Iodine trichloride properties

The 5,6,7-cis-triene configuration of vitamin D3 is important for its biological activity as the 5,6-trans form has very low activity. Exposure to iodine in non-polar solvents under diffuse light (39,40) or to mild acidic conditions (41) affords formation of the 5,6-trans isomer. The reverse transformation occurs photochemically (42). The 5,6-trans isomer can undergo further isomerizations upon exposure to heat (43) or acids or treatment with antimony trichloride. The cis and the trans forms of vitamin D3 display characteristic ultraviolet properties. The cis form as shown earlier has a UV maxima at 265 nm. The trans form has a UV maxima at 273 nm. 

Nucleic acids can be labeled with a radioactive isotope in vitro by heating them in a mixture containing radioactive iodine and thallic trichloride (TICI3). The reaction is rapid and simple. It results in the formation of a stable covalent bond between the radioactive iodine atom and carbon atom 5 of cytosine in the nucleic acid. The biological properties of the nucleic acid are not significantly affected by this procedure. 

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