SIDNEY l\ I'ELICK 



125 



side-chains. Explanation (b) is ihc more likely one, since Iree 

 tryptophan in highly viscous media (35) and at low temperature (40) 

 also exhibits P values of about 25 per cent. In comjjaring polariza- 

 tions, P, measured with plane-polarized incident light, and polariza- 

 tions, P,„ measmed with unpolarized incident light, the relation is 



p — - 



" - 2 _ P 



(^) 



Since the theoretical maximum of P is 1/9 for right-angle emission, 

 the maximal value of P„ is 1/3. 



Energy transfer and quenching. When excitation energy is trans- 

 ferred from protein to bound coenzyme a new coenzyme excitation 



CO 



<: 



_i 



UJ 



300 400 500 



EMISSION, myU 



Fig. 11. The emission band of glycerophosphate dehydrogenase, excited at 280 

 ni/A. curve 1. 1 he other curves shcnv the eflecls on the emission spectrum of suc- 

 cessive increments of DPNH. 1 he bound coenzyme quenches the protein emis- 

 sion by the capture of excitation energy and reemits a portion of it in its own 

 emission band at 450 mfi. 



