids in water. Continuous improvement of spectrometers
facilitates and ameliorates the quality of vCd and RoA measurements. however,
measurements of the vCd spectra with a tunable external-cavity quantum cascade
laser (QCL), as recently done by Lüdeke et al. for proline [204], are especially
promising. QCLs provide at least 100 times more power in the mIR than ordinary
thermal IR light sources and permit the use of IR detectors that do not require N 2
cooling. A shortcoming of the method is in the very limited wavenumber range of
ca. 100 to 200 cm
−1
in the 800 and 2500 cm
−1
mIR interval. however, even now,
this spectral range would permit very valuable measurements of amino acid vCd
spectra.
Another promising innovation may be the introduction of a transient vibrational chiral spectrometer, capable of detecting photo-induced vCd changes
with picosecond time resolution, as recently described by Bonmarin and helbing
[224–228]. With this newly developed spectrometer, it was possible to detect vCd
changes in the Ch-stretch region following visible excitation of a cobalt spartein
complex (Co(sp)Cl 2 ) with picosecond time resolution [224]. Also, it was possible
to significantly increase the signal to noise ratio of the static vCd and voRd
spectra and ameliorate the quality of the transient chiral signals. It is expected that
this technique will make it possible to identify chiral intermediates in the course
of a chemical reaction or to probe with unprecedented detail the conformational
dynamics of amino acids and local secondary structure formation in peptides and
proteins [224].
Yet another technique which essentially extends the possibilities of vCd spectroscopy is application of the two-dimensional vCd (2d vCd) method, known for
more than a decade [184] but still rarely applied [176, 184–186]. As demonstrated
by ma et al. for alanine in a water solution, 2d vCd enables significant resolution
enhancement of overlapping bands and assignments of combination bands, especially in the NIR vCd region [176]. A variety of 2d spectra can be registered in
synchronous and asynchronous modes, providing the possibility of combination
of homo- and hetero-region spectra of both IR-IR, IR-vCd, and vCd-vCd techniques [176]. this seems to provide an extremely broad potential for applications
of 2d vCd spectroscopy in physico-chemical studies of amino acids and other
important biomolecules.
The α-cyclodextrin film technique introduced to VCD spectroscopy by Shanmugam and Polavarapu [151], and then applied by Zhang and Polavarapu [38, 153] and
combined with modern vCd spectrometers [55], is worth further exploration, since
137
configurational landscape and its dynamics is currently a challenging aspect of
computational work.
5.6.3 Promising Experimental Techniques
measuring vibrational chiroptical spectra meets two serious problems of an objective nature: (1) the sensitivity of the vCd and RoA spectra and (2) the very low solubility of several amino acids in water. Continuous improvement of spectrometers
facilitates and ameliorates the quality of vCd and RoA measurements. however,
measurements of the vCd spectra with a tunable external-cavity quantum cascade
laser (QCL), as recently done by Lüdeke et al. for proline [204], are especially
promising. QCLs provide at least 100 times more power in the mIR than ordinary
thermal IR light sources and permit the use of IR detectors that do not require N 2
cooling. A shortcoming of the method is in the very limited wavenumber range of
ca. 100 to 200 cm
−1
in the 800 and 2500 cm
−1
mIR interval. however, even now,
this spectral range would permit very valuable measurements of amino acid vCd
spectra.
Another promising innovation may be the introduction of a transient vibrational chiral spectrometer, capable of detecting photo-induced vCd changes
with picosecond time resolution, as recently described by Bonmarin and helbing
[224–228]. With this newly developed spectrometer, it was possible to detect vCd
changes in the Ch-stretch region following visible excitation of a cobalt spartein
complex (Co(sp)Cl 2 ) with picosecond time resolution [224]. Also, it was possible
to significantly increase the signal to noise ratio of the static vCd and voRd
spectra and ameliorate the quality of the transient chiral signals. It is expected that
this technique will make it possible to identify chiral intermediates in the course
of a chemical reaction or to probe with unprecedented detail the conformational
dynamics of amino acids and local secondary structure formation in peptides and
proteins [224].
Yet another technique which essentially extends the possibilities of vCd spectroscopy is application of the two-dimensional vCd (2d vCd) method, known for
more than a decade [184] but still rarely applied [176, 184–186]. As demonstrated
by ma et al. for alanine in a water solution, 2d vCd enables significant resolution
enhancement of overlapping bands and assignments of combination bands, especially in the NIR vCd region [176]. A variety of 2d spectra can be registered in
synchronous and asynchronous modes, providing the possibility of combination
of homo- and hetero-region spectra of both IR-IR, IR-vCd, and vCd-vCd techniques [176]. this seems to provide an extremely broad potential for applications
of 2d vCd spectroscopy in physico-chemical studies of amino acids and other
important biomolecules.
The α-cyclodextrin film technique introduced to VCD spectroscopy by Shanmugam and Polavarapu [151], and then applied by Zhang and Polavarapu [38, 153] and
combined with modern vCd spectrometers [55], is worth further exploration, since