Reactivity and selectivity in light-induced free radical reactions of 2-propanol with purine and pyrimidine mononucleotides and dinucleoside monophosphates

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Abstract
Photoalkylation reactions with 2-propanol, initiated with di-tert-butyl peroxide, of a variety of purine and pyrimidine mononucleotides and dinucleoside monophosphates lead to the substitution of an α-hydroxyisopropyl group for the H-8 atom of adenosine and the addition of the alcohol across the 5,6-double bond of the pyrimidines. Adenosine moieties blocked at their 3′-hydroxyl group are alkylated faster than those blocked at their 5′-hydroxyl. The reactivity of the uridine moieties of 3′-UMP, 5′-UMP, and uridylyl- (3′,5′)-uridine is not affected by the location of the phosphate group. However, the uridine moiety of uridylyl-(3′,5′)-adenosine is modified faster than that of adenylyl-(3′,5′)-uridine. It is suggested that steric hindrance imposed by the phosphate group determines the reactivity of adenosine moieties, while base stacking involving adenosine determines the reactivity of uridine moieties. These two effects play a major role in controlling the nature and degree of the selectivity of these photoalkylation reactions for either adenosine or uridine. Cytidine has been found to be inert in these reactions.