THE MECHANISM FOR FROZEN AQUEOUS SOLUTION IRRADIATION OF PYRIMIDINES*

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Abstract
Summary Upon freezing an aqueous solution, the formation of crystalline ice forces the solute molecules into solid aggregates. Therefore, identical photodimerimtion of pyrimidines occur in either frozen aqueous solutions or dry solid films. No photodimerization of pyrimidines was evidenced in a homogeneous glycerol glass. Upon freezing an aqueous solution containing solute and a small percentage of organic solvent, the solute molecules together with some organic solvent form ‘puddles’. Therefore, a photochemical reaction in this organic solvent was observed. Only those pyrimidines having an α,β‐unsaturated keto group in their structure undergo photodimerization. The mechanism through which the cyclobutyl ring is formed most probably involves a diradical intermediate. The unstable thymine monohydrate crystal has the most favorable orientation for two thymine molecules to form dimer. Therefore, the photodimerization process for thymine is less favored in dry solid film than in frozen aqueous solution because the thymine monohydrate crystalline lattice collapses under low humidity.