dichroism

(general discussion)

In certain anisotropic materials, the property of having different absorption coefficients for light polarized in different directions. There are few natural materials which exhibit strong dichroism. One of the first to be discovered was tourmaline. Light transmitted by thin plates of dark forms of tourmaline is almost completely polarized. See also Polarized light.

If the absorption in a dichroic material is different for different linear states of polarization, the material is termed linear dichroic. If it is different for right and left circularly polarized light, it is termed circular dichroic. Similarly, there can be elliptically dichroic crystals.

The study of dichroism allows conclusions as to the submicroscopic fine structure of cells. In visible light only a few cellular components, such as chloroplasts, show absorption. An absorption can, however, be produced by staining. The dichroic staining of plant fibers is especially simple. The elongate stain particles of benzidine dyes, for example, congo red, are deposited in an oriented manner in the spaces between the microfibrils and produce an intrinsic dichroism of the fiber: colored for a vibration plane parallel, colorless for a plane perpendicular to the stain particles and fibrils. Therefore, the direction of strongest absorption indicates the course (parallel or helical) of the microfibrils in the fiber.

Ultraviolet dichroism gives direct information as to the orientation of the absorbing molecules or molecular groups in cell structures. The method has been especially helpful for studies of orientation of deoxyribonucleic acid in nuclei and chromosomes. Lignifed plant cell walls show ultraviolet dichroism. It is pure form dichroism, a fact which eliminates the possibility that lignin is in an anisotropic state in the wall.

By irradiation with ultraviolet light, various compounds of the cell are caused to fluoresce. The fluorescent light is polarized if the object is anisotropic. This phenomenon, called difluorescence, is observable in lignifed cell walls, and leads to the same conclusions as to lignin deposition as emerge from dichroism studies.

Dichroism (biophysics)

In absorbing, anisotropic objects, the reference index and the absorption vary with direction. There are two main directions of anisotropy, which are perpendicular to each other. This phenomenon is called dichroism and may be studied in the polarizing microscope with one polarizer. By rotating the microscope stage the object is brought in different directions to the vibration plane of the polarized light, and the change of light absorption is observed. Study of dichroism thus allows conclusions as to the submicroscopic fine structure of cells.

In visible light only a few cellular components, such as chloroplasts, show absorption. An absorption can, however, be produced by staining. The dichroic staining of plant fibers is especially simple. The elongate stain particles of benzidine dyes are deposited in an oriented manner in the spaces between the microfibrils and produce an intrinsic dichroism of the fiber: colored for a vibration plane parallel, colorless for a plane perpendicular to the stain particles and fibrils. Therefore, the direction of strongest absorption indicates the course (parallel or helical) of the microfibrils in the fiber. Dichroic staining may also be carried out with animal tissue. Such studies have been made mainly with fibrous protein structures (collagenous fibers, myofibrils, neurofibrils) and with anisotropic nuclei of certain sperms.

In ultraviolet light many cell components show absorption. Ultraviolet dichroism gives direct information as to the orientation of the absorbing molecules or molecular groups in cell structures. The method has been especially helpful for studies of orientation of deoxyribonucleic acid in nuclei and chromosomes. The maximum absorption here is perpendicular to the molecular direction. Lignified plant cell walls show ultraviolet dichroism. It is pure form dichroism, a fact which eliminates the possibility that lignin is in an anisotropic state in the wall. See also Cell walls (plant).

By irradiation with ultraviolet light, various compounds of the cell are caused to fluoresce. The fluorescent light is polarized if the object is anisotropic. This phenomenon, called difluorescence, is observable in lignified cell walls, and leads to the same conclusions as to lignin deposition as emerge from dichroism studies. See also Dichroism.

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