Studies of MCD spectra from UV to NIR region by using Near Infrared CD Spectrometer

Introduction

Circular dichroism (CD) spectral measurements are indispensable for structural analysis of chiral compounds. Electronic CD (ECD), which is associated with electronic transitions in the vacuum ultraviolet (VUV) to visible regions, is often used for secondary structure estimation of proteins, and for structural analysis of chiral molecules. On the other hand, in the mid-infrared (mid-IR) region, vibrational CD (VCD) due to vibrational transitions can be used to determine the absolute configuration and conformation of chiral compounds in combination with molecular orbital calculations.

In the near infrared (NIR) region, CD signals from metal complexes and colored proteins can be observed due to d-d transitions, and CD signals from the first and second overtones of vibrational transitions can also be observed¹⁾.

However, there is no commercial CD spectrometer that is capable of measuring CD spectra in the entire VUV to NIR region. To overcome this problem, JASCO has developed the J-1700 CD spectrometer which enables CD measurements in the VUV to NIR region. By combining this with JASCO’s FVS-6000 VCD spectrometer, full VUV-to-mid-IR (163-13333 nm) CD measurements can be performed. In this section, we describe the features of the J-1700 CD spectrometer and show example of CD spectra obtained using the J-1700.

J-1700 CD Spectrometer

Unique applications of NIR CD measurements
-Structural analysis of metal complexes and colored proteins
-CD measurements of the first and second overtones of vibrational transitions
-Magnetic CD spectra measurements of transition metals

Features of J-1700 CD spectrometer
-Wavelength range is from 163 to 2500 nm.
-Light sources and detectors can be changed automatically.
Light sources: Xenon lamp and halogen lamp
Detectors: PMT and InGaAs detector (optional: InSb detector)
-Triple monochromator in NIR region for low stray light and high light intensity.

Wavelength ranges of JASCO CD spectrometers

Experimental

MCD measurements of neodymium glass and holmium glass
Magnetic field: 1.6 T
Bandwidth: 1 nm (350 – 950 nm), 10 nm (950 – 2500 nm)
Data interval: 0.2 nm
Scan speed: 200 nm/min
Response: 2 sec
Number of scans: 1

Results

MCD spectra of neodymium glass and holmium glass are shown in Figure 1 and 2 respectively.

Figure 1. MCD Spectra of neodymium glass (Green: Light path from N to S pole, Blue: Light path from S to N pole)

Figure 2. MCD Spectra of holmium glass (Green: Light path from N to S pole, Blue: Light path from S to N pole)

MCD spectra of neodymium and holmium glass can be measured from UV to NIR region. NIR MCD spectra for heme chromophores are very sensitive to oxidation, spin and ligand states, so NIR MCD method is applicable to structural analysis of hemoproteins.