5 α-Amino Acids In Water: A Review of VCD and ROA Spectra
113
VCD spectra: Calculations Probably the first calculations of the vCd spectra
of three isotopomers of alanine were done by diem et al. and Lal et al. in 1982
[171, 172] at the semiempirical CNdo/2 (Complete Neglect of differential overlap) level [189]. It is not strange that at that time “a very close relative agreement”
between semi-empirical and experimental spectra in the Ch stretching vibrations
region was stated and the discrepancy between theoretical and experimental spectra
was attributed to the presence of water influencing the spectra. It is noteworthy
that after 30 years of theoretical calculations of alanine vibrational optical activity
spectra, a proper inclusion of the influence of water on the voA spectra is still the
most important issue [141, 154, 190, 191]. the first ab initio hF/SCRF/6-31g*
calculations of the vCd (and RoA) spectra of alanine were presented by Nafie
et al. in 1994 [179]. the influence of surrounding water was simulated using the
self-consistent reaction field (SCRF) which modelled the water environment by setting the relative permittivity to 80 outside the cavity with a radius of 3.5 Å in which
the alanine molecule was closed. the calculated spectra were compared with those
measured by diem [174]. It is not surprising that, at that level of theory, some bands
were misordered and produced assignment problems. Nevertheless, according to
the prevailing standards of the 1990s, reproduction of the experimental spectra was
again evaluated as being very good [179]. the strength of the Polarised Continuum
models in theoretical studies of the solvent effect on the vCd spectra of alanine
was presented by gontrani, mennucci, and tomasi in 2000 [180]. they demonstrated that PCm combined with the B3LYP/6-31g* level of theory was able to
reproduce both the stability of zwitterionic forms of alanine in water solution and
the vCd spectra, except for the modes in which alanine h-atoms form hydrogen
bonds with water. the authors suggested that a supermolecular approach such as
that already used a few years earlier by tajkhorshid et al. [181] could improve the
predictions of the vCd intensity of those modes.
the supramolecular model of the solvent effect on the vCd spectra of L-alanine
was repetitively applied by tajkhorshid et al., [181] Frimand et al., [182] and Jalkanen et al. [144, 154] at gradually increasing levels of theory or complexity of
the studied supermolecular system. In the first of this series of papers [181], it was
found that the onsager solvent model cannot stabilise the alanine zwitterion against
conversion into the neutral tautomer, whereas a discrete set of four water molecules
stabilises zwitterionic alanine in the B3LYP/6-31g* calculations. to calculate the
vCd spectra, the distributed origin gauge atomic axial tensors and the electric dipole electric-dipole polarisability derivatives were calculated at the RhF/6-31g**
level of theory for the B3LYP calculated geometries. the as-calculated vCd spectra appeared to be very sensitive to the relative arrangement of alanine and the
neighbouring water molecules. In the next study [182], also performed at the RhF
and dFt levels combined with the 6-31g* basis set, the AAts were calculated with
the B3LYP functional. the alanine zwitterion was considered in three ways: (1)
as surrounded by explicitly four or nine water molecules only, (2) embedded in a
self-consistent reaction field expressed by the onsager model, and (3) surrounded
by explicit water molecules embedded in a self-consistent reaction field. It was
found that most bands of the vCd and IR spectra in the range of 1650–840 cm
−1
,
113
VCD spectra: Calculations Probably the first calculations of the vCd spectra
of three isotopomers of alanine were done by diem et al. and Lal et al. in 1982
[171, 172] at the semiempirical CNdo/2 (Complete Neglect of differential overlap) level [189]. It is not strange that at that time “a very close relative agreement”
between semi-empirical and experimental spectra in the Ch stretching vibrations
region was stated and the discrepancy between theoretical and experimental spectra
was attributed to the presence of water influencing the spectra. It is noteworthy
that after 30 years of theoretical calculations of alanine vibrational optical activity
spectra, a proper inclusion of the influence of water on the voA spectra is still the
most important issue [141, 154, 190, 191]. the first ab initio hF/SCRF/6-31g*
calculations of the vCd (and RoA) spectra of alanine were presented by Nafie
et al. in 1994 [179]. the influence of surrounding water was simulated using the
self-consistent reaction field (SCRF) which modelled the water environment by setting the relative permittivity to 80 outside the cavity with a radius of 3.5 Å in which
the alanine molecule was closed. the calculated spectra were compared with those
measured by diem [174]. It is not surprising that, at that level of theory, some bands
were misordered and produced assignment problems. Nevertheless, according to
the prevailing standards of the 1990s, reproduction of the experimental spectra was
again evaluated as being very good [179]. the strength of the Polarised Continuum
models in theoretical studies of the solvent effect on the vCd spectra of alanine
was presented by gontrani, mennucci, and tomasi in 2000 [180]. they demonstrated that PCm combined with the B3LYP/6-31g* level of theory was able to
reproduce both the stability of zwitterionic forms of alanine in water solution and
the vCd spectra, except for the modes in which alanine h-atoms form hydrogen
bonds with water. the authors suggested that a supermolecular approach such as
that already used a few years earlier by tajkhorshid et al. [181] could improve the
predictions of the vCd intensity of those modes.
the supramolecular model of the solvent effect on the vCd spectra of L-alanine
was repetitively applied by tajkhorshid et al., [181] Frimand et al., [182] and Jalkanen et al. [144, 154] at gradually increasing levels of theory or complexity of
the studied supermolecular system. In the first of this series of papers [181], it was
found that the onsager solvent model cannot stabilise the alanine zwitterion against
conversion into the neutral tautomer, whereas a discrete set of four water molecules
stabilises zwitterionic alanine in the B3LYP/6-31g* calculations. to calculate the
vCd spectra, the distributed origin gauge atomic axial tensors and the electric dipole electric-dipole polarisability derivatives were calculated at the RhF/6-31g**
level of theory for the B3LYP calculated geometries. the as-calculated vCd spectra appeared to be very sensitive to the relative arrangement of alanine and the
neighbouring water molecules. In the next study [182], also performed at the RhF
and dFt levels combined with the 6-31g* basis set, the AAts were calculated with
the B3LYP functional. the alanine zwitterion was considered in three ways: (1)
as surrounded by explicitly four or nine water molecules only, (2) embedded in a
self-consistent reaction field expressed by the onsager model, and (3) surrounded
by explicit water molecules embedded in a self-consistent reaction field. It was
found that most bands of the vCd and IR spectra in the range of 1650–840 cm
−1
,