72 



RADIATION BIOLOGY 



The absorption o{ coiijusatcd-boiul groups separated within a {^ivcn 

 molecule by two or more saturated bonds is usually independent and 

 simply additive. The absorption of such groups can, of course, be 

 affected by the addition of side chains or auxiliary groups, especially if 

 the latter may be charged (Nils and OH). Such groups may distinctly 

 affect the spectral position and intensity of absorption of a given conju- 

 gated system (Fig. 5-4). 



In large polar macromolecules, such as proteins and nucleic acids, the 

 near-ultraviolet absorption spectrum of the polymer is often not strictly 



WAVE LENGTH, A 

 6000 4000 3000 2500 

 I I I 



2000 



6000 



WAVE LENGTH, A 

 4000 3000 2500 



2000 



20 



>- 

 (/) I 5 



u I 



Q. 

 O 



05 



15 20 25 30 35 40 45 50 



-3 



Q I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 



15 20 25 30 35 40 45 50 



WAVE NUMBER, Cm"'xi0"' WAVE NUMBER, Cm''xiO" 



(a) (b) 



Fig. 5-5. (a) The ultraviolet dichroism of hexamethylbenzene crystals. I, Electric 

 vector perpendicular to the plane of the ring; II, electric vector parallel to the plane of 

 the ring, (b) The ultraviolet dichroism of tobacco mosaic virus particles, oriented by 

 .streaming. I, Electric vector perpendicular to the direction of streaming; II, electric 

 vector parallel to the direction of streaming. (Scheibe et al., 1943; see also Wilkins 

 et al, 1950.) 



the linear sum of the absorption of its component conjugated groups, 

 even though these component groups are separated by appreciable 

 lengths of saturated bonds. This nonadditivity appears to be due to the 

 formation of labile intergroup bonds to the conjugated groups, either of 

 ionic or hydrogen-bond type. Thus the ultraviolet absorption spectra 

 of proteins are often not simply the sum of the absorption of the compo- 

 nent amino acids. The phenolic group of tyrosine, for instance, fre- 

 quently appears to be involved in some type of loose bond (Crammer and 

 Neuberger, 1943; Sizer and Peacock, 1947; Finkelstein and McLaren, 

 1949; Schauenstein and Treiber, 1950). The ultraviolet absorption of 

 highly polymerized deoxyril^onucleic acid is about 25-30 per cent less 

 than that of the depolymerized form, which form is very nearly the linear 

 sum of the absorption of the component nucleotides (Kunitz, 1950; 



