128 



LIGHT AND LIFE 



of intramolecular energy transfer between tyrosine and tryptophan 

 is negligible. 



30 



20 



10 



o EXCITATION \ 



• ABSORPTION 



m/x 260 280 300 



Fig. H. Normali/ed excitation aiui absorption .spectra of LDH and its DPNH 

 complex. Curve 1 is the aljsorpiion band of LDH, and curve 2 the absorption 

 band of its constituent trvptophan on the assumption that the peptide-bound tryp- 

 tophan aI)sorl)s hkc the free amino acid in dihue aikah. Cmve 3 is the excitation 

 spectrum band of the bound Dl'NH normali/ed to coincide with the absorption 

 band of the complex at .8.3.') m/^i; the latter band and the point of normalization 

 are not shown. 1 his normaii/ation takes account of the (pianlum yield of the 

 DPNH emission and puts the absorption and excitation bands on a comparable 

 basis. The units on the ordinate are arbitrary. The ratio of areas, curve 3 to 

 curve 1, is the (juanium vieid of energy transfer from protein to DPNH; the ratio 

 of areas, curve 3 to curve 2. is the quantum yield of the energy transfer if the 

 constituent tvrosine is neglected and only the light absorbed directly in the trypto- 

 phan is transferred t(» the bound Dl'NH. It is the latter ratio that approximates 

 the fractional quenching of the protein emission. 



