15 



there can be wanderings of agents produced at the primary ionization point and 

 that these will get into the molecule and cause an inactivation. However, only 

 a certain class of agents may wander. 



The kind of primary event that occurs at one point cannot wander 

 through the cytoplasm and arrive at another spot. Something that has a charac- 

 teristic more related to chemistry than to the direct physical event that occurs 

 must wander. This must be something with quite a long half-life, one that lasts 

 long enough to produce an effect. In addition, it must be something that is free 

 to diffuse. That is to say, the entity itself must move and not merely the prop- 

 erty possessed by the entity. So this will be a different class of process from 

 the one of which I have been speaking. 



That is well-illustrated by the fact that, roughly speaking, for an en- 

 zyme molecule, with the exception of the very sensitive sulfhydryl enzymes that 

 Dr. Barron has worked with, it takes of the order of 10 ion pairs to produce an 

 effect by indirect means. In the other process that may occur, which is not the 

 passage at a distance through a liquid medium, the ionization event seems to 

 somehow distribute its effect right through the molecule. It then ultimately pro- 

 duces an effect at some critical place, or alternatively causes the molecule to 

 split open, so that it no longer acts as a molecule but becomes an opened-up 

 system of some kind that is no longer specific. 



The part that is my primary concern this afternoon concerns this direct 

 effect. This is the only subject of which we have really made a study, and the 

 extent to which one unit of this type of radiation action can cause an inactivation 

 of a molecule is remarkable. Looking at this thing in general, one is forced to 

 accord the process some respect in radiobiological materials because it does 

 seem to be as potent as it could possibly be. 



I should like to reiterate what I was discussing when I talked about the 

 migration of the positive charge. If we take a polypeptide chain and, let us say, 

 we ionize a nitrogen atom, it loses an electron, and so we actually have at the 

 moment in this chain, a "carbon" atom. It has lost its electron and it has a 

 positive charge. 



Why is it necessary that it retain its inherent new carbonlike proper- 

 ties? As a matter of fact, it won't have the right valence. Nothing will be real- 

 ly fitting right. Is it not possible for this erroneous valence to migrate instead 

 of staying in place? That is to say, why won't this ersatz carbon nucleus that 

 is, after all, a nitrogen nucleus with a right to crystallize 7 electrons around 

 it, take 1 of these 7 electrons from the next atom, so that it is now restored as 

 a nitrogen atom and we no longer have a carbon atom as the next neighbor, but 

 we have effectively a boron atom which will now be plus? 



That may not last. The electron may come back or it may wander on, 

 and you have, therefore, a random walk migration of this plus outward from the 

 center. It is a random walk that is not in an area, but along lines and so it will, 

 in time, move anywhere on the chain. The motion will be very rapid, because 

 the exchange of an electron between 2 atoms like this takes place at the velocity 

 of electronic motion and over the distance of 1 X. • 



The decision to transfer takes something of the order of 10" ° seconds. 

 The number of decisions to transfer that can occur in 10"^ seconds is 10 . This 

 would mean to my mind that the migration would have had a chance to cover the 

 whole long chain and possibly even to branch out through the residues on the side 

 in some sort of way. This is the exchange that I was talking about before. 



