EFFECT OF X RAYS ON BIOLOGICAL SYSTEMS 295 



LZ ^ K 



(-0 



in which K" is a constant and t is the time of radiation exposure. Protein 

 sokitions (0.07 per cent plasma alVjumin in water), which become cloudy 

 after 2 hours irradiation at 10°C with 72,000 r, remain clear for hours 

 when kept at 3°C. This large temperature effect on denaturation may 

 be due to the spiral structure of the protein (Pauling and Corey, 1950), in 

 which the plane of the conjugated system C — CO — NH — C is nearly 

 parallel to the spiral axis, and hydrogen bonds are formed between each 

 carbonyl and imino group and an imino or carbonyl group of a residue 

 nearly one turn forward or back along the spiral. 



A decrease in the viscosity of gelatin solutions on irradiation has been 

 reported (Fernau and Pauli, 1915). When aqueous solutions of serum 

 albumin are irradiated with 100,000 r, the effect on the viscosity depends 

 on the pH of the solution. At pH 3 there is a large increase in the 

 intrinsic viscosity; at pH 9, the increase ia smaller, whereas at the iso- 

 electric point of the protein there is almost no change (Barron and Finkel- 

 stein, 1952). 



Sedimentation velocity measurements do not show changes on irradia- 

 tion up to 90,000 r because of the rather large concentration of protein 

 necessary for such measurements. When serum albumin is irradiated 

 with 100,000 r at pH 3, 25 per cent of the total area is in a faster sedi- 

 menting peak, indicating the presence of a dimer. Irradiation in the 

 presence of 6 X 10~^ M cysteine prevents the formation of this com- 

 ponent. Irradiation in the absence of oxygen causes a slight reduction in 

 the amount of this faster component. With irradiation at pH 7 the 

 amount of the faster component diminishes markedly. The protection 

 with cysteine and with oxygen-lack is aa indication that the process may 

 be a dimerization through disulfide formation. 



No marked changes in electrophoretic mobilities are observed on irra- 

 diation of proteins up to 100,000 r or on acid-base titration. 



A number of side groups of proteins may be studied by attaching to 

 them easily recognizable ions, such as organic dyestuffs or metals. A 

 preliminary survey of this field has shown that amino groups are easily 

 detached on irradiation with X rays, whereas carboxyl groups are more 

 resistant. The first are studied with organic anionic dyes, and the second 

 with metals, such as calcium and uranium. It is well known that pro- 

 teins can be split into smaller components, the so-called "subunits," by 

 being dissolved in concentrated solutions of urea, by having the tempera- 

 ture lowered, or by small pH changes. Such a split of some protein mol- 

 ecules can be produced on irradiation. When hemocyanin is irradiated 

 with a particles (radon), there is a split into half-molecules, the process 

 being independent of the temperature of irradiation. Hemoglobin and 



