HALL AND DOTY 31 



sedimentation distribution; the change is nearly completed during the first 5 

 minutes of heating. The intrinsic viscosity showed a similar behavior. Both 

 these changes indicate a fall in apparent molecular weight to a limiting value. 

 The sharpening of the sedimentation distribution suggests a sharpening of the 

 molecular-weight distribution as well. Consequently the fall in molecular 

 weight appears to result from the dissociation of an aggregate and not the hy- 

 drolysis of phosphate ester bonds. 



In order to proceed with the investigation of RNA stabilized by heating, a 

 standard procedure was adopted: solutions in 0.01 M phosphate buffer were 

 heated at 83° C for 10 minutes and then cooled to room temperature for 

 measurement. 



PROPERTIES OF RNA 



The intrinsic viscosity and sedimentation constant measured for a number 

 of preparations are listed in table 1. Recent results, shown in the lower part 

 of the table, have consistently given sedimentation constants near 7 S, in con- 

 trast to variable and lower results obtained earlier, which are listed in the upper 

 part of the table. RNA preparations having these higher and more consistent 

 sedimentation values were obtained by improving the separation of the micro- 

 somal particles from other cellular fractions (thereby reducing ribonuclease 

 contamination) and by maintaining a temperature of 5° or less at all stages 

 of the preparation in which RNA might be attacked. 



For a typical, improved preparation of stabilized RNA with s 2 j = 6.6 S and 

 the intrinsic viscosity [•/]] =0.29, a molecular weight (weight average) of 106,000 

 can be calculated from the Mandelkern-Flory equation [6] using 2.35 X 10 6 for 

 (4> 1/3 /P) and 0.55 for the partial specific volume. Light-scattering measure- 

 ments on the same sample yielded the same value for the weight-average 

 molecular weight. 



TABLE 1. Sedimentation Constant, Intrinsic Viscosity, and Molecular 

 Weight of RNA Preparations 



Measured in 0.01 M phosphate, pH 7.0, after heating to 83° for 10 minutes. 



*°25 M25 M s 3 w 



2.9 0.14 21,700 



4.1 0.17 40,700 

 4.6 0.19 50,400 



5.0 ... 



6.2 0.26 93,000 

 6.6 0.29 106,000 

 6.6 ... 



7.1 ... 



7.4 0.27 120,000 



6.8 ±0.4 0.27 106,000 



