Two things are needed to produce fuel from sunlight: an antenna that harvests light, and a light-driven catalyst. The most efficient antennae contain bacteria. An international team, headed by Leiden University professor Huub de Groot, modified chlorophyll from an alga so that it resembles the extremely efficient light antennae of bacteria.
The team was then able to determine the structure of these light antennae, which is the first step to converting sunlight into energy using an artificial leaf.
The fastest light harvesters can be found in nature: in green leaves, algae, and bacteria. The light antennae of bacteria – chlorosomes – are the fastest of all. They have to be capable of harvesting minimal quantities of light particles in highly unfavorable light conditions, such as deep in the sea. These chlorosomes are made up of chlorophyll molecules. The art is to imitate these systems very precisely. German colleagues in Huub de Groot’s team modified chlorophylls from the alga Spirulina, such that they resembled the pigments of bacteria. De Groot’s Leiden group then studied the structure of these semi-synthetic light antennae.
Says De Groot, “Nanotechnology and supramolecular systems are becoming increasingly important, but it is very difficult to determine their structure.” De Groot and his colleagues successfully determined the detailed molecular and supramolecular structure of their artificial self-assembled light antennae. They did this using a combination of solid state NMR and X-ray diffraction, which are both methods for the structural research of materials. X-ray diffraction enabled them to determine the overall structure and NMR allowed them to penetrate deeply into the molecules.
“We already knew that the light antennae in bacteria form a structure rather like the annual rings of a tree trunk,” says De Groot. “The molecules in these semi-synthetic antennae seem to stack in a different way; they are flat. But this, too, is one of four ways we had thought in advance were possible.”
The researchers still have to determine how the light antennae of modified Spirulina chlorophylls work in practice, but new insights are coming in quick succession. With a combination of NMR and electron microscopy, De Groot recently resolved the structure of the light antennae of the bacteria themselves. This allowed the researchers to explain how the antennae were able to function so quickly and so efficiently.