The reactivity of alkyl radicals containing leaving groups in the imposition and the mechanism of copper phenanthroline induced DNA strand scission

Abstract

The reactivity of alkoxy substituted alkyl radicals containing bromide (96) and diphenylphosphate (97) as leaving groups in the β-position has been studied in 4:1 methanol/acetonitrile solution with a hydrogen atom donor (1,4-cyclohexadiene) and a proton scavenger (2,6-di-t-butylpyridine). Product and laser flash photolysis studies provide evidence for the formation of olefin cation radical 103. Subsequent reaction of 103 forms dimethylketal 102 and vinyl ether 105. Olefin cation radical 103 is proposed to arise from heterolytic cleavage of the β-leaving group in radicals 96 and 97. The rate constants for heterolytic loss of the leaving group to form 103 in acetonitrile are 8.0 x 107 s-1 for the loss of bromide and > 1 x 108 s-1 for loss of diphenylphosphate. The rate constants for reaction of 103 with 1,4-cyclohexadiene and methanol are 6.0 x 105 M-1 s-1 and < 1 x 103 M-1 s-1 respectively. Dimethylketal 102 is proposed to arise from hydrogen atom transfer to the benzylic carbon in 103 followed by trapping with methanol or through trapping of 103 with methanol followed by hydrogen atom transfer. The reaction of 103 with 1,4-cyclohexadiene is proposed to occur through hydrogen atom transfer to the ether oxygen in 103 followed by deprotonation to generate 105. This is the first example of this reaction motif in solution. Evidence for a copper 1,10-phenanthroline induced β-elimination of the 2-deoxyribonolactone (1) lesion has been obtained. Through the use of copper phenanthroline conjugates coupled with site specific incorporation of (1) in DNA, rate constants for this process have been measured. Comparison of the rate of cleavage of independently prepared 1 (k21 = (5.63 ± 0.69) x 10-4 s-1) to the rate of decay of alkali labile lesions (kdecay = (4.70 ± 0.90) x 10-4 s-1) indicates that 1 is the major alkali labile lesion generated from reaction of the conjugates with DNA. The rate of DNA oxidation (kOX) by the CuOP conjugates is slower than kdecay (kOX < kdecay). Similar ratios of direct strand breaks to alkali labile lesions are observed for the conjugates and Cu(OP)2, indicating that kdecay:kOX is similar. This leads to the conclusion that Cu(OP)2 produces direct strand breaks through a Cu(OP)2 induced cleavage of 1.