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A step closer to image capture using biological photoreceptors
The red/purple colour of salt beds comes from the patches in the outer membrane of salt-loving micro-organism Halobacterium salinarum, in which the coloured light-receptor protein bacteriorhodopsin is embedded (Click to enlarge)
Light capture in biology is all around us, but putting this to good use technologically has proved challenging. Professor Anthony Watts and Olivia Berthoumieu, together with collaborators in Oxford, have brought this ambition a bit closer, as reported in four recently published papers (1-4) and as a result of an eight year effort.
‘Coupling biomolecules into nanotechnological devices is a major hurdle,’ says Tony Watts, ‘but by strategically engineering a specific amino acid into a robust bacterial photoreceptor, and coupling this to a gold electrode, we have been able to detect molecular photoswitching in an unprecedented way.’
The bacterial photoreceptor bacteriorhodopsin has long been the focus of potential biotechnological development. Under low oxygen conditions, it uses sunlight to generate an electrical potential across the outer membrane of the Archae in which it is found (Halobacterium salinarum). That then acts as the energy source to drive metabolism, live and reproduce.
A high-resolution image of specially prepared bacteriorhodopsin immobilised on a gold surface through a strategically engineered linking group in the protein, which is around 4nm (4 billionths of a metre) across (Click to enlarge)
Bacteriorhodopsin is responsible for the red-purple colour of marine salt-reclamation beds. The natural purple membranes in which the photoreceptor protein normally exists can be produced in the laboratory from H.salinarum. These membranes have long been recognised as the best environment to enable possible technological development for light or energy capture, and for memory storage (it has been suggested that proteins like bacteriorhodopsin can store more than 20 times the information of the same amount of conventional magnetic recording material).
But progress has been slow in realising the aims of achieving energy capture using these membranes. Previous efforts to immobilise bacteriorhodopsin have been plagued with difficulties. Foremost amongst these is the difficulty of achieving good electrical coupling to an electrode. It has also not been possible to immobilise the membrane to an electrode asymmetrically, so that the electrical generation can flow in the same direction.
Now, by introducing two major molecular modifications to the membranes and receptor protein, these difficulties have been overcome and a highly responsive light capture configuration has been achieved.
