542 G. H. BEAVEN, E. R. HOLIDAY, AND E. A. JOHNSON 



however, the dichroism to be of opposite sign from that reported by Bute- 

 nandt et al., i.e., absorption is greatest with the electric vector parallel to 

 the flow (or shear) direction. This alteration in sign is confirmed by Perutz, 

 Jope, and Barer,^'° using the microspectrographic technique of Barer et al.,^^^ 

 adapted for work with polarized radiation. These three groups of workers 

 are mainly concerned with the significance to be attached to the parallel 

 dichroism of the tryptophan fine-structure band at ca. 290 m/i of the protein 

 moiety. Seeds and Wilkins reject the conclusion of Butenandt et al. that 

 the dichroism of similar sign and magnitude in the 260-mM region is an 

 indication of orientation of the nucleic acid, and suggest that it is due to 

 form dichroism. The optical properties of the hexagonal crystals present 

 in the leaf hairs and other cells of plants infected with tobacco mosaic 

 virus have also been studied, '°^ but the results obtained are too complex 

 to be reviewed here. Calf thymus nucleoprotein (containing DNA) gives 

 oriented films and fibers (though less readily than the derived DNA) which 

 are negatively birefringent. As with nucleic acid, the ultraviolet dichroism 

 is negative, with maximum absorption when the electric vector is perpen- 

 dicular to the direction of shear, and dichroic ratios of 1.5-1.7 have been 

 observed for air-dried specimens of birefringence ca. —0.01 4. ^"^-'-^ 



6. Influence of Dichroism on the Microspectrographic Estimation 

 OF Nucleic Acid in Intact Cells 



The use of a refracting microscope for the ultraviolet microspectrometry 

 of intact cells, was initiated by Caspersson^^^ ^^ 1936 and has since been 

 intensively developed^^* and exploited*' by him and his co-workers as a 

 valuable cytochemical tool. [Cf. Swift, Chapter 17.] More recently, fully 

 achromatic reflecting microscopes have been devised for such work; the 

 results obtained have been reviewed by Davies and Walker,'^ and instrumen- 

 tal problems elsewhere ^°* 



Commoner and Lipkin^^'* suggest that orientation, which is known from 

 birefringence observations to be often present to a greater or lesser extent in 

 biological material, may be expected to invalidate absorption measure- 

 ments with unpolarized light because of Beer-Lambert absorption law 

 deviations which are, according to their arguments, inherent in such sys- 

 tems. Their views have been the subject of much comment. 



Thorell and Ruch'" have measured the absorption at 260 m^i of sodium- 

 's" M. F. Perutz, E. M. Jope, and R. Barer, Discussions Faraday Soc. No. 9, 423 (1950). 

 I'l R. Barer, E. R. Holiday, and E. M. Jope, Biochim. et Biophys. Acta 6, 123 (1950). 

 '32 T. Caspersson, Skand. Arch. Physiol. 73, Suppl. 8 (1936). 

 '" T. Caspersson, Exptl. Cell. Research 1, 595 (1950); T. Caspersson, F. Jacobsson, 



and G. Lomakka, ibid. 2, 301 (1952). 

 '3< B. Commoner, Science 110, 31 (1949); B. Commoner and D. Lipkin, ibid. 110, 41 



(1949). 

 '" B. Thorell and F. Ruch, Nature 167, 815 (1951). 



