Measurement of Vibrational Circular Dichroism spectra using the FVS-6000
It is generally understood that chiral compounds have different bioactivities depending upon the absolute configuration of each compound. Some familiar examples include glutamic acid and thalidomide. Lglutamic acid demonstrates the “Umami” taste*1, while D-glutamic acid has a bitter taste, similarly, the R form of thalidomide is a sedative, but the S form has teratogenic activity. Thus, the separation and study of chiral compounds is critical for many reasons.
The functionalities of chiral compounds have been studied for the development of advanced molecules for many applications. The study of chiral compounds has spread to several fields such as natural products, pharmaceuticals and other functional molecules, and it can be pointed out that among those studies, the structural analysis of chiral compounds is a very important topic. X-ray Diffraction (XRD), Nuclear Magnetic Resonance (NMR) and Electronic Circular Dichroism (ECD) using UV/Vis light are employed as primary methods for the structural analysis of chiral compounds. In this paper, the measurement of chiral compounds by Vibrational Circular Dichroism (VCD) using infrared light will be outlined.
Figure 1 Principles of VCD spectroscopy
The FVS-6000 VCD system has a high sensitivity detector, suitable optical filter technology and a thermostatted Photo Elastic Modulator (PEM) to accurately measure the weak VCD peaks.
The measurement results of typical chiral compounds and hemoglobin as a model protein using the FVS-6000 are reported.
*1 Umami taste is the fifth taste sensation in addition to sweet, acid, salty and bitter taste.
Figures 3 through 6 illustrate the measurement results for alpha-pinene; 1,1-Bi-2-naphthol; proline and hemoglobin, respectively. Both IR and VCD spectra can be obtained by the FVS-6000. The identification of the absolute configuration of chiral compounds can be determined from both the IR and the VCD spectra as well as the analysis of the molecular structure.
Figure 4 outlines measurement results for 1,1-Bi-2-naphthol which is used as a ligand for transition-metal catalyzed asymmetric synthesis and is a precursor for chiral ligands such as BINAP. The small peaks due to the anisotropy factor “g value” (VCD peak/IR peak) around 1600 and 1500 cm-1 attributed to the benzene ring are clearly shown. These results demonstrate that the FVS-6000 is a very effective system for evaluation of chiral compounds which have similar structures.
Figure 3: Spectra of alpha-pinene (neat, 50 micron pathlength BaF2 liquid cell)
Figure 4: Spectra of 1,1’-Bi-2-naphthol (solvent: CHCl3, concentration: 0.162 M, 50 micron pathlength BaF2 liquid cell)
Figure 6 contains the measurement results for hemoglobin. Hemoglobin is known as a spherical model protein which contains rich alpha-helix structures, and in addition, its VCD spectrum shape is very different from that of concanavalin A, which contains a rich beta-sheet structure.
The addition of VCD spectral results to information obtained from ECD and IR spectra can provide much more accurate secondary structure analysis of proteins in solvents. We also believe that VCD can also be a powerful tool for the analysis of DNA and chiral polymers other than proteins.
In this paper, the standard performance and measurement results of typical chiral compounds using the FVS-6000 were reported and we believe the FVS-6000 can be an essential and indispensable tool for analysis of chiral compounds.
Figure 5 Spectra of proline (solvent: D2O, concentration: 0.9 M, 25 μm pathlength CaF2 liquid cell)
Figure 6 Spectra of hemoglobin (solvent: D2O, concentration: 50 mg/mL, 25 μm pathlength CaF2 liquid cell)
- Model name: FVS-6000
- Resolution: 4 cm-1
- Detector: MCT-V
- Accumulation: 1000 (alpha-pinene and 1,1’-Bi-2-naphthol), 2500 (proline), 2000 (hemoglobin)