Reaction mechanisms of trans-1,2-dihydroxy-anti-3,4-epoxy-1,2,3,4-tetrahydro-5-methylchrysene with DNA in aqueous solutions

Reactions of trans-l,2-dihydroxy-anti-3,4-epoxy-l,2,3,4-tetrahydro-5-methylchrysene(anti-5-MeCDF) with DNA and the effects of ionic strength onthe reaction were studied in aqueous buffer solution (5 mM sodiumphosphate, pH 7) by means of absorption and fluorescence spectroscopy.The results are compared with those obtained with the widelystudied metabolite model compound trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene(BaPDE5.The rate constant of hydrolysis of 5-MeCDE is k = 1.0 ? 10⁴s,which is {small tilde}35 times smaller than the value of k forBaPDE under similar conditions. As in the case of BaPDE, therate of reaction of 5-MeCDE is accelerated in the presence ofDNA. This effect is attributed to the rapid formation of physicalassociation complexes (binding constant K) and the subsequentslower formation (rate constant k₃) of carbocations at DNA bindingsites, which in turn decay rapidly via hydrolysis to tetraols(l,2,3,4-tetrahydroxytetrahydro-5-methylchrysene, 5-MeCT) andto covalent adducts. The values of K and k₃ are 2800 ? 300/Mand 8.7 ? 10^–3/s respectively, and are reduced to 450? 100/M and 1.8 ? 10^–3/s in the presence of 0.1 M NaCl.The fraction of 5-MeCDE molecules which bind covalently to DNAis, on the other hand, constant under these conditions and liesin the range of 5–8%. Similar values for the covalentbinding are observed for BaPDE, even though the physical associationconstant K is {small tilde}10 times larger than for 5-MeCDEunder similar conditions. This difference in the values of Kare attributed to the larger aromatic ring system in BaPDE whichallows for a higher interaction of this molecule with the basesof DNA. Finally, the tetraol derived from the hydrolysis of5-MeCDE also binds non-covalently to DNA, but the value of Kis {small tilde}3 times smaller than for the diol epoxide.