92 ELECTROMAGNETIC RADIATIONS AND MATTER 



Because they carry more energy than photons in the visible region, the 

 photons in the ultraviolet region are less likely to be absorbed. They pene- 

 trate deeper into the absorbent and excite molecules at the point at which 

 they are finally caught. 



Of all the synthetic biological reactions whose rate is sensitive to ultra- 

 violet light, probably the photosynthesis of simple organic sugars from C0 2 

 and 2 in plant leaves is the best understood; and yet the understanding of 

 this basic process is not completely satisfactory. Of course if it were, we 

 should be able to reproduce the syntheses in a test tube; but we cannot. 



More important to present considerations is our knowledge of photo- 

 catalyzed syntheses of the vitamins from basic components. Some of the 

 vitamins have been purified, crystallized, and synthesized, and hence their 

 chemical composition and structure are known. Consider the antirickets 

 vitamin D 2 (calciferol) for instance. Its structure is well known: two six- 

 membered rings and a five-membered ring attached to an unsaturated 

 aliphatic side chain of six carbon atoms, with a molecular weight of 393. 

 This molecule is formed through the absorption of ultraviolet radiation of 

 2500 to 3000 A by ergosterol, a sterol molecule whose structure also is well 

 known. The synthesis occurs in at least two steps. The absorption is con- 

 sidered to take place at a carbon-carbon double bond, and the absorbed 

 energy to go into excitation of the t electrons which form the bond. The 

 opening of a benzene-like ring follows, and further rearrangements of the 

 atoms and bonds give the biochemically active vitamin B 2 structure. The re- 

 action will not occur at all unless photolyzed. 



This synthesis takes place in the human body at a location to which both 

 the molecular components and ultraviolet radiation are accessible: that is, 

 just beneath the surface of the skin in the living tissue serviced by the blood 

 capillaries. Thus the principle upon which ultraviolet therapy is based, and 

 the advantages of moderate exposure to sunlight, both become apparent. 



Phototherapy 



Prolonged sun bathing can damage skin pigments and can cause ery- 

 thema. For instance, on the average it takes only 20 microwatts (/xw) of 

 ultraviolet of wavelength 2537 A (from a mercury vapor lamp) falling upon 

 the skin for 15 min to produce erythema. It is fortunate that the very in- 

 tense ultraviolet radiation from the sun is attenuated (scattered, absorbed, 

 converted into radiation of longer wavelength) by the ozone and nitrogen 

 compounds in the upper atmosphere. Ultraviolet radiation would be a prob- 

 lem in space travel if it were not so readily reflected by metallic surfaces. 

 The effects on the eye are well known and have been implied in the discus- 

 sion of the chemistry of the eye: the higher-energy photons of the ultraviolet 

 in falling on the retina can keep the rod and cone cells devoid of rhodopsin 



