Researchers tap terahertz spectroscopy to probe photoactive proteins

Kevin Robinson

A recent study of rhodopsin and bacteriorhodopsin using terahertz spectroscopy has led researchers to conclude that the technique may be useful in studying the conformational changes caused when biological pathways are activated.

Spectra from the terahertz range are sensitive to the three-dimensional arrangement of the atoms in a molecule, making it a useful frequency region for probing conformational changes. Andrea G. Markelz of the University at Buffalo in New York, Susan K. Gregurick of the University of Maryland, Baltimore County, in Baltimore and their colleagues studied rhodopsin and bacteriorhodopsin because of their similarities: Both are seven-member, photoactivated proteins containing retinal. Rhodopsin is found in human eyes, and bacteriorhodopsin is a proton pump in the bacterial energy cycle.

Using terahertz spectroscopy at roughly 10.5 cm^–1, researchers probed the vibrations of rhodopsin (left) and bacteriorhodopsin (right). Although the proteins have different biological functions, they are similar in structure and respond with nearly identical motion. The ribbon colors imply different secondary molecular structures, and the colors of the arrows imply the amplitude of the vibration. Courtesy of Susan K. Gregurick and Andrea G. Markelz.

The researchers calculated the effect of the absorption of wavelengths from 8 to 50 cm^–1 on the shape of the various parts of the two molecules, which are arranged in several helices. They also reported tests on an additional mutant bacteriorhodopsin molecule, D96N.

They found that the proteins react in similar ways, despite having very different biological functions. The results showed that the lowest frequencies corresponded to movement in the cytoplasmic region of the proteins and that, as the frequency increased, the normal mode movements flowed down the helices toward the extracellular region of the protein systems. The overall movement of the helices was similar in both proteins.

Terahertz radiation already is used commercially for product inspection, and Gregurick and Markelz noted that it holds promise for detecting denatured proteins in pharmaceuticals.

Their research continues in several areas. They are working to address questions about using the system with wet samples because of the potential interactions between water and the terahertz signal. On the mathematical side, they are working to extend their calculations to better understand how vibrational energy is transferred within the protein. In addition, they are trying to remove sources of inhomogeneous broadening to access specific vibrational modes and are working to understand the various contributions to the dielectric response.

Biophysics Journal, published online Jan. 16, 2008; doi:10.1529/biophysj.107.105163.