538 G. H. BEAVEN, E. K. HOLIDAY, AND E. A. JOHNSON 



Absorption measurements with the electric vector in and perpendicular to the ring 

 plane show very marked dichroism; D approaches 100 at 300 mn where the perpendic- 

 ular absorption is very small, while the in-plane absorption extends to a much longer 

 wavelength. The dichroism decreases with decreasing wavelength and at 250 m^ D 

 has fallen to ca. 2. By a special crystallization method Seeds has also been able to 

 make observations with the electric vector in the ring plane and parallel or perpendic- 

 ular to the pseudo-dyad axis through C2-C6 and finds a small but definite dichroism 

 below 300 m/i.This work appears to be the first in which the three absorption spec- 

 tra of a crystal have been measured and dichroism in the ring plane demonstrated; 

 Craig and Lyons''^ have since demonstrated that in naphthalene the transition 

 dipole moment for the weak transition at ca. 310 m/x lies predominantly in the mo- 

 lecular plane and along the shorter molecular axis. 



Lyons"* has examined 2-amino-4-chloro-6-meth3'lpyrimidine (II), for which X-ray 

 structure data are available, and 2-chloro-4,6-dimethylpyrimidine (III). Of the three 

 band systems generally shown by pyrimidines^' the weak longwave band (a) is ob- 

 scured (though not in III) by system (b) located at 250-290 m/i; the third sj'stem (c) 

 lies at 200-250 m/i. Band (a) disappears in acidic solution and is a transition of the 

 n— n- type'^' involving non-bonding electrons on the nitrogen atoms while bands (b) 

 and (c) are due to tt-tt transitions corresponding to the 260-mM and 200-m/i benzene 

 systems. For the type (b) band of II the electric vector is in the molecular plane for 

 strong absorption in agreement with the results for I and there is a favored direction 

 (x) in the plane parallel to a line through the two nitrogen atoms. For III, with un- 

 known crystal structure, the polarization of bands (a) and (c) are similar and different 

 from that of band (b). Since by analogy with hexamethylbenzene band (c) of II and 

 III will be polarized in the molecular plane, their polarization direction in the plane 

 must be mainly y, at right angles to that of the (b) band. From these findings Lyons 

 is able to give assignments to the type (b) and (c) systems and to draw some important 

 conclusions about the nature of the upper state of the type (a) n—n- transition. 



5. Dichroism of Nucleic Acid 



The optical anisotropy of biological materials is the subject of an ex- 

 tensive literature (see review^-") mainly concerned with birefringence stud- 

 ies. Sperm heads have long been known^-^ to show a large optical aniso- 

 tropy, indicating parallelism of the nucleoprotein molecules, and it has 

 very recently been shown by X-ray diffraction studies that this anisotropy 

 arises from a true three-dimensional crystallinity.^^- The birefringence of 

 cytoplasm has been the subject of much work, but it has recently been 

 pointed out^-" that form and intrinsic birefringence have not always been 

 distinguished, while the sign of the birefringence is certainly not sufficient 



"7 D. P. Craig and L. E. Lyons, /. Chem. Phys. 20, 1499 (1952). 



"s L. E. Lyons, /. Chem. Phys. 20, 1814 (1952). 



113 M. Kasha, Discussions Faraday Soc. No. 9, 14 (1950). 



12" M. M. Swann and J. M. Mitchison, Birefringence of cytoplasm and cell membranes, 



Progr. Biophys. and Biophys. Chem. 2, 1-16 (1951). 

 121 W. J. Schmidt, "Die Doppelbrechung von Karyoplasma, Zytoplasma und Meta- 



plasma." Borntraeger, Berlin, 1937. 

 1" M. H. F. Wilkins and J. T. Randall, Biochim. et Biophys. Acta 10, 192 (1953). 



