5. Triplets and Water 



Electronic excitations gi\e us valuable information about prop- 

 erties and reactions of molecules, but whether these excitations are 

 involved in biological reactions and take any part in energy trans- 

 missions is a different question. There are reasons to doubt it. 

 Singlet excitations are much too short-lived to allow their utiliza- 

 tion or regulation. Moreover, pure electronic excitation, which 

 corresponds approximately to the visible part of the spectrum, de- 

 mands energy quanta which, as a rule, are too high to be available 

 in biological systems. The longest visible wavelength corresponds 

 to 40 Calories while the biological energ}' unit, the energy of the 

 ^P, is about 10 Calories. 



These difficulties are not present in the case of triplets, of which 

 the lifetime is longer and the energies lower; their energy levels 

 may lie even in the near infrared. However, the forbidden nature, 

 which makes the singlet-triplet transition attractive, seems also to 

 make it useless for the biologist. An excitation which has a small 

 probability of occurring can be of no use to the cell. So we are left 

 empty-handed. 



There is one possible hitch in this. The biological solvent is 

 water, not borax or glycerol, and water has many queer and unique 

 properties, such as its strong dipole character. The reasons why the 

 physicist has shunned water as a solvent are evident. On freezing, 

 water cracks, becomes inhomogeneous and unfit for optical meas- 

 urements. It becomes inhomogeneous both optically and chemi- 

 cally, having a tendency to crystallize out and leave dissolved 

 molecules behind. So the physicist has had good reason not to 

 touch water. The biologist, however, is inseparably linked to it. 



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