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



of tuberculin proteins and thymus DNA over a wide range of pH. At low pH values 

 the measurements are complicated by opalescence, but above pH 5 the differences 

 between the calculated and measured mixture absorbances are much greater than 

 could reasonably be ascribed to spectrophotometric errors, and indicate that the 

 total absorbance is reduced in the mixture, deficits of 10% or more being observed. 



The limited experimental work on this subject does not reveal which 

 component of a nucleic acid-protein system gives rise to the absorbance 

 deficit, or if both components are affected. In particular it is not yet known 

 if combination with a protein has the same effect as Mg++ or Na+ ions (in 

 appropriate concentrations) in restoring the low e(P) value characteristic 

 of the native state to a DNA preparation that has previously been partly 

 denatured (with consequent increase in €(P)) by exposure to low salt con- 

 centration, etc. Clearly there is scope for accurate spectrophotometric 

 studies on nucleoproteins in relation to the absorption properties of their 

 constituent proteins and nucleic acids and the effects of denaturation. 



IV. Ultraviolet Dichroism 



The dichroism of the ultraviolet absorption of oriented fibers and sheets 

 of high-molecular-weight nucleic acid may be used to obtain information 

 concerning the orientation of the purine and pyrimidine rings with respect 

 to the long axis of the macromolecule. This axis may be taken as parallel to 

 the fiber axis or direction of shear in specimens which have been oriented 

 by stretching or shearing, respectively. Dichroism measurements may also 

 be used to study the state of nucleoprotein as it exists in intact cells. The 

 possibility that the dichroism of nucleic acid in biological material may 

 give rise to serious errors in the microspectrometric estimation of nucleic 

 acid has been the subject of much recent comment. For these reasons it 

 has seemed advisable to review. briefly the present status of dichroism 

 studies, although it must be emphasized that a full treatment would require 

 an extensive development of the underlying principles of crystal optics. 

 Here it is only possible to give references to some standard texts on the 

 subject and to state the appropriate concepts and equations without de- 

 tailed exposition. ^''^"^ 



89 H. Ambronn and A. Frey, "Das Polarisationsmikroskop." Akad. Verlagsgesell- 



schaft, Leipzig, 1926. 

 1"" C. W. Bunn, "Chemical Crystallography," 1st ed. Oxford University Press, 



London, 1945. 

 i"! N. H. Hartshorne and A. Stuart, "Crystals and the Polarising Microscope," 



2nd ed. Edward Arnold, London, 1950. 

 102 E E. Jelley, Microscopy, in "Physical Methods of Organic Chemistry," Vol. 1, 



Part 1 of "Technique of Organic Chemistry," (A. Weissberger, ed.), 2nd ed. In- 



terscience Publishers, New York, 1949. 



