Theoretical study on the characterization of the ribosyl C4 '-radical observed in irradiated crystals of uridine

The angular dependence of the beta-hydrogens hyperfine splitting (hfs) constants of the 1'-amino 2'-deoxyribosyl C4'-radical (1) has been computed at the B3LYP/6-311G**//UHF/6-31G** level for the S-type (C2' endo ring puckering) and N-type (C3' endo ring puckering) configurations. Good agreement between the theoretical hfs constants and the three large experimental P-hydrogen hfs constants of the radical species observed in irradiated single crystals of uridine has been found only for the N-type configuration with the beta5'-oxygen in the staggered conformation. It is concluded that the observed radical species is the uridine C4'-radical (2) that adopts the C3' endo ring puckering as found in single crystals of uridine by means of neutron diffraction. This conclusion is in contrast with that reached in a previous theoretical study.