Efficient Inhibition of Photo[2 + 2]cycloaddition of Thymidilyl(3‘−5‘)thymidine and Promotion of Photosplitting of the cis-syn-Cyclobutane Thymine Dimer by Dimeric Zinc(II)−Cyclen Complexes Containing m- and p-Xylyl Spacers

Abstract

Monomeric and dimeric zinc(II) complexes of cyclen (cyclen = 1,4,7,10-tetraazacyclododecane) inhibited the photo[2 + 2]cycloaddition of thymidilyl(3‘−5‘)thymidine (d(TpT)) at neutral pH in aqueous solution for a novel mechanism. Comparison of the initial rates and the product yields of the photodimerization of d(TpT) (in Tris buffer at pH 7.6 with I = 0.10 (NaNO3)) at 3−5 °C by high-pressure mercury lamp indicates that the dimeric zinc(II) complexes, p- and m-xylyl-bis(Zn2+−cyclen) (Zn2L2 and Zn2L3), are effective inhibitors (70−85% inhibition compared with the control reaction at [d(TpT)] = [bis(Zn2+−cyclen)] = 0.2 mM after 20 min irradiation). This inhibition is due to the extremely strong 1:1 complexation of two deprotonated thymidine (dT-) moieties with two Zn2+−cyclen moieties (apparent complexation constants, log Kapp (Kapp = [Zn2L2 (or Zn2L3)−d(T-pT-)]/[Zn2L2free (or Zn2L3free)][d(TpT)free] (M-1)), of 6.4 ± 0.1 at pH 7.6 (50 mM HEPES, I = 0.1 (NaNO3)) and 25 °C, as determined by the isothermal calorimetric titration). A major product, cis-syn-cyclobutane thymine dimer (T[c,s]T), was also found to form complexes with Zn2+−cyclens. The apparent affinity constants for 1:1 complexes of one of the imide sites of T[c,s]T with a monomeric Zn2+−benzylcyclen (ZnL1) and with each Zn2+−cyclen unit (ZnL) of m-xylyl-bis(Zn2+−cyclen), log Kapp (Kapp = [ZnL1 (or ZnL)−(T[c,s]T)-]/[ZnL1free (or ZnLfree)][T[c,s]Tfree] (M-1)), were 3.7 ± 0.1 and 3.8 ± 0.1, respectively, at pH 7.6 (50 mM HEPES, I = 0.1 (NaNO3)) and 25 °C. The photosplitting of T[c,s]T, a reverse reaction of the photodimerization, at pH 7.6 (5 mM Tris buffer with I = 0.1 (NaNO3)) was kinetically and thermodynamically promoted by m-xylyl-bis(Zn2+−cyclen). The 1H NMR measurement showed that 78% of the cyclobutane of T[c,s]T (1 mM) was split after 1 h of UV exposure in the presence of an equivalent amount of m-xylyl-bis(Zn2+−cyclen), whereas the control reaction showed 54% splitting. The kinetic and thermodynamic stability of the 1:1 m-xylyl-bis(Zn2+−cyclen)−d(T-pT-) complex also accounts for the acceleration of photosplitting of T[c,s]T. The inhibitory effect of m-xylyl-bis(Zn2+−cyclen) on the photoreaction of poly(dT) was also revealed.