14 RADIATION BIOLOGY 



(HiicluT and Kaspcr.s, 1947). Myoglobin is iin enzyme, consistinf^ of two 

 heniin groups attached to a protein having a molecular weight of about 

 32,000. The absorption at o4()0 A is due almost entirely to the hemin 

 group, and the corresponding photochemical dissociation is presumably 

 either predissociation or direct optical dissociation. At 2800 A, much of 

 the absorption (about 40 per cent) is due to the tyrosine and tryptophane 

 groups which are presumably distributed throughout the protein mole- 

 cule. It is very probal)le that the energy is transferred from these pri- 

 marily excited groups to the hemin-carbon monoxide complex by classical 

 resonance. The elTect is almost certainly not caused by a general degra- 

 dation of the protein, since the quantum efficiencies of such reactions are 

 smaller by several orders of magnitude (Finkelstein and McLaren, 1941)). 



SOLVENT EFFECTS 



The presence of a solvent may affect the primary process in several 

 different ways; namely, it may decrease, leave unaltered, or even increase 

 the quantum yield of the photochemical reaction. The solvent may also 

 bring about a change in the reaction products. What the effect will be in 

 a specific instance cannot, in general, be predicted. However, a knowl- 

 edge of the possible influences of a solvent on the primary step is of great 

 value in interpreting experimental results. 



In one sense the simplest, and perhaps the most important, effect of the 

 solvent on the primary process is its influence on the normal ecjuilibrium 

 state of the reactant (i.e., absorbent) molecules. Acidic or basic media 

 often determine the charge on solute molecules and thus alter their 

 absorption spectra and their relative probabilities of fluorescing, dissociat- 

 ing, undergoing internal changes, or degrading their energy of excitation. 

 Changes in the molecular state, such as dimerization or the formation of 

 molecular complexes with the solvent, are of even more common occur- 

 rence and are likewise effective in changing the photochemical properties 

 of the solute. Usually such changes can be detected by measurements of 

 nonkinetic properties, such as absorption spectra, conductivity, or trans- 

 ference, and the coUigative properties of the solution. 



Since solute molecules are continuously in a state of multiple impact 

 with the surrounding molecules, any oscillational energy which the solute 

 molecule may possess, in excess of the thermal eciuilibrium amount, is 

 quickly drained off by successive collisions of the second kind. This loss 

 of oscillational energy may stabilize an electronically excited molecule by 

 reducing its probability of predissociating or of undergoing internal con- 

 version. Conversely, the solvent may "induce predissociation" of a 

 molecule by light of wave lengths too long to bring about dissociation of 

 the isolated molecule (Herzberg, 1950). After an act of internal conver- 

 sioji, a complex molecule has momentarily a large fraction of its energy of 

 excitation in its generalized oscillational degrees of freedom. If all its 



