Formation of C1' located sugar radicals from X-irradiated cytosine nucleosides and -tides in BeF2 glasses and frozen aqueous solutions.

Free radical formation in nucleosides and nucleotides containing cytosine as base was studied after X-irradiation at 77 K of samples prepared in frozen aqueous BeF2 glasses and in frozen aqueous solutions by means of electron spin resonance (ESR) spectroscopy. By comparison with radical patterns from the cytosine base and from 1-CH3-cytosine, by using specifically base-deuterated nucleosides and by comparison between the 2'-deoxy- and the ribonucleotide it could be demonstrated that a radical at the C1'-position of the sugar was formed in nucleosides and nucleotides in both matrices. Quantitative analysis showed that in the BeF2 glass an initial population of about 10% of substrate located species due to this radical was present at 77 K and developed to about 25% after decay of the .OH (.OD) radicals at about 140 K. This was taken as proof that at least part of these radicals were formed from .OH radicals. In frozen aqueous solutions about 20% of C1' located sugar radicals were found to be present at 77 K, the population remaining roughly constant with increasing temperature to 140 K. The mechanistic findings of these unexpected results are discussed in terms of mobile .OH radicals and/or hole transfer in the glass and in the glassy regions of the frozen aqueous solutions.