122 LIGHT AND LIFE 



viously we need to know more about the details of the inner complex 

 before we can interpret the above result. The selectivity in non- 

 enzymatic reactions and the relative stability of the inner complex 

 in water are easiest to account for if we postulate a face-to-face orien- 

 tation of the adenine and pyridine rings. Energy transfer would also 

 occur if the rings were aligned edge to edge in the same plane. This 

 would be an unstable arrangement in free solution, even with the 

 help of a hydrogen bond between the adenine amino group and the 

 carbonyl oxygen of the nicotinamide, but such an orientation could 

 be stabilized in the enzyme complex. In such a case, or in inter- 

 mediate orientations, the substrate could have access to either side 

 of the pyridine ring and the specificity would again be determined 

 by the protein and the reaction that it catalyzes. 



Protein Fluorescence and the Transfer of Excitation Energy 

 Protein fluorescence is described in detail in this symposium by 

 Weber (40) , and I will deal with the subject only in so far as it 

 applies to the analysis of the enzyme-coenzyme complexes. The trans- 

 fer of excitation energy from the aromatic amino acid residues of the 

 proteins to the bound coenzyme is established by the excitation spec- 

 tra of Figs. 7 and 8. It will be profitable to consider the protein 

 emission and the transfer process in more detail in order to see what 

 can be learned about the properties of the proteins, the geometry of 

 the complexes, and the mechanism of the excitation energy transfer. 

 We will confine our attention chiefly but not entirely to LDH be- 

 cause the pertinent properties of LDH are extreme and most favorable 

 for the analysis. 



The number and nature of the emitting groups. LDH contains 

 approximately 28 tryptophan and 56 tyrosine residues per molecule. 

 Both tyrosine and tryptophan have absorption maxima near 280 m^i 

 and the free amino acids emit respectively at 306 and .S50 ny with 

 quantum yields of 20 per cent. The LDH emission maxinumi is near 

 350 ni/x and appears to be predominantly tryptophan emission. The 

 emission bands of LDH, free tryj^tophan, and GPD obtained from 

 solutions at the same optical density at the wavelength of excitation 

 are shown in Fig. 9. The areas under the curves obtained in this 

 way represent relative quantum yields, li may l)e seen that the 

 quantum yield of GPD is low. However, the emission looks like 

 tryptophan emission: and il we assume that about a third to one- 

 half of the light absorbed goes into tyrosine and is dissipated as heat, 

 then the tryptoi)han emission from C;PD is approximately normal. 



