5 α-Amino Acids In Water: A Review of VCD and ROA Spectra
119
VCD spectra: Experiment and Calculations L-proline is a relatively small, rigid
and water-soluble amino acid which, probably, is the main reason why the number
of vCd and RoA studies dedicated to it is greater than for the other natural amino
acids except for L-alanine. the vCd C*h stretching and bending vibrations bands
of L-proline (along with 4-hydroxy and allo-4-hydroxy proline) were measured in
water in the 1980s, revealing a pattern characteristic of all L-amino acids [147–149].
therefore, it is surprising that the first systematic study of the vCd spectra of
L-proline in water was a computational study by Cappelli et al. in 2005 [201]. the
authors investigated the vCd and IR spectra of L-proline in water using the dFt
method, with three water molecules in the first solvation sphere with or without the
IEF-PCm version of the polarisable continuum model of bulk water. Four forms of
L-proline (neutral, zwitterionic, cationic, and anionic) were considered. the authors
found that hydrogen bond effects on spectra can only be partially reproduced by the
sole continuum solvent model, whereas the introduction of explicit water molecules
shifted several modes towards lower frequencies, changed the intensities and, for
some of vCd bands, also changed the sign. obviously, protonation was also shown
to noticeably change the spectra. Interestingly, the environment affected the geometry relatively little, but sometimes dramatically changed the Boltzmann population
of the conformers.
In 2006, the vCd spectra of L-proline (and several other natural amino acids)
were measured in the form of aqueous films with α-cyclodextrin (α-CD, Fig. 5.8a)
[38]. A negative vCd band at 1628 cm
−1
due to stretching vibrations of the C = o
group could be registered more easily than the bands at ca. 1550 cm
−1
, because
the L-proline spectra are weak and the absorbance of α-CD bands in the 1200–
1000 cm
−1
region is strong. therefore, in this range, the vCd bands of L-proline
were not very reliable. the 1550–1200 cm
−1
region of the vCd bands could be seen
in the subtracted spectra in which the appropriate solution of racemate proline in
α-CD film was used as a reference background. Between 1400 and 1300 cm
−1
, a
negative-positive doublet, reported earlier [149], was clearly seen in the subtracted
vCd spectrum and confirmed in the IR absorption spectra (Fig. 5.8a). It worth adding that in 2004 and 2006, the vCd and IR spectra of N-acetylproline in water were
measured and calculated to determine its conformation in water and to verify the
md force fields used for larger amides and polypeptides [202, 203].
Quite recently, Lüdeke et al. recorded the vCd spectra with a tunable externalcavity quantum cascade laser (QCL) [204]. they showed that difficulty in standard
vCd spectroscopy measurements due to strong IR absorption of solvents can be
overcome by using QCLs, first demonstrated in 1994 [205]. 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. the wavelength of a QCL
can be tuned within a window of ca. 8 % around the central wavelength, yielding a
ca. 100 to 200 cm
−1
spectral range lying between 800 and 2500 cm
−1
. however, the
spectral range of 100 cm
−1
may be sufficient to cover important difference bands
such as amide I or amide II bands of peptides. the authors demonstrated the 1320
and 1220 cm
−1
range of the QCL-vCd spectra of L- and d-proline taken in water with background correction by a racemic mixture. In agreement with previous
119
VCD spectra: Experiment and Calculations L-proline is a relatively small, rigid
and water-soluble amino acid which, probably, is the main reason why the number
of vCd and RoA studies dedicated to it is greater than for the other natural amino
acids except for L-alanine. the vCd C*h stretching and bending vibrations bands
of L-proline (along with 4-hydroxy and allo-4-hydroxy proline) were measured in
water in the 1980s, revealing a pattern characteristic of all L-amino acids [147–149].
therefore, it is surprising that the first systematic study of the vCd spectra of
L-proline in water was a computational study by Cappelli et al. in 2005 [201]. the
authors investigated the vCd and IR spectra of L-proline in water using the dFt
method, with three water molecules in the first solvation sphere with or without the
IEF-PCm version of the polarisable continuum model of bulk water. Four forms of
L-proline (neutral, zwitterionic, cationic, and anionic) were considered. the authors
found that hydrogen bond effects on spectra can only be partially reproduced by the
sole continuum solvent model, whereas the introduction of explicit water molecules
shifted several modes towards lower frequencies, changed the intensities and, for
some of vCd bands, also changed the sign. obviously, protonation was also shown
to noticeably change the spectra. Interestingly, the environment affected the geometry relatively little, but sometimes dramatically changed the Boltzmann population
of the conformers.
In 2006, the vCd spectra of L-proline (and several other natural amino acids)
were measured in the form of aqueous films with α-cyclodextrin (α-CD, Fig. 5.8a)
[38]. A negative vCd band at 1628 cm
−1
due to stretching vibrations of the C = o
group could be registered more easily than the bands at ca. 1550 cm
−1
, because
the L-proline spectra are weak and the absorbance of α-CD bands in the 1200–
1000 cm
−1
region is strong. therefore, in this range, the vCd bands of L-proline
were not very reliable. the 1550–1200 cm
−1
region of the vCd bands could be seen
in the subtracted spectra in which the appropriate solution of racemate proline in
α-CD film was used as a reference background. Between 1400 and 1300 cm
−1
, a
negative-positive doublet, reported earlier [149], was clearly seen in the subtracted
vCd spectrum and confirmed in the IR absorption spectra (Fig. 5.8a). It worth adding that in 2004 and 2006, the vCd and IR spectra of N-acetylproline in water were
measured and calculated to determine its conformation in water and to verify the
md force fields used for larger amides and polypeptides [202, 203].
Quite recently, Lüdeke et al. recorded the vCd spectra with a tunable externalcavity quantum cascade laser (QCL) [204]. they showed that difficulty in standard
vCd spectroscopy measurements due to strong IR absorption of solvents can be
overcome by using QCLs, first demonstrated in 1994 [205]. 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. the wavelength of a QCL
can be tuned within a window of ca. 8 % around the central wavelength, yielding a
ca. 100 to 200 cm
−1
spectral range lying between 800 and 2500 cm
−1
. however, the
spectral range of 100 cm
−1
may be sufficient to cover important difference bands
such as amide I or amide II bands of peptides. the authors demonstrated the 1320
and 1220 cm
−1
range of the QCL-vCd spectra of L- and d-proline taken in water with background correction by a racemic mixture. In agreement with previous