From a fundamental point of view, Raman spectroscopy is based on the inelastic scattering of light by matter in one of its phases: gas, liquid and solid. The scattered light can have an energy (or a frequency) either higher, or lower than that of the incident light, giving rise to anti-Stokes or Stokes Raman effects, respectively. In the framework of quantum physics, the interaction of the light (photons) with matter, induces transitions between its vibrational states. Consequently, the bands observed on Raman spectra (representing the scattering intensity versus wavenumber, i.e. inverse of wavelength, routinely expressed in cm^-1 units) reflect in fact these vibrational transitions.

An important fact in the field of molecular Raman spectroscopy is that the Raman spectrum of a molecule and its conformational properties are closely related. This is of a fundamental interest in the case of organic molecules, in which rotations around the chemical bonds (torsions), generally lead to the generation of a variety of conformers. Many of these conformers may coexist at normal conditions (temperature and pressure), because of the small amount of energy separating them in the molecular energy landscapes. Conformational transitions may be induced by varying temperature, pressure, or other physicochemical parameters, and can be detected by Raman spectroscopy through the variation of Raman intensity and/or vibrational wavenumbers. This is the reason why we generally speak of Raman conformational markers, which recall the Raman bands considered as the fingerprints of different conformations. This spectroscopic tool can also be used to recognize the interaction sites of two molecular partners, through the changes observed in the intensity or wavenumbers of the molecular groups involved in the interactions. Simultaneously, one can also follow the conformational changes of interacting molecular partners by means of their Raman conformational markers.

The application of Raman spectroscopy to biological molecules has rapidly grown up since the early 1970s, presumably because of the arrival of lasers (monochromatic and coherent sources of light) and modern dispersive spectrometers. The first period of this application was completely devoted to the constitution of a spectral database recorded from different important biological molecules (proteins, nucleic acids, phospholipids, etc) in aqueous solutions, i.e. natural medium of biological molecules and systems. Then the time arrived for studying more complex systems such as cells and tissues. This was especially made possible by the setup of microRaman spectrometers. Furthermore, the progress in microcomputer manufacturing together with the elaboration of rapid mathematical and statistical algorithms, could offer to Raman spectroscopy the possibility of acting as a powerful imaging tool. A Raman image, collected from a complex biological system, i.e. cell or tissue, can provide useful information on the localization or distribution of different constituting molecules. This is the point where global information (image) can also carry local (molecular) information. It was also natural to use Raman spectroscopy as a tool for diagnosis, for instance in analyzing metabolic effects in biological fluids, or alterations induced in tissues upon tumor formation, such as changes in DNA, protein and lipid conformations, or molecular degradation. Thanks to the continuing technical progress, portable miniature Raman spectrometers can be used in the future as a non-invasive tool for medical diagnosis.

Raman spectroscopy is widely recognized as a label-free technique; the collected signal in a spectrometer directly arises from the vibrational motions of different chemical groups of the analyzed molecule. Moreover, the contribution of water to Raman signal is rather low compared to infrared absorption. The weakness of this technique arises from its low intensity: only one scattered photon out of one million incident ones, contributes to the Raman scattering process. As a consequence, the samples used for recording Raman spectra are generally much more concentrated than those needed for other optical spectroscopic methods such as fluorescence and circular dichroism. To overcome this difficulty, two different approaches have been developed during the last years. The first one is the resonance Raman spectroscopy (RRS), which is based on the excitation of a sample by means of a monochromatic light, of which the wavelength falls within its molecular absorption spectrum. This phenomenon leads to a considerable enhancement of Raman intensity, and allows the sample concentration to be decreased. However, the absorption of light by biological molecules corresponding to their absorption spectra, generally located in the ultra-violet range of electromagnetic radiation, can cause irreversible damages due to their degradation. The second possibility is to use the Raman signal enhancement effect by the so-called plasmon resonance, when the analyzed molecule is adsorbed on the surface of a noble metal, i.e. gold or silver, colloid or nanoparticle. This method, known as surface enhanced Raman spectroscopy (SERS) has found a large audience for analyzing the biological molecules. SERS signal was shown to be enhanced up to several orders of magnitude, depending on the molecular affinity for binding to the metal surface, as well as on the molecular group involved in the adsorption. This technique is gradually becoming a routine tool in pharmacology and molecular biology for analyzing molecular samples at very low concentrations.

Mahmoud Ghomi

Paris, 2011