5 α-Amino Acids In Water: A Review of VCD and ROA Spectra
109
spectra of L-alanine in d 2 o were recorded in the mid-IR (800–2000 cm
−1
) region,
the hydrogen-stretching region (2000–4000 cm
−1
), and, for the first time, the nearIR region (4000–6000 cm
−1
) as a function of pd (Fig. 5.1). A variety of different
2d-CoS plots were demonstrated based on IR and vCd spectral data, as well as
the combination of both IR and vCd spectra. Analysis of homo- and hetero spectral
regions of vCd–vCd and IR–IR 2d-CoS plots revealed the correlation of mutual
changes of methine and methyl bending, methine and methyl stretching and carboxylate stretching regions with changes in the acidity and basicity of the solution. the
2d maps evidently enhanced the resolution of overlapping bands and assignments
of combination bands especially in the NIR vCd region.
In 2006, the vCd bands of L-alanine, described before as weak [174], were
called strong [176]. the characteristic negative/positive intensity pattern of two orthogonal C*h bending modes at 1337 cm
−1
and 1291 cm
−1
, respectively, as well as
the weak negative Ch 3 umbrella mode at 1377 cm
−1
, exhibit most of the spectral
changes at low pd between 3.0 and 1.5, whereas in the pd range from 3.0 to 13.0,
the changes occur smoothly (Fig. 5.1a, b). For higher frequency regions, only the
spectra for high pd were measured (Fig. 5.1c, d)
In comparison to IR, the corresponding vCd intensities are four to five orders of
magnitude smaller, and, as a consequence, the vCd–vCd 2d-CoS plots are eight
to nine orders of magnitude smaller than the corresponding IR–IR plots. moreover,
the synchronous 2d-CoS plots are usually at least an order of magnitude larger than
the associated asynchronous plots. here, we mention only some of the 2d maps
generated in Ref. [176]. the negative cross-peaks at 1291 and 1337 cm
−1
of the
synchronous vCd–vCd 2d-CoS plot (Fig. 5.2a) represent the pd-induced correlation of the two bending motions of the C*h group. they indicate that the two
correlated bands changed intensity in the opposite direction. this means that they
both decreased in intensity. the corresponding asynchronous cross-peaks at 1291
and 1337 cm
−1
were positive (yellow, Fig. 5.2b), meaning that, over the range of the
pd perturbation, the lower frequency C*h bending motion changed earlier than the
higher frequency motion. high pd changed the charge on the amino group. thus,
C*h movement towards the Nh 3
+
group was more sensitive to pd changes than
those directed towards the carboxylate group. this indicates that the C*h bending
positioned at 1291 cm
−1
corresponds to modes directed towards the Nh 3
+
group,
whereas the mode at 1337 cm
−1
can be assigned to motions towards the Co 2
−
group.
Similarly, the weak positive synchronous and asynchronous cross-peaks (1377 and
1337 cm
−1
) indicate that the umbrella δ
s
(Ch 3 ) mode (1377 cm
−1
) is more sensitive to high pD than the higher frequency bending δ(C*H) mode. The synchronous
and asynchronous vCd–IR 2d-CoS of L-alanine bands at 1291 and 1337 cm
−1
(Fig. 5.3) lead to the same conclusion as those derived from the vCd–vCd 2d
spectra i.e., that the methine group is more sensitive to high pd than the methyl
group to pd changes.
In comparison to conventional 1d vCd spectra, a significant resolution enhancement was obtained for the 2d NIR vCd spectra (Fig. 5.4). thanks to this, by
comparing the signs of synchronous and asynchronous 2d vCd peaks, the authors
could obtain the following sequences of apparent change as a result of increasing
109
spectra of L-alanine in d 2 o were recorded in the mid-IR (800–2000 cm
−1
) region,
the hydrogen-stretching region (2000–4000 cm
−1
), and, for the first time, the nearIR region (4000–6000 cm
−1
) as a function of pd (Fig. 5.1). A variety of different
2d-CoS plots were demonstrated based on IR and vCd spectral data, as well as
the combination of both IR and vCd spectra. Analysis of homo- and hetero spectral
regions of vCd–vCd and IR–IR 2d-CoS plots revealed the correlation of mutual
changes of methine and methyl bending, methine and methyl stretching and carboxylate stretching regions with changes in the acidity and basicity of the solution. the
2d maps evidently enhanced the resolution of overlapping bands and assignments
of combination bands especially in the NIR vCd region.
In 2006, the vCd bands of L-alanine, described before as weak [174], were
called strong [176]. the characteristic negative/positive intensity pattern of two orthogonal C*h bending modes at 1337 cm
−1
and 1291 cm
−1
, respectively, as well as
the weak negative Ch 3 umbrella mode at 1377 cm
−1
, exhibit most of the spectral
changes at low pd between 3.0 and 1.5, whereas in the pd range from 3.0 to 13.0,
the changes occur smoothly (Fig. 5.1a, b). For higher frequency regions, only the
spectra for high pd were measured (Fig. 5.1c, d)
In comparison to IR, the corresponding vCd intensities are four to five orders of
magnitude smaller, and, as a consequence, the vCd–vCd 2d-CoS plots are eight
to nine orders of magnitude smaller than the corresponding IR–IR plots. moreover,
the synchronous 2d-CoS plots are usually at least an order of magnitude larger than
the associated asynchronous plots. here, we mention only some of the 2d maps
generated in Ref. [176]. the negative cross-peaks at 1291 and 1337 cm
−1
of the
synchronous vCd–vCd 2d-CoS plot (Fig. 5.2a) represent the pd-induced correlation of the two bending motions of the C*h group. they indicate that the two
correlated bands changed intensity in the opposite direction. this means that they
both decreased in intensity. the corresponding asynchronous cross-peaks at 1291
and 1337 cm
−1
were positive (yellow, Fig. 5.2b), meaning that, over the range of the
pd perturbation, the lower frequency C*h bending motion changed earlier than the
higher frequency motion. high pd changed the charge on the amino group. thus,
C*h movement towards the Nh 3
+
group was more sensitive to pd changes than
those directed towards the carboxylate group. this indicates that the C*h bending
positioned at 1291 cm
−1
corresponds to modes directed towards the Nh 3
+
group,
whereas the mode at 1337 cm
−1
can be assigned to motions towards the Co 2
−
group.
Similarly, the weak positive synchronous and asynchronous cross-peaks (1377 and
1337 cm
−1
) indicate that the umbrella δ
s
(Ch 3 ) mode (1377 cm
−1
) is more sensitive to high pD than the higher frequency bending δ(C*H) mode. The synchronous
and asynchronous vCd–IR 2d-CoS of L-alanine bands at 1291 and 1337 cm
−1
(Fig. 5.3) lead to the same conclusion as those derived from the vCd–vCd 2d
spectra i.e., that the methine group is more sensitive to high pd than the methyl
group to pd changes.
In comparison to conventional 1d vCd spectra, a significant resolution enhancement was obtained for the 2d NIR vCd spectra (Fig. 5.4). thanks to this, by
comparing the signs of synchronous and asynchronous 2d vCd peaks, the authors
could obtain the following sequences of apparent change as a result of increasing