Violeta Iancu and Saw-Wai Hla used scanning tunnelling microscope (STM) to obtain the most detailed images yet of single chlorophyll-a molecules bound to a gold surface. They then demonstrated controlled four-step conformational switching in the molecule. The work is reported in the Proceedings of the National Academy of Sciences
“Single molecule switches using STM manipulation have been demonstrated before by us and by other groups but mostly these switches operate in two levels,” says Hla. “To our knowledge, this is the first multi-step switching process realized by STM manipulation on a single molecule [and] the first time that a single chlorophyll-a molecule could be imaged…but what I’m really satisfied with about this work is the controlled switching of four molecular conformations with atomic scale precision.”
Chlorophyll-a is one of several pigments involved in photosynthesis, the process by which plants convert light energy to chemical energy. Conformational changes in chlorophyll molecules, induced by the absorption of light energy, drive the photosynthesis reaction.
The chlorophyll-a molecule consists of a porphyrin ‘head’, which has a magnesium atom at its centre, and which absorbs light, and a phytyl carbon chain ‘tail’.
Hla’s team was able to manipulate the conformation of the chlorophyll-a molecule. They used the tip of the scanning tunnelling microscope to inject single electrons into the molecules, resulting in the controlled four-step conformation switching.
The red panel in figure on the left shows diagrams of the chlorophyll-a molecule in each of the four conformations, superimposed on STM images of individual molecules. The black panel on the right of the figure shows calculated images of how each of the conformations is produced.
The straight tail conformation (a) can be switched to a bent tail conformation (b) by injection of an electron, which causes a counterclockwise rotation of the phytyl chain at the point marked by a red arrow in the top right of the figure. Injection of another electron then switches the molecule to a third conformation (c), by a 60° rotational bending at the point marked by the blue arrow. The fourth conformation is obtained by a clockwise 60° rotation at the point marked by the orange arrow. All four conformations can be seen in this short film clip:
“We can change the switching frequency by changing the tunnelling current…and we could also determine the energy barrier to switch the conformation of the molecule,” explains Hla. “[Because] conformation changes of this molecule are supposed to play a vital role [in photosynthesis],” he continues, “understanding its mechanical and electronic properties will be an initial stage to studying photosynthesis at a single molecule level. The natural plant molecules may also be useful in developing solar energy conversion devices.”
When in conformation 2, the molecule can be switched to either conformation 1 or conformation 3, depending on where an electron is injected. Similarly, conformation 3 can be switched to conformation 2 or conformation 4. Hla and his team speculate that the injection of an electron probably provides the energy needed for the conformational change.
Because of the non-toxic nature of chlorophylls, the authors envision that their work can be applied to the development of environmentally-friendly nanoscale energy devices. It may also be used to make mechanical switches in future medical devices, and will have a host of other applications in technology.