160 



LIGHT AND LIFE 



-0.1 



500 



700 



900 



Fig. 3. Spectrum of complex between dihydrothioctate and flavin mononucleotide. 

 Curve A, F) minutes after mixing. Curve B, 20 minutes after mixing. Curve C, 

 200 minutes after mixing. 



10-3 M FMN at pH 6.4 was measured against the same concentration 

 of FMN at intervals. A peak at 535 m^t^ appeared immediately after 

 mixing; visually it is seen as an intensification of the orange color 

 of FMN. On standing at room temperature, two new broad peaks 

 centered around 565 m^ and 880 ni/x appeared gradually and the solu- 

 tion turned olive green. The 565 and 880 m^ peaks correspond re- 

 spectively to the peaks of the flavin semiquinone and its dimer (1). 

 It is likely that the 535 m^ peak in the model reaction corresponds 

 to the 530-540 ny peak of the reduced dihydrothioctyl dehydrogenase, 

 although the possibility that the 565 nifx peak in the model system 

 shifts to lower wavelengths as a result of binding to the enzyme 

 protein cannot be ruled out. 



Isenberg and Szent-Gyorgyi have obtained spectral evidence for the 

 formation of a complex between FMN antl tryptophan and several 

 other electron acceptors (4, 5) . They derived the kinetic equation 

 describing the complex formation and found that, as expected, a 

 straight line resulted from a plot of the reciprocal of the absorbancy 

 of the complex against the concentration of one of the reactants while 

 holding the secontl reactant constant. In the following equations, 

 which are similar to those above, T (SH) 2 is the reduced thioctate, 



