PHOTOCHEMISTRY OF N-[9-(2',3',5'-TRI-O-ACETYL-BETA-D-RIBOFURANOSYL)PURIN-6-YL]-PYRIDINIUM CHLORIDE IN AQUEOUS-SOLUTIONS - MECHANISM OF THE FORMATION OF TRI-O-ACETYLLUMINAROSINE

The photochemistry of N-[9-(2',3',5'-tri-O-acetyl-beta-D-ribofuranosyl)purin-6-yl]pyridinium chloride (1) has been investigated in aqueous solutions in the pH range 6-8 under aerobic and anaerobic conditions. A multistep mechanism of the photochemical transformation of 1 into the highly fluorescent nucleoside 2',3',5'-tri-O-acetylluminarosine (2) is proposed in which, as the first step, 1 undergoes light-induced, hydrolytic ring opening in the imidazole portion of the purine ring to form N-[5-formamido-6-[(2',3',5'-tri-O-acetyl-beta-D-ribofuranosyl)amino]pyrimidin-4-yl]pyridinium chloride (3). At pH > 7, 3 exists partly in the electrically neutral, zwitterionic form, which undergoes electron-transfer-induced ring closure to give luminarosine 2. This process is sensitized by the excited triplet state of 1, which serves as an electron acceptor. Under anaerobic conditions, the resulting pyridinyl radicals undergo dimerization.