STRUCTURK OF PROVITAMINS D 749 



j^m '^'he peak at 2815 A. ha.s the greatest absolution. The molecular 

 absorption coefficient K at 2815 A. is reported" to be at about 30 X 10^ 

 Earlier reviews of the role of absolution spectra in the investigation of the 

 vitamin D precursors is included in (he papers of Ijoofbourow"" "^ and 

 Brockmann.'"** Th(> al)S()rption maxima and minima of ergosterol and 7- 

 dehydrocholesterol are shown in Table 2, while Figure 1 gives a graphic 

 representation of their absorption spectra. 



The melting points of the several provitamins vary sufficiently to be of 

 use in characterization. All exhil)it optical rotation. These values are 

 summarized in Table 3. 



TABLE 3 

 Melting Points and Specific Rotations of Provitamins D 



" C. E. Bills and E. M. Honeywell, /. Biol. Chem., 80, 15-23 (1928). 



* A. G. Boer, E. H. Reerink, A. van Wijk, and J. van Niekerk, Proc. Acad. Sci. Amster- 

 daiti, 39, 622-632 (1936). 



•= A. Windaus and J. Naggatz, Ann., 542, 204-218 (1939). 

 •^ A. Windaus and R. Langer, Ann., 508, 105-114 (1933). 



* H. R. Rosenberg, Chemistn/ and Phi/siologi/ of the Vitamins, Interscience, New York, 

 1945. 



^ A. G. Boer, J. van Niekerk, E. H. Reerink, and A. van Wijk, U. S. Patent No. 

 2,163,659 (June 27, 1939). 



" A. G. Boer, J. van Niekerk, E. H. Reerink, and A. van Wijk, U. S. PatentNo. 2,216,- 

 719 (Oct. 18, 1940). 



3. Structure of the Provitamins D 



The natural provitamins are all sterols, although some of the synthetic 

 compounds prepared by irradiation can no longer be classed in this cate- 

 gory. The primary irradiation products — the lumisterols — still retain the 

 steroid nucleus, although they have assumed a stereoisomeric configuration. 

 On the other hand, the tachysterols, w^hich are the irradiation products next 

 in sequence, have lost their typical steroid ring structures; however, they 



'»» T. R. Hogness, A. E. Sidwell, Jr., and F. P. Zscheile, Jr., ./. Biol. Chem., 120, 239- 

 256 (1937). 



1'° J. R. Loofbourow, Bull. Basic Sci. Research, 3, 101-156, 201-236 (1931). 

 Ill J. R. Loofbourow, Bull. Basic Sci. Research, 4, 59-112 (1932). 

 "2 J. R. Loofbourow, Bull. Basic Sci. Research, 3, 257-262 (1931). 



