78 



this 540 m/A level 64 calories would be needed, which are not 

 likely to be available. As has been shown (Fig. 6), in water the 

 excited riboflavine goes into the triplet, the emission of which has 

 its maximum at 605 m/x which corresponds to an energy of 47 

 calories which is somewhat closer to biological values but seems 

 still to be too high. However, we must not forget that riboflavine 

 fulfills its role in the cell "activated" by the protein and we do 

 not know how far its energy relations are altered by its being 

 linked to the protein. We know that they are altered because the 

 riboflavine bound to its protein loses its fluorescence (which may 

 mean that it goes into the triplet) and that the wavelength of its 

 absorption becomes lengthened by 20 m/x (which means that less 

 energy is needed for its excitation ) . How far the energies needed 

 for the triplet excitation are altered we do not know. It is possible 

 that they are lowered more considerably. 



To return once more to the experiment in Fig. 6, the riboflavine 

 in th frozen tube, on excitation, went quantitatively into the triplet 

 state. Even so this tube did not change color, as observed in day- 

 light. This indicates that it absorbed light by its singlet excitation 

 and if a triplet was formed it was formed through a transition 

 from the singlet into the triplet, as symbolized by the arrow Si -^ 

 Ti in Fig. 5. The tube containing KI showed a strong brownish 

 tint which was observable even better if the tube was removed 

 from the freezing mixture and was allowed to warm up to some 

 extent.^ This suggests that the iodide made a direct transition into 

 the triplet possible, an observation which is interesting for the 

 biologist because iodine, in the form of thyroid hormones, is one 

 of the main regulators of metabolism and its action on E* is inde- 

 pendent of its charge or binding. Iodine is not the only substance 

 which is capable of this action. Serotonin shows this action con- 

 siderably stronger. Its effect is noticeable even in a 5.10'^ M con- 

 centration. There are tvv'o circumstances which make this effect 



^ On freezing, first the peripheral sheets freeze and crack up making it 

 more difficult to see what is inside the tube. On warming up the opposite 

 happens. 



