if I understand correctly. 



POLLARD: That is correct. 



CHARGAFF: That is not to say that if you break it in half it is no 

 longer transforming or is that the minimum? I really don't know what you are 

 measuring when you say 6, 000, 000. Is it the molecular weight? 



POLLARD: What is done is what I call a mental transformation and 

 one has to undergo this before he can understand it. You take a preparation of 

 the transforming principle, dry it and then take part of the dried specimen out 

 as a control. You irradiate it with fast electrons and fast and slow deutronsand 

 you measure, after irradiation, the amount of activity which is left. In the case 

 of the early experiments, this was a very difficult thing to determine. One had 

 to determine the concentration of the irradiated material on which a fixed num- 

 ber of transformations would take place. It was rather nasty, and the observa- 

 tions were not very precise. In the modern experiments, it is much easier. 

 You can simply measure the number of antibiotic-resistant forms that are trans- 

 formed. These colonies can be measured as a definite number, and you can get 

 some estimate of the activity of the t" insforming principle that is left at the end. 

 This loss of activity follows approximately a logarithmic function. 



I must say with regard to the transforming principle, in view of the 

 crudeness of the assay, that we did take the logarithmic inactivation on faith. If 

 you believe then that the logarithmic inactivation requires that there be a con- 

 stant, which is volume in one case and area in another, the volume corresponds 

 to the volume of the sensitive unit. This can be £e-expressed as a molecular 

 weight. Bombardments that measure the volume give a value of six million for 

 the equivalent molecular weight. Bombardments measuring area do not agree 

 with this unless the substance is very long and thin. Agreement between the vol- 

 ume and the area can be obtained by saying that it is 45 % units across and 

 3800 ^ units long. That is all we can say about this. 



CHARGAFF: That is roughly a ratio of about 100? 



POLLARD: Roughly, 100. 



I have spent too long on this since I am not too sure that our work is at 

 its best in these two cases because the assays are somewhat an open question. 

 But the substances that we have studied show remarkable radiosensitivity in the 

 dry state, sensitivity that is apparently confined to a region that is approximate- 

 ly that of the molecule. 



I should like to summarize the facts as we know them. I think I have 

 nine. Incidentally, I must stress that I have a very fine group that is doing all 

 this work, and I am on top of a pinnacle that they support. 



1. All the inactivation volumes are within a factor of 4 of the molecu- 

 lar volume, on the basis that a single ionization will inactivate the 

 molecule. Just a single ionization, not primary, but any ionization. 



2. The cross-section of a molecule measured with densely ionizing 

 radiation, such as alpha particles or deuterons, is a varying func- 

 tion of what we call the ionization density. Dr. Zirkle and Dr. 

 Tobias call it linear energy transfer. Either is all right. General- 

 ly, this shows a trend to a maximum value and that value corre- 

 sponds ordinarily to the diameter and area of the molecule. 



