II. CHEMISTRY 



177 



manifest, the shorter the wavelength involved. Hence it is to be expected 

 that the longer wavelengths in the critical region will exhibit greater over-all 

 efficiency than the shorter ones. This is borne out experimentally. Bunker 

 and Harris-^ found that line 296.7 was the most effective for producing 

 vitamin D in depilated rats. Knudson and Benford-'^ also used rats, but 

 under the conditions of their experiment line 280.4 was the most efficient. 

 The findings with animals represent a special situation, which has little 

 bearing on the physics of activation in vitro. 



The uniformity in quantum efficiencies for the production of vitamin D 

 is especially noteworthy, when it is considered that the conversion is not 

 a single step, but an over-all effect involving the production of several 

 substances in succession, of which vitamin D is not the last (Fig. 9). No 



Fig. 9. Absorption spectra of ergosterol and its irradiation products. (After 

 Windaus et al.^^ and Velluz et aZ.^sa) 



studies have been reported on the energy requirements of the intermediate 

 reactions, but it is assumed that the requirements are less uniform than 

 for the over-all activation. 



Before the energy relations in activation were understood, the suggestion 

 was occasionally advanced''^-*' -'^^ that the yield of vitamin D might be 

 improved by the use of filtered light or of light sources predominating in 

 certain wavelengths. The goal was a light which would activate provitamin 

 D, but not destroy the vitamin D produced. Why this has never been 

 reached is evident from the spectra in Fig. 9, which show that ergosterol 

 and calciferol, as well as the intermediate products, absorb light over 

 essentially the same range, 230 m/x to 300 van. There are, however, some 

 differences in the absorption maxima, w^hich may account for the fact that 



"3" L. Velluz, G. Amiard, and A. Petit, Bull. soc. chrm. Francs 16, 501 (1949). 



224 T. A. Webster and R. B. Bourdillon, Biochem. J. 22, 1223 (1928). 



"4» I. M. Heilbron, E. D. Kamm, and R. A. Morton, Nature 120, 617 (1927). 



