Raman Optical Activity: Principles and Applications
RNDr. Václav Profant, Ph.D., Institute of Physics of Charles University, Faculty of Mathematics and Physics, Charles University,
One of the main interests of field of biophysics is the determination of structure of biologically active molecules and consecutive clarification of the relation between their structure and functionality. The molecular structure is traditionally studied by methods of x-ray crystallography and nuclear magnetic resonance (NMR). Both methods can provide data within atomic resolution; however, they also suffer from some intrinsic limitations. X-ray crystallography is used for identification of atomic and molecular structure of crystals, i.e. solid phase samples, and absolute configuration determination is possible only in the case of monocrystalline samples. Also, the crystal based molecular structure may differ from the structure in solution which is the favored environment for biomolecules. NMR studies reveal the solution structure of molecules; nevertheless, the measurements require larger amount of samples and there is an upper limit for the size of the studied systems.
Independently, the information about the molecular structure can be provided by the vibrational spectroscopy which can verify, supplement and broaden findings based on the before mentioned techniques. Both main methods of vibrational spectroscopy - infrared absorption (IR) and Raman scattering (RS) - are nondestructive, suitable for measurements in solutions, and they enable to study molecules under different experimental conditions (pH, T, concentration, etc.). As vibrational transitions have fast photon response, the acquired spectra are sums of spectra of individual conformers. Therefore, vibrational spectra represent dynamical image of studied molecules in solution. On the other hand, the vibrational spectra suffer from low resolution and they are generally difficult to interpret even for relatively small systems. The detailed interpretation of vibrational spectra should be preferably done on the basis of quantum mechanical (QM) calculations.
Chirality, i.e. distinguishability of an object from its mirror image, is an asymmetric property characteristic for majority of biomolecular systems. Chiral molecules exist in two forms called enantiomers, which are chemically identical (with exception of reaction with other chiral molecules) but differ in their 3D spatial arrangement. This difference is macroscopically manifested by the different meaning of rotation of the plane of linearly polarized light about the direction of propagation, i.e. different optical activity. In living systems many biomolecules are composed selectively from the specific enantiomers, such as D-saccharides in nucleic acids, and L-amino acids in proteins. However, common methods of vibrational spectroscopy are not sensitive to the chirality and provide same spectra for both enantiomers.
Fortunately, in mid-1970's it was shown that the enantiomer selectivity can be incorporated into the vibrational spectroscopy, which led to the development of the whole new field of Raman optical activity (ROA). ROA conveniently combines the stereochemical sensitivity of optical activity with the high structural content of Raman scattering spectroscopy, which all lead to increased sensitivity to local spatial arrangements of atoms in molecules. During the last decade the field of ROA experienced a substantial expansion. It is currently transforming from a specialized area of research into important new spectroscopic techniques with a wide scope of applications. This was made possible by availability of commercial instrumentation as well as software for quantum mechanical (QM) calculation of ROA spectra. The prime application of ROA remains the determination and verification of molecular chirality during organic synthesis, analysis, and natural product isolation, especially in the pharmaceutical industry. Additionally, ROA is also extremely sensitive to conformation of all classes of biological molecules, covering saccharides, nucleic acids, and proteins. For these types of studies, ROA tends to be more advantageous than complementary absorption based technique of vibrational circular dichroism (VCD) as it enables easy measurements in water in the whole region of fundamental vibrations.