56 RADIATION BIOLOGY 



reacting system show a much higher population of electrons than would 

 correspond to black-body radiation (Thomas and Eyring, 1951). The 

 concentration of such highly populated states depends upon the potential- 

 energy curves for the system. Using the simplified diagram of Fig. 1-20, 

 the anomalous populations of upper states may be explained in the follow- 

 ing way: Most molecules pass by radiationless transitions from ^ to / 

 by way of F, liberating energy equal to the amount by which state / lies 

 below A. This heats the system and correspondingly promotes the 

 black-body number of molecules in all states. However, unreacted mole- 

 cules at state A also have the choice of passing to H and thus greatly 

 overpopulating the latter state as compared with the black-body number 

 of molecules arriving from /. The result is a strong chemiluminescent 

 band due to the transitions from // to /. In the same way, molecules 

 may pass by radiationless transition from A by way of B to the ionic 

 state G, or they may pass through C by absorbing radiation. As a result 

 of these and analogous processes, the ionic population will greatly exceed 

 the number that equihbrium theory would lead one to calculate as arising 

 from /. The population of the ions in the exhaust of hot flames from 

 jet engines (Sanger et al, 1949) could be readily calculated if the detailed 

 surfaces were known. No new principles are required to calculate the 

 statistical behavior of systems on known surfaces. 



6. ENERGY TRANSFER IN BIOLOGICAL REACTIONS 



Since molecules behave the same way in the same local environment 

 whether within or without a hving organism, we may expect to under- 

 stand energy exchange in biological reactions in terms of the discussions 

 already given. The very large molecules of biological importance — 

 viruses, proteins, genes, and structural substances — give some possibility 

 for anomalous behavior simply through the complexity of their size and 

 their internal linkages. Of those mentioned, only proteins are at all well 

 studied, and the literature of protein investigations affords only a very 

 few clear-cut examples of surprising behavior in energy transfer. Fur- 

 thermore none of these examples has been adequately investigated, and 

 it would be out of place to discuss them in any detail. Three cases will 

 be mentioned briefly, two of which are of special interest in this chapter 

 because they involve photoreactions. 



Carbon monoxide is liberated in unit quantum yield from carbonyl- 

 myoglobin, independent of wave length in the region from 2800 to 5460 A 

 (Biicher and Negelein, 1942; Biicher and Kaspers, 1946). In the visible 

 region of wave lengths the light is entirely absorbed by the prosthetic 

 group of myoglobin, which is an iron-porphyrin, or heme. Carbon mon- 

 oxide is bound at the iron atom of the heme. At 2800 A, 50 per cent of 

 the absorbed radiation is taken up by tyrosine and tryptophan amino 



