True Blue: Electron jumps make protein shine like an LED

By Davide Castelvecchi
From Science News, March 1st, 2008; Vol.173 #9 (p. 131)

A protein known to chemists for its bright blue fluorescence may not be fluorescent after all. Instead, it gives off light by a mechanism similar to that of light-emitting diodes (LEDs), chemists report. The finding suggests that some of the oceans’ many bioluminescent animals may have been using the principle behind LEDs for millions of years.

The protein, antibody EP2-19G2, works in concert with an artificial organic molecule called stilbene, and is often used to label DNA molecules and to detect mercury contamination. Stilbene likes to lodge in a cozy hollow within the antibody’s structure. When ultraviolet rays strike, they excite one of stilbene’s electrons. In its free form, stilbene would then release its extra energy by letting a ring-shaped arm spin. But if stilbene is locked into place inside the antibody, it will instead release the energy by giving off a blue photon.

Scientists had assumed that this was banal fluorescence—an excited electron releasing a photon as it falls back to its normal state, says Richard Lerner of the Scripps Research Institute in La Jolla, Calif.

However, Lerner and his collaborators noticed that the antibody behaves differently than the common fluorescent dyes used in biology and chemistry labs do. For example, at lower temperatures it gives off less light, while fluorescent dyes give off more. Perhaps, the researchers thought, the luminescence could be the result not of an electron falling back to its lower-energy state, but of an electron jumping between molecules.

When an electron in stilbene jumps to an excited state, the lower-energy state is left with a void, Lerner explains. To fill the void, an electron could jump from the protein—specifically, from a tryptophan amino acid within it—to stilbene, leaving behind a positive charge. An electron would then jump from the stilbene to the tryptophan to fill that void. That electron would be jumping to a lower-energy state (in another molecule), and so emit the blue photon. This would make the complex an analog of LEDs—semiconductors that shine when a voltage helps some of their electrons fall into positively charged spots.

To test this assumption, the researchers tried a mutant version of the antibody in which a different amino acid replaced the tryptophan. “If you took out tryptophan, the whole phenomenon disappeared,” Lerner says, which points to the amino acid’s role in luminescence. The chemical reactivity of the light-emitting mixture also indicates that electrons are jumping between the two molecules, the researchers report in the Feb. 29 Science.

Nicholas Turro of Columbia University says that non-protein organic molecules are known to emit light by transferring charges. This case is unprecedented because it is a protein and is orders of magnitude brighter.

If proteins can shine like LEDs, says Lerner, perhaps nature has already discovered the trick and has been using it all along. “In biology, everything that can happen, will.”

Finding natural LEDs might be a long shot, says Mikhail Matz of the University of Texas at Austin. But, “we’ll be on the lookout.”

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