II. CHEMISTRY 179 



dictory to the conclusion that the temperature coefficient of activation is 

 small ; it merely reflects the better exposure of molecules in agitated solution 

 and perhaps also the driving of the precalciferol ^ calciferol equilibrium 

 to the right (see below). 



Ergosterol can be activated in the solid state, in the vapor phase, or in 

 solution. When it is irradiated in the solid state the first irradiation products 

 which form on the exposed surfaces act as filters which prevent light from 

 reaching the under layers until the surface products are destroyed. The 

 degree of antiricketic potency attainable by the irradiation of solid ergos- 

 terol is therefore relatively low, and under practical conditions generally 

 not more than 10% of that attained in solution. Few studies have been 

 made of activation in the vapor phase, a procedure which is handicapped 

 by the fact that the melting point of ergosterol is higher than the tempera- 

 ture at which calciferol begins to undergo transformation into its inactive 

 pyro-isomers. 



Irradiation in quiet solutions is less effective than in agitated solutions, ^^ 

 because agitation breaks up the formation of strata of irradiation products 

 like those on the surface of crystals and promotes uniform exposure of all 

 portions of the liquid. Thus it is that in commercial practice the solutions 

 are always agitated by stirring, flowing, or boiling during exposure to the 

 activating rays. 



Apart from any light-filtering action of solvents, and from any role which 

 they may play as carriers of dissolved oxygen, there appears to be what 

 Bills ei alP^ termed a specific solvent effect on activation. This was revealed 

 by parallel spectrographic and biologic examinations on the course of ac- 

 tivation in three transparent solvents — alcohol, cyclohexane, and ether. 

 The importance of the specific solvent effect was shown by the fact that 

 the time required for the attainment of maximum potency in ether was 

 longer than the time required for the entire sequence of activation and 

 destruction in alcohol (Fig. 10). The maximum potency reached in ether 

 was higher than in alcohol or cyclohexane, but certain spectral changes 

 (see below) were more conspicuous in alcohol. Dasler-'^ reported that the 

 maximum potency obtained by the irradiation of ergosterol in alcohol 

 represented a 30% conversion of ergosterol to calciferol, while the maxi- 

 mum potency obtained in ether represented a 70 % conversion. 



4. Chemistry of Activation 



When a provitamin D is exposed to ultraviolet light of suitable wave- 

 length, the transformation into the corresponding \atamin D begins at once. 



^" A. Windaus, K. Westphal, F. von Werder, and O. Rygh, Nachr. Ges. Wiss. Got- 



tingen, Math, -phijsik. Kl. 45 (1929). 

 "8 C. E. Bills, E. M. Honeywell, and W. M. Cox, Jr., J. Biol. Chem. 92, 601 (1931). 



