ORIGIN OF MOLECULAR ABSORPTION BANDS 163 



small amount of an impurity which produced the selective absorption. 



In December, 1926, reports fram three separate laboratories confirmed 

 the contamination hypothesis. The contamination was subsequently 

 identified as ergosterol. The names of Rosenheim, Webster, Heilbron, 

 Kamm, Morton, and Pohl are associated with the work of identifying the 

 absorption spectrum of ergosterol. Heilbron, Kamm, and Morton 

 [1927] reported that the fractional crystallization of cholesterol led to the 

 accumulation in the least-soluble fraction of the substance responsible 

 for the characteristic absorption spectrum, and identified three absorp- 

 tion bands at 2935, 2820, and 2690 A. Irradiation destroyed the three 

 bands, leaving only a general absorption. 



Subsequently, Bills, Honeywell, and MacNair [1928] showed that 

 ergosterol was the contaminant provitamin. With the aid of a continu- 

 ous ultraviolet source of radiation supplied by a hydrogen discharge 

 tube, they found that ordinary cholesterol possessed a fourth absorption 

 band at 2600 A (Fig. IV-19), and that ergosterol possessed similar 



o 



absorption bands at 2935, 2820, and 2700 A. This absorption dis- 

 appears under irradiation, which produces activation that yields thera- 

 peutically valuable vitamin D. 



In Fig. IV-19, the spectral antirachitic efficiency curve of ultraviolet 

 radiation is plotted. Along with this curve, the absorption curve of 

 ergosterol and the erythema curve are given. Note how the antirachitic 

 curve follows the general contours of the absorption curve of ergosterol, 



o 



and that the most effective antirachitic wavelength, 2804 A, gives the 

 least erythema. One may conclude that a measurement of the erythema 

 effectiveness does not give an index to the effectiveness of ultraviolet 

 irradiation in the cure or prevention of rickets. 



Origin of Molecular Absorption Bands 



If the irradiated atoms or molecules of the tissue become ionized, it is 

 possible that this ionization will lead to chemical changes resulting in the 

 destruction of the cell. On what part of the living cell the radiation 

 acts and what primary changes result are still some of the questions to be 

 answered. The primary process may consist in absorption by a com- 

 plex molecule which is part of the nucleus. Since the molecules are 

 relatively close together and interact strongry with each other, the 

 absorption will not be limited to a sharply defined wavelength, but will 

 spread over a more or less narrow band of wavelengths. 



A single absorption band is characterized by a group of absorption 

 lines so close together that in a spectrogram obtained with a spectro- 

 graph of small dispersion the lines fuse together. With higher disper- 



