FORMATION OF C1' LOCATED SUGAR RADICALS FROM X-IRRADIATED CYTOSINE NUCLEOSIDES AND NUCLEOTIDES IN BEF2 GLASSES AND FROZEN AQUEOUS-SOLUTIONS

Free radical formation in nucleosides and nucleotides containing cytos ine as base was studied after X-irradiation at 77 K of samples prepare d in frozen aqueous BeF2 glasses and in frozen aqueous solutions by me ans of electron spin resonance (ESR) spectroscopy. By comparison with radical patterns from the cytosine base and from 1-CH3-cytosine, by us ing 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 nucleo tides in both matrices. Quantitative analysis showed that in t he 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)-O-. ((OD)-O-.) radicals at about 140 K. T his was taken as proof that at least part of these radicals were forme d from (OH)-O-. radicals. In frozen aqueous solutions about 20% of Cl' located sugar radicals were found to be present at 77 K, the populati on remaining roughly constant with increasing temperature to 140 K. Th e mechanistic findings of these unexpected results are discussed in te rms of mobile (OH)-O-. radicals and/or hole transfer in the glass and in the glassy regions of the frozen aqueous solutions.