19 



excitable than the bonded ones. Not being strongly bonded to any 

 nucleus in particular, they are also less likely to cause nuclear 

 vibrations. However, if the molecule contains any weak bonds 

 which can be broken or distorted by £* (if the dissociation energy 

 is lower than the excitation energy), then the £* communicated 

 to the molecule will be used up in breaking or distorting that bond. 

 Evidently, such molecules would be equally unfit to function as 

 transmitters of E*. If the molecule does not contain such weak 

 spots and is unable to dissipate its E* (and if there was no colli- 

 sional deactivation), then it has to shoot out its £* as a photon, 

 for molecules can hold E* only for a very short period of the 

 order of 10"* to 10"^ seconds. This means that the molecule will 

 be fluorescent. Fluorescence thus tells us that the molecule is ca- 

 pable of accepting energy and does not dissipate it. These are two 

 qualities any molecule must have to be able to act as an energy 

 transmitter. Thus, fluorescence becomes a most important indicator 

 for our studies, though in itself it may have no direct biological 

 meaning, fluorescent light not being different from any other 

 light. Naturally, the biological role of these molecules will not be 

 to emit but to transmit energy and so in their natural setting they 

 should not be fluorescent at all or should be so only to a small ex- 

 tent (some of the energy being spilled) . For example, chlorophyll, 

 the most important energy transmitter, shows a very poor fluores- 

 cence in the plant while it displays a most wonderful one in ex- 

 tracted condition. 



Naturally, a fluorescence in a tissue extract does not necessarily 

 mean the presence of an energy transmitter, nor does the absence 

 of fluorescence necessarily exclude one. The fluorescence may be 

 covered up by other substances which absorb the light or interfere 

 with excitation. Fluorescence may also be absent because a mole- 

 cule acquires the structure necessary for fluorescence only in con- 

 junction with other cell constituents such as metals or proteins. 



Organic molecules, fluorescent in solution, may belong to dif- 

 ferent groups of substances but they all can be expected to have a 



