Authors: Dr. Nicholas R. Conley1, Dr. Anca Dragulescu-Andrasi2, Prof. Jianghong Rao2, Prof. W. E. Moerner1,
Journal: Angewandte Chemie International Edition
Affiliation: 1Department of Chemistry, Stanford University; 2Department of Radiology, Molecular Imaging Program at Stanford, Stanford University School of Medicine
I hope most of us have experienced the wonder of watching fireflies lighting up on a warm summer evening. I remember watching with awe as trees suddenly illuminated with dozens specs of light. It isn’t surprising that scientists have taken advantage of the chemistry behind this natural wonder. (If you are more interested in fireflies than chemistry right now please take a few minutes to listen to the first part of this Radiolab episode.) Bioluminescence imaging uses luciferase enzymes (i.e. firefly luciferase) and their substrates (i.e. ᴅ-Luciferin), in order to study living subjects noninvasively. It has found many applications in cancer research and infection studies through the use of bioluminescent cancer cell lines and bioluminescent pathogens, respectively. Additionally, it is possible to study gene expression, gene delivery, and protein-protein interactions using bioluminescence imaging.
One problem with bioluminescence imaging is that the light emitted is absorbed and scattered by tissues in the subject. This effect is particularly noticeable for light below 600 nm. In order to help alleviate this issue, the authors have developed a simple modification to ᴅ-Luciferin that red-shifts its emission. Specifically they replaced a sulfur atom with selenium, because they hypothesized that the emission maximum would be red-shifted due to the difference in polarity between sulfur and selenium. Selenium analogues have been shown to work as well as their natural sulfur counterparts, so the authors felt confident it would be a viable alternative. Additionally, the inclusion of an amino-substituent on the six-membered ring in ᴅ-Luciferin causes a red-shift in emission, so the authors chose to make a selenium analogue (aminoseleno-ᴅ-Luciferin) of the amino-substituted ᴅ-Luciferin (amino-ᴅ-Luciferin) .
After successfully synthesizing aminoseleno-ᴅ-Luciferin, the researchers studied the absorbance of the selenium analogue and found it was almost identical to that of amino- ᴅ-Luciferin. However, the emitted light was significantly red-shifted – 55% of the emission was above 600 nm in comparison to only 41% for amino-ᴅ-Luciferin and 23% for ᴅ-Luciferin. The authors completed in vitro studies and found that the aminoseleno-ᴅ-Luciferin has a lower rate of emission than amino-ᴅ-Luciferin; however they had similar rates in vivo. The authors believe that a trade-off between quantum yield and tissue penetration can explain much of this difference – the aminoseleno-ᴅ-Luciferin has a lower quantum yield, but larger tissue penetration.
This work demonstrates the use of selenium to red-shift the emission of bioluminescent probes. Substitution with 77Se and 125Te could yield MRI detectable compounds – furthering the application of this approach.