Microscope Detects Chirality to Make Solid-Tissue Imaging Possible

Researchers at the University of Illinois’ Beckman Institute for Advanced Science and Technology developed a spectroscopic microscope enabling optical measurements of molecular conformations and orientations in biological samples. The device is the instrumentational component of a measurement technique that the researchers said allowed them to increase the speed and accuracy at which they obtained images of such samples at the microscopic level.

The advancement stems specifically from a previously introduced infrared spectroscopic imaging technique, and, said Rohit Bhargava, a professor of bioengineering and the director of the Cancer Center at Illinois, aims to incorporate the concept and study of molecular chirality into microscopy. Molecular chirality refers to atomic spatial orientation in molecules or multimolecular assemblies.

In certain biological systems, a molecule may elicit a cellular response, where a mirror image could be inactive. Though vibrational circular dichroism (VCD) can be used to determine a molecule’s chemical structure, the measurements that process delivers are time intensive.

Previously, those measurements also could not be used to image complex biological systems and/or solid tissue samples.

Using the research team’s microscope to accelerate the image acquisition time and improve the signal-to-noise ratio of traditional VCD techniques, the Illinois researchers successfully imaged biomolecule chirality. “When you send light down a microscope from a spectrometer, you are essentially throwing away a lot of it,” Bhargava said. “For VCD measurements, you also have to send the light through a photoelastic modulator, which changes its polarization to left- or right-handed. At that point, you do not have a lot of light left, which means you have to average your signal for a long time to see just one pixel within an image.”

The researchers built the instrument around a quantum cascade laser (QCL) rather than a traditional thermal light source. The QCL increased the amount of deliverable power, which allowed the researchers to acquire faster measurements. The team ultimately achieved both rapid and concurrent infrared and VCD measurements from the framework of their discrete frequency infrared imaging microscope.

“The laser source motivated the whole design,” said Yamuna Phal, a graduate student researcher in electrical and computer engineering. “Previously, you could only perform VCD on liquid samples, but we can image solid tissues as well.”

“We initially envisioned the discrete frequency infrared microscope as a platform on which other techniques could be built,” said Kevin Yeh, a postdoctoral research associate who co-led the development of the microscope. “We have solved one of these extensions, which is VCD, but we could envision many others.”

Additional applications for the discrete frequency infrared imaging microscope-enabled technique, team members said, are likely to span the biological sciences.

The research was published in Analytical Chemistry (www.doi.10.1021/acs.analchem.0c00323).