PHYSICAL PROPERTIES OF NUCLEIC ACIDS 



469 



gives an X-ray diagram with sixty-six independent reflections distributed 

 on nine well-defined layer-lines, have calculated the cylindrically averaged 

 Patterson function (Fig. 11). The theoretical curves of the Patterson 

 function for a smooth two-chain helix of radius 9.0 A. and in which the two 

 chains are separated by one-half the length of the c axis of the unit cell are 

 found to pass through a large proportion of the Patterson peaks. Using the 

 cylindrical Patterson function, Franklin and Gosling determined that the 

 unit cell is face-centered monoclinic having a = 22.0 A., b = 39.8 A., c = 

 28.1 A. and j8 = 96.5°. In order to calculate the number of nucleotides in 

 the unit cell, it is necessary to know both the density and the water con- 

 tent. These are interdependent and can only be measured on a polycrystal- 

 line mass, and some doubt must rest on the values. For a density of 1.47 

 g./cc. at 75% relative humidity and a water content of about 40% of the 

 dry weight, a value of twenty-three nucleotides per lattice point results, so 

 that the unit cell will contain eleven nucleotides on each of two chains. 



Origin __ 



of curvc/ii) 



Orig in 



o,curve(i),. ■ '" [-- "■■[' ^ p^pT 



5 lO 15 20 25 



Fig. 11. Cylindrical Patterson function of crystalline sodium deoxypentose 

 nucleate. 



