60 LIGHT ABSORPTION EFFECTS 



for these properties. In the anisotropic region, the result of the bire- 

 fringence measurement led to the conclusion that the actomyosin fibers 

 must be oriented along the muscle fiber axis. 



The second case is that of chloroplasts, whose birefringence showed 

 them to be composed of a stack of discs long before the electron micro- 

 scope made the structure visible. 



The third case is that of DNA. By slowly drying a solution of DNA 

 stretched over a hole by capillary forces, a fiber can be produced. The 

 fiber birefringence shows that the individual elements of the DNA are 

 lined up in the disc configuration, even though it was known from vis- 

 cosity studies that the individual DNA molecules must be long and thin. 

 Thus it was possible to conclude that the elements composing the DNA 

 — the nucleotides — must be arranged as flat discs perpendicular to the 

 long axis of the molecule. 



3. Dichroism 



The section on birefringence has dealt with the transmission and 

 scattering of light by substances which do not absorb the light. By 

 examining the light scattered in two mutually perpendicular directions, 

 we were able to make inferences about the orientation of the subunits of 

 the substances. 



In a very similar manner, we could examine the light transmitted by 

 substances which absorb appreciable amounts of the incident plane polar- 

 ized light. The results of the analysis are of the same type as for bire- 

 fringence. For example, DNA has been studied by shining plane polar- 

 ized light of wavelength 260 mfi, the wavelength of maximum absorption 

 by nucleic acids. There is twice as much absorption perpendicular to the 

 fiber axis as there is parallel to it. Thus one concludes that the absorbing 

 elements themselves lie chiefly perpendicular to the fiber axis, as already 

 deduced from birefringence studies. 



If more light is absorbed at right angles to the fiber than parallel to 

 it, the fiber is said to exhibit negative dichroism; if the converse obtains 

 for the absorption, the fiber exhibits positive dichroism. 



The energies involved in molecular vibrations and oscillations are such 

 that the wavelengths of light absorbed by these movements lie in the 

 infrared region of the spectrum. Polarized infrared sources now exist, 

 and many substances have been studied by this method. Such studies 

 have been of considerable use because of the characteristic absorption by 

 atomic groupings. For example, the stretching of the N — Ff bond involves 

 light of 3.0 fi wavelength, C — O stretching is at 6.0 fx, and the bending of 

 the N — H bond is at 6.5 fx. By measuring the dichroism at each of these 

 frequencies, the relative orientations of the groups may be inferred. For 



