C. WlAHUi 



97 



chymotrypsin they arc not in contact with water but with a medium 

 of mainlv non-polar character. 



We have been unable to discern any contribution from tyrosine 

 to the Class B proteins. Teale (Biochem. /., in press) has studied 

 with particular care the albumins, human and bovine. Both of 

 these proteins have the lowest (non-zero) relative content of trypto- 

 phan, which in human albumin accounts for 16 per cent of the 

 absorption at 280 ni/x, ^\hile in bovine albumin it accounts for 31 

 per cent of it. Photooxidation of the tryptophan residue with 

 methylene blue in visible light results in complete disappearance 

 of the fluorescence without any restoration of tyrosine fluorescence. 

 Urea and heat denaturation fail to induce any appearance of tyrosine 

 emission. Direct evidence of the loss of the quanta absorbed by 

 tyrosine is obtained by a measurement of the changes in absolute 

 quantum yield of the protein with exciting wavelength (Fig. 12) . At 



Absorption Edges of 

 Tyrosine and Tryptophan 

 in Proteins 



Absolute Quantum Yield of 

 Proteins against Exciting 

 Wavelength 



• 280 290 300 3IO 320 

 Wavelength X(m\i) 



•B.S.AIbumin 

 ■yptophan 

 SAIbumin 



CarboxIpeptidcK 



260 270 280 290 300 

 Wavelength ACmp) 



Fig. 12. -Absorption spectra of tyrosine and tryptophan in proteins, and absolute 

 quantum yield of proteins at various exciting wavelengths. 



wavelengths longer than 280 m/^ the absolute yield increases as the 

 relative number of quanta absorbed by the tryptophan increases. 

 At wavelengths longer than 300 m^u, where virtually all the absorp- 

 tion is due to tryptophan, the yield levels off, with similar values in 

 both albumins. 



Although the exact nature of the process by which the tyrosine 



