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| B is for Birefringence |
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| B is for Birefringence
The technical mumbo jumbo first. The physical properties of isotropic materials, such as glasses, liquids and amorphous materials, do not depend on direction. However, most properties of a wide variety of crystals (including liquid crystals) do show such variation. This anisotropy of physical properties originates in the anisotropic build-up of the materials (crystal structure). Anisotropy in the optical properties of uniaxial crystals is referred to as either birefringence or dichroism, depending on whether the index of refraction or the absorption coefficient is concerned. Birefringence means that there are two distinct speeds with which light can propagate, depending on the direction of propagation. When a light ray splits into two beams as it passes through a material, the effect is called birefringence (or double refraction) and the material is birefringent. If you look at something through a birefringent material, you'll see double. The word birefringence comes from the Latin bi- (twice) plus refringere (to break up). Thus, the light rays are "broken in two" by a birefringent material. One well-known example of a birefringent medium is crystalline calcite (calcium carbonate). If you look at the world through a clear crystal of calcite (calcium carbonate), you will see double. Place such a crystal on a drawing, and you'll see two overlapping copies of the drawing. The molecular structure of calcite causes double refraction, in which each light ray is split into two rays that emerge from the crystal at slightly different angles. Calcite shows this more clearly than most crystals, but quartz and many other crystalline minerals also split light ray.
Now, the practical interpretation for crystal growers. You might hear the word birefringence used quite often by crystal growers when viewing crystals under a microscope. Here, crystal growers are stretching the definition of the term birefringence to describe the colorful display produced by biological macromolecular crystals when polarized light is passed through the crystal.
A light microscope with polarizing optics is required to observe birefringence. The following path is a typical set-up. Light passes from the light source through the first polarizing lens, then the specimen (crystal) then the second polarizing optic, the magnifying optics and then into your eye. On many typical polarization set ups, the second polarizing filter can be rotated while the specimen is stationary. Rotating the polarizing optic without something to rotate the plane of polarized light in the path (i.e. a crystal) will result in one seeing light, dark, light, dark, as the filter is rotated. But if a crystal with birefringent properties (i.e. a biological macromolecular crystal) is positioned in between the two polarizing filters, one will observe changing colors as the polarizing filter is rotated. Specifically, when the polarizing filters are aligned such that the field is dark, a birefringent object (crystal) will glow with color.
Birefringence is one way we can differentiate amorphous precipitate from microcrystals in a drop when viewed under a microscope. Precipitate does not have birefringent properties while most biological macromolecular crystal do have birefringent properties.
One drawback with using birefringence in today's crystal growth world is that most of the crystallization devices utilized are made from plastic such as polystyrene and polypropylene. These plastics are optically active can be birefringent. In fact, often times the colors we see displayed in crystals are contributions from the plastic birefringence. However, it is still possible to observe microcrystalline birefringence in the plastic trays, but there is usually a contributory effect from the plates used to grow the crystals. One way to avoid this is to grow crystals in a glass device or at least observe the crystals in a path that is free of plastic.
Birefringent precipitates will glow, sparkle, or glisten.
To test for birefringence, position the polarizers so the field of view is dark WITHOUT they crystallization plate or set up. Place the tray into position on the microscope. If a crystal is birefringent, some of the light passing through the crystal will be rotated and passed through the second (analyzing) polarizing filter. The intensity of the transmitted light will increase and decrease as the crystal is rotated or the polarizer is rotated. Remember, birefringence is not ALWAYS clearly visible when plastic is in the light path (i.e. when you use plastic slides or crystallization plates). However, a birefringent crystal viewed in a plastic tray or plastic cover slide will have a different color than the background (i.e. plastic plate) or foreground (plastic slide). Finally, birefringence is a property of crystals, both biological (proteins, peptides, and nucleic acids) and inorganic crystals (salts). Birefringence is MORE pronounced in inorganic (salt).
A quick comment on what to do with birefringent precipitates. Streak seeding is a common and often successful method of taking advantage of microcrystalline precipitate to grow large single crystals. But that starts with an S so we cannot talk about that this time! |
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