Model Studies of DNA C5‘ Radicals. Selective Generation and Reactivity of 2‘-Deoxyadenosin-5‘-yl Radical

Abstract

The reaction of hydrated electrons (e(aq)(-)) with 8-bromo-2'-deoxyadenosine has been investigated by radiolytic methods coupled with product studies and addressed computationally by means of DFT-B3LYP calculations. Pulse radiolysis revealed that this reaction was complete in approximately 0.3 mus, and, at this time, no significant absorption was detected. The spectrum of a transient developed in 20 mus has an absorbance in the range 300-500 nm (epsilon(max) congruent with 9600 M(-1) cm(-1) at 360 nm), and it was assigned to aromatic aminyl radical 3. Computed vertical transitions (TD-UB3LYP/6-311+G) are in good agreement with the experimental observations. Radical 3 is obtained by the following reaction sequence: one-electron reductive cleavage of the C-Br bond that gives the C8 radical, a fast radical translocation from the C8 to C5' position, and an intramolecular attack of the C5' radical at the C8,N7 double bond of the adenine moiety. The rate constant for the cyclization is 1.6 x 10(5) s(-1). On the basis of the theoretical findings, the cyclization step is highly stereospecific. The rate constants for the reactions of C5' and aminyl 3 radicals with different oxidants were determined by pulse radiolysis methods. The respective rate constants for the reaction of 2'-deoxyadenosin-5'-yl radical with dioxygen, Fe(CN)(6)(3)(-), and MV(2+) in water at ambient temperature are 1.9 x 10(9), 4.2 x 10(9), and 2.2 x 10(8) M(-1) s(-1). The value for the reaction of aminyl radical 3 with Fe(CN)(6)(3-) is 8.3 x 10(8) M(-1) s(-1), whereas the reaction with dioxygen is reversible. Tailored experiments allowed the reaction mechanism to be defined in some detail. A synthetically useful radical cascade process has also been developed that allows in a one-pot procedure the conversion of 8-bromo-2'-deoxyadenosine to 5',8-cyclo-2'-deoxyadenosine in a diastereoisomeric ratio (5'R):(5'S) = 6:1 and in high yield, by reaction with hydrated electrons in the presence of K(4)Fe(CN)(6).

Selective Generation and Reactivity of 5′‐Adenosinyl and 2′‐Adenosinyl Radicals

“…It is worth mentioning that in the case of the deoxyribosides the build up of TMPDC + was not observed. [7] This strengthens our mechanistic suggestion because a heterolytic cleavage of b-C,O-bonds in secondary alkyl radicals has never been observed.…”

“…[7] The reaction of an aqueous solution of 7 (1 mm) and tBuOH (0.25 m) at pH~7 with e À aq in the absence of O 2 was complete within approximately 300 ns. At this time, no significant absorption was detected in the 300±750 nm region.…”

“…[11] From the slope of k obs versus [7], the bimolecular rate constant was determined to be 1.1 î 10 10 m À1 s…”

“…[9] In analogy to the reactions of 8-bromo-2'-deoxyadenosine 3, [7] we assigned the transient in Figure 1 to the conjugated aminyl radical 10, and the observed rate to the cyclization of radical 9 (Scheme 3). Compared with the 2'-deoxyribosides, in the ribo case the spectrum containing the two bands developed more slowly (100 vs 20 ms) and the absorbance at the maximum (350 vs 360 nm) was about two thirds.…”

“…1 î 10 4 m À1 cm À1 ), [7,14] it is reasonable to assume a similar extinction coefficient also for radical 10. This analysis suggests that about 60 % of the radicals, produced by reaction of the hydrated electron with 7, leads to radical 10 (Scheme 3).…”