BIPHOTONIC PHOTOIONIZATION OF CYTOSINE AND ITS DERIVATIVES WITH UV-RADIATION AT 248 NM - AN EPR STUDY IN LOW-TEMPERATURE PERCHLORATE GLASSES

High-intensity laser photoionization of cytosine and its derivatives h as been used to generate the parent radical cations and to study the p athways by which they react. The parent radical cations are generally unstable at 77 K and undergo deprotonation to neutral radicals. The ra dicals formed on photoionization of cytosine, 3-methylcytosine, and 5- methylcytosine all give EPR spectra consistent with N1 alpha-deprotona tion, In contrast, photoionization of 1-methylcytosine, 5'-dCMP, 5'-CM P, and poly C in neutral glasses gives rise in each case to EPR spectr a at 77 K that are consistent with loss of a proton from the N1 carbon , which in the case of the nucleosides and nucleotides generated the C 1' radical. In alkaline glasses photoionization of 5'-dCMP gives a sex tet spectrum that has been tentatively assigned to the C3' radical. Fi nally photoionization of N1-substituted 5-methylcytosine derivatives i n neutral glasses gives spectra consistent with loss of a proton from the base methyl group as observed previously with thymine nucleosides. The direct formation of sugar radicals from the radical cations of cy tosine derivatives at 77 K is an important observation and contrasts w ith the properties of the radical cations derived from the other DNA n ucleosides, and its relevance to radiation damage to DNA is discussed.