DEHYDRATION OF U.V. IRRADIATED URIDINE AND ITS DERIVATIVES

Abstract— Ultraviolet irradiation of uridine, oligonucleotides and long chain Poly U produces a variety of photoproducts including a hydrated base arising from the addition of water to the 5,6 double bond. Both the hydration and dehydration reactions in a nucleotide are sensitive to the position of the phosphate linkage. The 5' mononucleotide is less easily hydrated than the 3'. Conversely, the dehydration process in the nucleoside and the nucleotides is most rapid for uridine and decreases progressively in rate as a phosphate is added to the 2', 5' or 3' position respectively. In the compound UpU the dehydration is a two component reaction with the two dehydration rates similar to those of 5' and 3' mononucleotides respectively. In higher order oligonucleotides the dehydration rate is slower than any of the rates for 5' and 3' monophosphates or 3', 5' diphosphates. The dehydration rate decreases rapidly for oligonucleotides up to chain lengths of 6 or 8 and then decreases at a diminishing rate. Long chain Poly U (7gt; 200 nucleotides) dehydrates at approximately 1/5 the rate of Up. In addition the initial alkaline treatment to dehydrate irradiated Poly U may result in chain breakage but this effect is not seen in subsequent cycles of irradiation and dehydration. The chain length effect on dehydration rate is only slightly decreased by the presence of 7M urea in the dehydration mixture. Quantitative data is presented on the effect of ternpcrature, and pH on the rate constant for dehydration of Up and its derivatives.