INTRODUCTION 639 



epimerization, the biopotency is greatly decreased, although the epi-com- 

 pounds still possess some activity. 



The presence of a side chain on carbon 17 appears to be another require- 

 ment for biologic activity. When the side chain is completely absent, as 

 in the case of A^''-androstadiene-3,17-diol,''^' or even when the chain is 

 shortened to a length of four carbons, as in the case of 3-hydroxy-A^'''- 

 choladienic acid,*^ the activated compound is almost without biologic 

 potency. Slight changes in the normal side chains, such as replacement of 

 methyl groups by ethyl groups or the variation in the spatial arrangement 

 of such ethyl side chains, can result in profound alterations in biologic 

 activity. For a more complete discussion of the structural relationships of 

 the provitamins D, of the vitamins D, and of their derivatives, the reader is 

 referred to pages 749 to 756, and pages 782 to 791 of Volume I, The Lipids. 



b. Production of the Vitamins D from the Provitamins D. In order to 

 effect the change from a physiologically inert compound to one possessing 

 the high biologic activity exhibited by the vitamins D, it is necessary to 

 impart a definite quantity of energy to the provitamin D molecule. This 

 can be brought about by the application of the ultraviolet light present in 

 sunlight, or of that supplied by artificial sources of radiant energy, as well 

 as by several types of physical treatment. It was originally believed 

 that light having wave lengths between 230 and 305 m/x (which is the princi- 

 pal absorption range of ergosterol) activated ergosterol with equal effi- 

 ciency. ^^-"^^^ Rosenberg"'^ is of the opinion that the greatest efficiency re- 

 sults when light having a wave length of 281 m/z is employed; this coincides 

 with the position of the chief absorption band of ergosterol. Bunker et 

 al.^"^ obtained the best results with 7-dehydrocholesterol when monochro- 

 matic fight having a wavelength of 296.7 m^ was used. These investi- 

 gators'^^ reported that the most effective treatment of rachitic rats with 

 ultraviolet radiation was by the use of rays having this same wave length 

 (296.7 m/i). This finding is disputed by Knudson and Benford,^^^ who ob- 

 served that light with a wave length of 280.4 m^t was most effective in 

 curing rickets. In addition to sunlight, the ultraviolet light obtained from 

 magnesium and carbon arcs has been used, as well as the mercury vapor 



'" A. Butenandt, E. Hausmann, J. Paland, D. v. Dresler, and U. Meinerts, Ber., 71, 

 1316-1.322(1938). 



132 S. K. Kon, F. Daniels, and H. Steenbock, J. Am. Chem. Soc, 50, 257.3-2581 

 (1928). 



1" T. A. Webster and R. B. Bourdillon, Biochem. J., 22, 1223-1230 (1928). 



i'" A. L. Marshall and A. Knudson, J. Am. Chem. Soc, 52, 2304-2314 (1930). 



»=» J. W. M. Bunker, R. S. Harris, and L. M. Mosher, /. Am. Chem. Soc, 62, 508-511 

 (1940). 



"6 A. Knudson and F. Benford, /. Biol. Chem., 124, 287-299 (1938). 



