BIREFRINGENCE 59 



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, e.g. nucleic acids) there may w T ell occur directions of 

 easier and harder electron movability in the structure as a w r hole. 



(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 important instances of the use of birefringence which 

 permitted significant deductions about structures in biology. The case of 

 muscle is the first one, in which the measurement of the birefringence 

 permitted the conclusion that there are both isotropic and anisotropic 

 regions in striated muscle — the bands called J and A are actually named 



