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  • Optical Retarder Aids Protein Crystal Growth Study

Photonics Spectra
Dec 1997
Ruth A. Mendonsa

In September a device for studying protein crystal growth made its first flight in space on the STS-86 space shuttle mission to Mir Space Station. The Microgravity Research Program Office's Biotechnology Project Office at NASA's Marshall Space Flight Center is managing the project, which will study how protein molecules move through fluid and form crystals. A liquid crystal variable retarder from Meadowlark Optics is playing an important role in the experiment.

A liquid crystal variable retarder is playing an important role in studying protein crystal growth in space. An interferometer created the fringes in this image of a Lysozyme protein crystal in the experiment on Mir.

Protein crystal growth relies on the movement of large molecules through a fluid. When the protein molecules contact each other and form a crystal, the protein concentration is depleted. Density differences affect the index of refraction of the fluid. An optical system that reveals these variations helps distinguish how temperature, concentration and pH differences promote crystal growth. In Earth's gravity, however, the less dense, depleted volume tends to rise, causing the solution to flow across the crystal surface. This problem is eliminated by the low g environment of space.

Laser light is divided

In operation, a cube beamsplitter with two Meadowlark Optics polymer retarders divides light from a deep red diode laser. The light travels to the test cell and a reference mirror and then back to the beamsplitter, where it proceeds as two beams, one vertically and one horizontally polarized. The new beam is enlarged by a long-working-distance microscope that directs it into a Meadowlark Optics liquid crystal variable retarder. This device retards the horizontally polarized beam without affecting the vertically polarized beam. The retardation is controlled by an electric voltage applied across the liquid crystals. A Dove prism folds the light through a 180° turn and delivers it though a polarizing filter to a video camera. The polarizing filter is tilted evenly to both beams so they produce an interference pattern. The computer controls the variable retarder to produce varying interference patterns.

During an experiment, the crewman selects a crystal, adjusts the focus and the tilt platform to minimize fringes and then the system automatically collects images and data. Results will come from about 4050 video frames captured and stored in the computer until STS-86 returns it to Earth in early 1998.

The principal investigator on this project is Alexander McPherson of the University of California at Riverside, and investigators are William Witherow and Marc Pusey of the Marshall Space Flight Center.

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