. Elementary biophysics: selected topics . oftfla Line-up Axis Fig. 25. Disc-shaped and rod-shaped molecules lined up so that their bi- refringence may be studied. (From G. E. Oster, "Birefringence and Dichroism," in Physical Techniques in Biological Research, Vol. I, p. 439, 1955; courtesy Academic Press, Inc., New York.) This bond, being a covalent bond due to shared electrons lying primarily between the atoms involved, is difficult to distort at right angles to the bond. On the other hand, the triple bond C = C has the atoms bound about three times as strongly, so that the distort
. Elementary biophysics: selected topics . oftfla Line-up Axis Fig. 25. Disc-shaped and rod-shaped molecules lined up so that their bi- refringence may be studied. (From G. E. Oster, "Birefringence and Dichroism," in Physical Techniques in Biological Research, Vol. I, p. 439, 1955; courtesy Academic Press, Inc., New York.) This bond, being a covalent bond due to shared electrons lying primarily between the atoms involved, is difficult to distort at right angles to the bond. On the other hand, the triple bond C = C has the atoms bound about three times as strongly, so that the distortion of electron positions which can be effected is mainly at right angles to the bond direction. (b) Form (or organizational) birefringence. As a result of grouping molecules in a regular array (or in a composite structure with an in- herent regularity, nucleic acids) there may wTell occur directions of easier and harder electron movability in the structure as a wrhole. (c) Flow birefringence. If a solution is caused to flow, the viscous forces will tend to orient particles with their long axes in the direction of the flow. Spherical particles may even be distorted by these forces. Thus, during the flow, there will be organizational birefringence whose magnitude will depend on the speed of the flow. Consider the two separate cases of a collection of discs and a collection of rods, as sketched in Fig. 25. In the case of the discs, electrons can be moved more readily along the planes of the discs than out of the plane; therefore polarized light inci- dent at right angles to the lineup axis will be affected more than polarized light incident along the axis. The case of the rods is precisely opposite, since electrons can more readily be moved along the lineup axis. Thus, the measurement of the birefringence for lined-up molecules tells us whether the molecules have a rodlike or a disclike shape and how they are oriented with respect to the lineup axis. There are several import
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