Scientists Capture 'Spooky Action' In Photosynthesis
Guoyan Wang and Daping Sun
(Inside Science) -- Photosynthesis and other vital biological reactions depend on the interplay between electrically polarized molecules. For the first time, scientists have imaged these interactions at the atomic level. The insights from these images could help lead to better solar power cells, researchers added.
Atoms in molecules often do not equally share their electrons. This can lead to electric dipoles, in which one side of a molecule is positively charged while the other side is negatively charged. Interactions between dipoles are critical to biology -- for instance, the way large protein molecules fold -- often depend on how the electric charges of dipoles attract or repel each other.
One process where dipole coupling is key is photosynthesis. During photosynthesis, dipole coupling helps chromophores -- molecules that can absorb and release light -- transfer the energy that they capture from sunlight to other molecules that convert it to chemical energy.
Intriguingly, a consequence of dipole coupling is that chromophores may experience a strange phenomenon known as quantum entanglement. Quantum physics suggests that the world is a fuzzy, surreal place at its very smallest levels. Objects experiencing quantum entanglement are better thought of as a single collective than as standalone objects, even when separated in space. Quantum entanglement means that chromophore properties can strongly depend on the number, orientations and positions of their neighbors.
Understanding the effects that dipole coupling might have on chromophores might help shed light on photosynthesis and light-harvesting applications such as solar energy. However, probing these interactions requires imaging chromophore activity with atomic levels of precision. Such a task is well beyond the capabilities of light-based microscopes, which are currently limited to a resolution slightly below 10 nanometers or billionths of a meter at best, said Guillaume Schull, a physicist at the University of Strasbourg in France. In comparison, a hydrogen atom is roughly one-tenth of a nanometer in diameter.
Instead of relying on light to illuminate and image chromophores, scientists in China used electrons. Scanning tunneling microscopes bring extremely sharp electrically conductive tips near surfaces they scan. Quantum physics suggest that electrons and other particles do not have one fixed location until they interact with something else, and so has some chance of existing anywhere. As such, electrons from the microscope's tip can "tunnel" to whatever the microscope is scanning. A fraction of these electrons lose energy during tunneling, which gets emitted as light that the microscope uses to image targets with atomic-scale resolution.
The researchers experimented with chromophores made of a purple dye known as zinc phthalocyanine. These chromophores were each about 1.5 nanometers wide, and they shone red light when the microscope excited them.
The scientists used the microscope's tip to push chromophores together. When the chromophores were roughly 3 nanometers apart, the spectra of light they gave off began shifting.
"I was quite surprised by the dramatic spectral change when two isolated molecules were simply pushed together," said study co-senior author Zhenchao Dong, a physical chemist at the University of Science and Technology of China in Anhui.
The research team's theoretical simulations suggest these changes were a direct visualization of dipole coupling between the chromophores.
"Dipole-dipole interactions play an important role in many biological and photophysical processes. To my knowledge, it is the first time that one has directly imaged them with sub-molecular resolution," said Schull, who did not participate in this study. "This is really impressive."
The scientists experimented with clusters of up to four chromophores. The varieties of light from these chromophores suggested they might have been entangled. Future research can explore dipole coupling in more complex arrangements -- for example, in 3-D systems, Dong said. He and his colleagues detailed their findings in the March 31 issue of the journal Nature.
By analyzing how molecules interact and exchange energy, "the most important implication of these findings is the possibility to understand and therefore engineer molecular structures for efficient solar-energy conversion devices," said Elisabet Romero, a biophysicist at VU University Amsterdam, who did not take part in this research. A better understanding of dipole coupling might also yield insights into the function of structures of molecules such as catalysts, Romero added.