138 VITAMIN D GROUP 



B. THE PROVITAMIN THEORY AND SYNTHESES 

 1, History 



When it became evident that in foodstuffs the sterol fraction contains the 

 acceptor of the activating rays, investigators tried to explain chemically the 

 changes induced in the sterols by irradiation. Hess and Weinstock-^ intro- 

 duced the use of the quartz spectrograph for investigating the chemistry 

 of activation. They found that ordinary cholesterol and wheat phytosterol 

 (sitosterol) were somewhat opaque to ultraviolet light, and that irradiation 

 decreased their opacity. On the other hand, dihydrocholesterol and dihydro- 

 sitosterol, which were not activatable, were practically transparent. Un- 

 fortunately, a mercury arc was used as the light source for the spectrograms. 

 Since this gives a discontinuous spectrum, nothing could be learned of the 

 spectral structure in the region of absorption. 



Schlutz and Morse," with better spectrographic technique, determined 

 that the absorption spectrum of ordinary cholesterol is banded. They rec- 

 ognized two maxima of absorption, at approximately 294 and 283 m/z and 

 were thus the first to record the provitamin absorption spectrum. They 

 noticed that after brief irradiation the inflections gave way to general ab- 

 sorption, and, considering Beer's law, they postulated that either the cho- 

 lesterol had been at least half metamorphosed, or else "the substance in 

 which the absorption spectrum is changed may be a small amount of im- 

 purity in the cholesterol which is not removed by repeated crystallizations 

 from alcohol, and which is exceedingly absorptive." 



Rosenheim and Webster^^ found that cholesterol which had been regen- 

 erated from cholesterol dibromide was so pure that it no longer showed the 

 characteristic absorption spectrum and was no longer activatable. Heilbron 

 ei al}'' reported that fractional crystallization of cholesterol led to the ac- 

 cumulation, in the least soluble fraction, of the substance responsible for 

 the characteristic absorption spectrum. They recognized a third absorption 

 maximum, X 269 m^u. Irradiation destroyed the three bands, leaving only 

 general absorption. Pohl,^^ by the technique of photoelectric photometry, 

 also detected three absorption bands in cholesterol, which faded upon ir- 

 radiation. With the knowledge that complete disappearance of the bands 

 corresponded to the destruction of only a trivial fraction of the cholesterol, 

 he concluded that the absorbing substance was present only in minute 



24 A. F. Hess and M. Weinstock, /. Biol. Chem. 64, 181 (1925); 64, 193 (1925). 



25 F. W. Schlutz and M. Morse, Am. J. Diseases Children 30, 199 (1925). 



26 O. Rosenheim and T. A. Webster, J. Soc. Chem. Ind. 45, 932 (1926); Biochem. J. 

 21, 127 (1927). 



27 I. M. Heilbron, E. D. Kamm, and R. A. Morton, /. Soc. Chem. Ind. 45, 932 (1926) ; 

 Biochem. J. 21, 78 (1927). 



28 R. Pohl, Nachr. Ges. Wiss. Gottingen, Math, physik. Kl. 142 (1926). 



