ULTRAVIOLET ABSORPTION SPECTRA 



185 



6000 



2400-3000 A. All proteins show absorption below 2400 A, increasing 

 rapidly toward shorter wave lengths. Although certain amino acids, 

 histidine, and trj^ptophane (Coulter el al., 1936) and tyrosine (Smith, 

 1928) show characteristic peaks in this region, these peaks are generally 

 submerged by the fast rising "end" absorption; this absorption is due in 

 large part to the peptide bond (Magill et al., 1937; Setlow and Guild, 

 1951 ; Goldfarb et al., 1951) and to a lesser degree to absorption represent- 

 ing the long wave limb of the vac- 

 uum ultraviolet absorption peaks of 

 single C — C, C — N, etc., bonds 

 (Marenzi and Vilallonga, 1941a). 

 The 223-m;u peak of tyrosine, which 

 shifts to 242 m/x in alkaline medium 

 (Kretchmer and Taylor, 1950), should 

 then become apparent, but it does not 

 always become apparent, probably 

 because of binding of the phenolic 

 group within the protein (Sizer 

 and Peacock, 1947; Finkelstein and 

 McLaren, 1949). 



Absorption in the region 2400 3000 

 A is due to the presence of the aro- 

 matic amino acids, phenylalanine, 

 tyrosine, and tryptophane, especially 

 the last two (Figs. 5-14, 15) (Goodwin 

 and Morton, 1946). Proteins and 

 polypeptides that lack these amino 

 acids, such as gelatin (Loofbourow 

 et al., 1949) and clupein (McLaren, 

 1949), have negligible absorption in 

 this region, although the peptide bond does have a very weak band at 

 about 2800 A (e = 1-5) (Setlow and Guild, 1951). 



lodination of tyrosine, as in diiodotyrosine, induces a shift of the alka- 

 line tyrosine band to 3115 A (Marenzi and Vilallonga, 1941b) ; the alkaline 

 absorption maximum of thyroxine is still further displaced to 3310 A 

 (Heidt, 1936; Marenzi and Vilallonga, 1941b, c). 



2400 



2600 2800 3000 

 WAVE LENGTH. A 



Fig. 5-14. Ultraviolet absorption 

 spectra of the aromatic amino acids. 

 I, tryptophane in AVlO HCl; II, 

 tyrosine in iV/10 NaOH; III, tyrosine 

 in AVlO HCl; IV, DL-0-phenylalanine, 

 (Loofhoiiroiv, 1940.) 



NUCLEIC ACIDS AND NUCLEOTIDES 



The ultraviolet absorption spectrum of nucleic acids and polynucleo- 

 tides is characterized by a strong absorption maximum at about 2600 A, 

 a minimum at about 2300 A, and continuously rising "end" absorption at 

 below 2300 A. (Loofbourow, 1940; Hotchkiss, 1948; Ploeser and Loring, 

 1949; Schlenk, 1949). As has been noted, the spectra of the highly 

 polymerized nucleic acids are not the linear sums of the spectra of their 



