Res. Chem. Intermed.
, Vol. 28, No. 2,3, pp. 257–264 (2002)
Also available online - www.vsppub.com
Theoretical study on the characterization of the ribosyl
-radical observed in irradiated crystals of uridine
I.Co.C.E.A. Consiglio Nazionale delle Ricerche, Via Gobetti 101, 40129 Bologna, Italy
Received 18 October 2001; accepted 15 November 2001
Abstract—The angular dependence of the ¯-hydrogens hyper ne splitting (hfs) constants of the 1
-radical (1) has been computed at the B3LYP/6-311G**//UHF / 6-31G**
level for the S-type (C2
ring puckering) and N-type (C3
ring puckering) con gurations.
Good agreement between the theoretical hfs constants and the three large experimental ¯-hydrogen
hfs constants of the radical species observed in irradiated single crystals of uridine has been found
only for the N-type con guration with the ¯5
-oxygen in the staggered conformation. It is concluded
that the observed radical species is the uridine C4
-radical (2) that adopts the C3
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.
: DNA radicals; hyper ne coupling constants; structure; DFT calculations.
It was recently suggested  that the poor agreement between the
constants computed for the 1
-radical (1), used as model
system, and those observed for the C4
-radical of the uridine 5
-phosphate (3) 
could be due either to alteration of the values of the
-hydrogens hfs constants
on replacing the 5
-phosphate with the hydroxy group in the calculations or to
the detection of a different radical. The rst hypothesis was based on the results
obtained by Colson and Sevilla  who found an anomalous phase shift between
-hydrogens hfs constants of the 2
-radical (4) that was attributed to a distortion of geometry at C5
owing to the
presence of the phosphate groups.
We have recently shown  that this anomalous phase shift is exclusively due to
the electronic effect of the
-oxygen of the 5
-phosphate and that phosphate groups