Early work identified three compounds, namely the c,s cyclobutane dimer, the so-called (6-4) photoproduct (5-hydroxy-6-4′-(5-methylpyrimidin-2′-one)-5,6-dihydrothymine) and a trimer hydrate, as products formed upon UV irradiation of thymine in frozen aqueous solution. More recent work has shown that an (α-4) product, namely α-4′-(5′-methylpyrimidine-2′-one)-thymine, is a likely product formed under these reaction conditions. During a thorough reinvestigation of the photochemistry of Thy in ice at −78.5°C, we found that a variety of other products could be detected. In addition to the c,s dimer, the other three known cyclobutane dimers, namely the c,a, t,s and t,a forms, are produced, although in considerably smaller amounts. The so-called “spore product” of thymine (5,6-dihydro-5-(α-thyminyl)thymine) is likewise formed. Two other dimers have been identified as minor products; one of these has been determined to be 5-(thymin-3-yl)-5,6-dihydrothymine and the other has been tentatively assigned to be a (5-4) adduct (6-hydroxy-5-4′-(5-methylpyrimidin-2′-one)-5,6-dihydrothymine). Compounds with the behavior expected of true trimeric compounds have been isolated via HPLC and characterized by mass spectrometry and photochemical behavior. One of these materials, putatively containing an oxetane ring, decomposes thermally to a secondary trimeric product that is then converted into the known trimer hydrate.
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