Polarimetry is a molecular spectroscopy technique that measures the properties of molecules using polarized light. Normally, light scatters in many directions, but polarized light is limited to a particular orientation. When polarized light is rotated by a sample, that sample is considered optically active. For example, dextrose, or D-glucose, rotates polarized light to the right (from the Latin dexter meaning “to the right”). The angle of rotation at a particular wavelength is proportional to the concentration of the sample. The technique has been used since the early 19th century to determine the concentrations of sugars and syrups. In a more contemporary context, the technique is still found in the food industry and is also used in life science applications to better understand the structure of biomolecules, often using circular dichroism (CD) systems. CD is a technique that utilizes circularly polarized light to analyze the structure of optically active molecules. If one could see the electromagnetic fields associated with light, circularly polarized light would appear to be rotating clockwise or counterclockwise, whereas linearly polarized light would appear to be oriented in one direction, usually horizontally or vertically (as in polarized sunglasses). Biomolecules can absorb left circularly polarized (LCP) light and right circularly polarized (RCP) light differently, and this difference gives insight to the molecular structure. Most modern CD instruments are the result of work done in the 1960s and remain largely similar today. Typically, a sample is exposed to alternating LCP and RCP light at a specific wavelength, and the differential absorption of the light is measured to produce a CD spectrograph. Magnetic CD works the same, but also applies a strong magnetic field parallel to the direction of light. Vibrational CD extends the range of CD into the IR and NIR ranges. These instruments are largely used in life science applications to understand the structure of proteins, nucleic acids and other biomolecules, but these instruments can also be used to analyze semiconductor thin films. In the life sciences, the secondary structure of proteins each have characteristic spectra, which change based on the conformation of the molecule. Due to the spectra being based on the conformation of the molecule, denaturation and structural changes from bonding can be studied using CD. This information can also be used to determine thermodynamic properties. Other analytical techniques like NMR or MS give more specific information on the molecular structure, but CD is much quicker and less expensive than those techniques. As such, CD is often used in combination with other analytical techniques to create a more detailed picture of a molecule’s structure and properties. One new innovation to better integrate techniques is a CD microplate reader introduced Hinds Instruments in July 2017. Despite CD being related to polarimetry in general, very few polarimeter vendors make CD-specific instruments. The overall CD market is relatively small at less than $10 million. The potential for growth is strong as the pharmaceutical and biotechnology sector continues to perform well and seeks to better understand biomolecular structures. JASCO is the leading vendor in the CD market, offering the J-1000 Series spectropolarimeters and FVS-6000 VCD spectrometer. Other vendors include Applied Photophysics, Bio-Logic, BioTools, Hinds Instruments and Olis.
CD at a Glance:
- Applied Photophysics