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



pounds. Cavalieri et al.^'^* have discussed the effects of methyl substitution 

 on the ultraviolet absorption spectra of xanthines and give absorption 

 curves for xanthine and xanthosine which agree well with those published 

 here. Hamer et al."^ have reported the effects on the ultraviolet absorption 

 spectra of purines and pyrimidines when these compounds are reduced 

 under acid and alkaline conditions. 



Nucleic Acids and Polynucleotides. 



Laland et al."^^ have determined 8(P) values for DNA preparations from 

 animal, plant and microbial sources, before and after degradation by acid, 

 alkali, heat, ultrasonic irradiation or desoxyribonuclease. From their re- 

 sults they conclude that any treatment of DNA which only leads to a de- 

 crease in intermolecular bonding does not alter the 8(P) value, whereas the 

 breaking of intramolecular hydrogen bonds results in an increase in e(P). 

 These workers therefore prefer to regard any alteration in the state of DNA 

 in solution which is not accompanied by an increase in 8(P) as a disaggrega- 

 tion, to distinguish it from the breakdown of the intermolecularly-bonded 

 structure which gives rise to the absorption anomaly. 



Ultraviolet Dichroism 



Franklin and Gosling's X-ray diffraction studies of oriented DNA fibers 

 have now been reported in greater detail."^ These authors recognize a 

 highly crystalline A-form, stable at 75% relative humidity, and a para- 

 crystalHne B-f orm which occurs at humidities of 92 % and higher. Riley and 

 Oster^" had previously studied the DNA-water system over a very wide 

 range of composition, and the micelle state in which DNA shows a partly 

 ordered liquid-crystalline structure, may correspond to the paracrystalline 

 B-f orm of Franklin and Gosling. The relation between the various forms of 

 hydrated DNA recognized by X-ray diffraction and those recognized mainly 

 by dichroism by Wilkins, Gosling, and Seeds^-^ (Section IV.5) is not yet 

 clear, but it seems certain that the macromolecular order, and presumably 

 the optical properties of the oriented DNA-water system, are critically 

 dependent on water content, as explicitly stated by Franklin and Gosling. 



"^ L. F. Cavalieri, J. J. Fox, A. Stone, and N. Chang, J. Am. Chem. Soc. 76, 1119 



(1954). 

 "* D. Hamer, Deirdre M. Waldron, and D. L. Woodhouse, Arch. Biochem. and Bio- 



phys. 47, 272 (1953). 

 "^* S. G. Laland, W. A. Lee, W. G. Overend and A. R. Peacocke, Biochirn. et Biophys. 



Acta, 14, 356 (1954). 

 "« R. E. Franklin and R. G. Gosling, Acta Cryst. 6, 673, 678 (1953). 

 1" D. P. Riley and G. Oster, Biochirn. et Biophys. Acta 7, 526 (1951). 



