124 



LIGHT AND LIFE 



UJ 



y 80 



EXCITATION EMISSION 



260 300 340 380 iTi, 



f 



Fig. 10. Fluorescence excitation and emission bands ol equal concentrations of 

 LDH in 0.1 M phosphate buffer, and in 6.0 M urea, showing the quenching effect 

 of denaturation upon the protein emission. Free tryptophan emission in 6 M urea 

 is enhanced about 40 per cent. 



The polarization of the protein emission, measured with j^lane- 

 polarized incident light, is about 25 percent for LDH, GPD, and 

 other proteins of coinparable size in dihite aqueous sohition. As 

 illustrated previously by the near maximal polarization of DPNH 

 emission in the LDH complex, the rotational relaxation of proteins 

 of this size is negligible chiring the lifetime of the excited state of 

 DPNH. It is also small during the lifetime of the excited state of 

 tryjjtophan. The partial dejjolarization of the tryptophan emission 

 in the large proteins therefore arises either (a) from internal energy 

 migrations, tyrosine to tryptophan or tryptophan to tryptophan, or 

 (b) by vibrational perturbation or oscillatory motion of the indole 



