Human eye proteins detect red beyond red

We all see red every now and again, but imagine seeing red beyond red. Researchers have altered the structure of a protein normally found in the human eye so that it can absorb a type of red light that we cannot normally see. The new protein could, in theory, give us the ability to see reds that are currently beyond our visible spectrum.

Colour vision in nearly all animals depends on specialised chemicals called chromophores, which sit inside proteins and absorb different wavelengths of light. Specific protein structures are thought to determine the absorption spectrum of the chromophores within.

To better understand the chemistry behind colour vision, Babak Borhan at Michigan State University in East Lansing and his colleagues engineered a series of mutations which altered the structure of human chromophore-containing proteins. These structural changes altered the electrostatic properties within the protein, which in turn changed the absorption spectrum of the chromophores.

The team created 11 different artificial protein structures and used spectrophotometry ? a technology that compares the intensity of light going in and out of a sample ? to identify which wavelengths they could absorb. Chromophores within one particular protein were able to absorb red light with a wavelength of around 644 nanometres ? tantalisingly close to the wavelength of infrared light, which starts at around 750 nanometres. This was unexpected since natural chromophores have a maximum absorption of around 560 nanometres.

"We were surprised," says Borhan. "But I still don't know if we're at the upper limit of absorption yet. I've speculated about 10 times and been proved wrong."

Green tinge

If these proteins were present in the eye you would be able to see red light that is invisible to you now, says co-author James Geiger, also at Michigan State University. But since objects reflect a mixture of light, the world would not necessarily always appear more red. "Something that looked white before would now look green with your new super red vision," he says.

Marco Garavelli at the University of Bologna in Italy says that in the far future, "one could dare to foresee mutations in visual receptors that extend our ability to see colours beyond the natural spectrum". However, he points out that it is not clear how these engineered proteins would affect neural signalling in the brain.

For now, Borhan is hoping that the modified proteins might prove useful in imaging technology. To track specific cells of interest in a body, researchers can currently attach green fluorescent proteins to them that fluoresce under ultraviolet light. Replacing these proteins with those in Borhan's experiment would allow absorption of longer wavelengths of light. Since longer wavelengths can penetrate further into the body, they may give a clearer view of deeper tissues.

Journal reference: Science, DOI: 10.1126/science.1226135

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