5 α-Amino Acids In Water: A Review of VCD and ROA Spectra
93
5.4 Basis of Molecular Activity in Vibrational Spectra
5.4.1 General Considerations
there are many methods which can be used to determine the structure and to understand the function of biomolecules. the methods of choice for biomolecules that
can be crystallised are X-ray and neutron diffraction. For several smaller and more
flexible biomolecules, such as some amino acids and small peptides, this is hardly
possible. In these cases, NmR techniques, using the nuclear overhauser effect, have
been used to supplement the diffraction methods and to solve the structure of biomolecules in the folding state in solution.
optical spectroscopy methods have also provided an effective way of investigating molecular structure. these techniques are characterised by lower resolution
than X-ray crystallography (as the wavelength of infrared or visible light is much
longer than typical molecular sizes) for the registration of signals from individual
conformers. Increased conformational sensitivity can be achieved with polarised
techniques by assessing absorption or scattering of circularly polarised light by chiral systems.
In recent years, a renaissance in the use of chiroptical spectroscopic methods
has taken place. Books and reviews are available on this topic [53–68]. vibrational
optical Activity (voA) encompasses two main forms of chiroptical spectroscopy:
(1) vibrational circular dichroism (vCd) and (2) vibrational Raman optical activity
(RoA). these methods are powerful techniques in stereochemical analysis (conformation as well as configuration), and in determining the absolute configuration
(AC) of molecules. Normal and chiral spectroscopies are highly structure-sensitive.
many changes have been observed in the spectra as a function of solvent polarity,
ph, and temperature. typical voA (vCd/RoA) intensities are four or five orders
of magnitude smaller than those of the parent IR/Raman methods. Nevertheless, use
of vCd/RoA spectra combined with quantum-chemical calculations is a method of
choice for the assignment of AC instead of intense but much less selective optical
rotatory dispersion (oRd) or electronic circular dichroism (ECd) spectra.
5.4.2 Chiroptical Methods
the chiroptical spectroscopic methods that are in wide current use include: 1. electronic circular dichroism (ECd), 2. optical rotatory dispersion (oRd), 3. vibrational circular dichroism (vCd), and 4. vibrational Raman optical activity (RoA).
Chiroptical vCd or RoA spectroscopies probe molecular vibrations to measure
the differential absorption (IR) or scattering (from NIR to uv) of right- and leftcircularly polarised light. In contrast to the ECd technique, these two methods can
be applied to molecules lacking chromophores. moreover, the numerous vibrational
modes give more data for comparison with the simulated spectra than a few broad
electronic spectroscopy bands.
93
5.4 Basis of Molecular Activity in Vibrational Spectra
5.4.1 General Considerations
there are many methods which can be used to determine the structure and to understand the function of biomolecules. the methods of choice for biomolecules that
can be crystallised are X-ray and neutron diffraction. For several smaller and more
flexible biomolecules, such as some amino acids and small peptides, this is hardly
possible. In these cases, NmR techniques, using the nuclear overhauser effect, have
been used to supplement the diffraction methods and to solve the structure of biomolecules in the folding state in solution.
optical spectroscopy methods have also provided an effective way of investigating molecular structure. these techniques are characterised by lower resolution
than X-ray crystallography (as the wavelength of infrared or visible light is much
longer than typical molecular sizes) for the registration of signals from individual
conformers. Increased conformational sensitivity can be achieved with polarised
techniques by assessing absorption or scattering of circularly polarised light by chiral systems.
In recent years, a renaissance in the use of chiroptical spectroscopic methods
has taken place. Books and reviews are available on this topic [53–68]. vibrational
optical Activity (voA) encompasses two main forms of chiroptical spectroscopy:
(1) vibrational circular dichroism (vCd) and (2) vibrational Raman optical activity
(RoA). these methods are powerful techniques in stereochemical analysis (conformation as well as configuration), and in determining the absolute configuration
(AC) of molecules. Normal and chiral spectroscopies are highly structure-sensitive.
many changes have been observed in the spectra as a function of solvent polarity,
ph, and temperature. typical voA (vCd/RoA) intensities are four or five orders
of magnitude smaller than those of the parent IR/Raman methods. Nevertheless, use
of vCd/RoA spectra combined with quantum-chemical calculations is a method of
choice for the assignment of AC instead of intense but much less selective optical
rotatory dispersion (oRd) or electronic circular dichroism (ECd) spectra.
5.4.2 Chiroptical Methods
the chiroptical spectroscopic methods that are in wide current use include: 1. electronic circular dichroism (ECd), 2. optical rotatory dispersion (oRd), 3. vibrational circular dichroism (vCd), and 4. vibrational Raman optical activity (RoA).
Chiroptical vCd or RoA spectroscopies probe molecular vibrations to measure
the differential absorption (IR) or scattering (from NIR to uv) of right- and leftcircularly polarised light. In contrast to the ECd technique, these two methods can
be applied to molecules lacking chromophores. moreover, the numerous vibrational
modes give more data for comparison with the simulated spectra than a few broad
electronic spectroscopy bands.