Studies on the Mechanism of Ribonucleotide Reductases

Ribonucleotide reductases are enzymes that catalyze the conversion of ribonucleotides to 2‘-deoxyribonucleotides. This important reaction is initiated by the generation of a C-3‘ nucleotide radical and subsequent loss of the 2‘-hydroxyl group. In order to model certain steps in this mechanism, selenol ester 23 was prepared and photolyzed providing the first selective chemical access to the 3‘-adenosyl radical. From product analysis it could be shown that elimination of the 2‘-OH function readily takes place under general base catalysis. The rate coefficient for this reaction was determined by competition kinetics to be 1.5·106 s-1 in the presence of 1 M triethylammonium acetate buffer at pH 7. Without catalyst the elimination rate is about 103 times slower. It can be concluded that a similar mechanism is also feasible for the key steps of the enzyme catalyzed reaction.