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

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 (NaNO 3 )) at 3−5 °C by high-pressure mercury lamp indicates that the dimeric zinc(II) complexes, p - and m -xylyl-bis(Zn 2+ −cyclen) (Zn 2 L 2 and Zn 2 L 3 ), are effective inhibitors (70−85% inhibition compared with the control reaction at [d(TpT)] = [bis(Zn 2+ −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 Zn 2+ −cyclen moieties (apparent complexation constants, log K app ( K app = [Zn 2 L 2 (or Zn 2 L 3 )−d(T - pT - )]/[Zn 2 L 2 free (or Zn 2 L 3 free )][d(TpT) free ] (M - 1 )), of 6.4 ± 0.1 at pH 7.6 (50 mM HEPES, I = 0.1 (NaNO 3 )) 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 Zn 2+ −cyclens. The apparent affinity constants for 1:1 complexes of one of the imide sites of T[ c,s ]T with a monomeric Zn 2+ −benzylcyclen (ZnL 1 ) and with each Zn 2+ −cyclen unit (ZnL) of m -xylyl-bis(Zn 2+ −cyclen), log K app ( K app = [ZnL 1 (or ZnL)−(T[ c,s ]T) - ]/[ZnL 1 free (or ZnL free )][T[ c,s ]T free ] (M - 1 )), were 3.7 ± 0.1 and 3.8 ± 0.1, respectively, at pH 7.6 (50 mM HEPES, I = 0.1 (NaNO 3 )) 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 (NaNO 3 )) was kinetically and thermodynamically promoted by m -xylyl-bis(Zn 2+ −cyclen). The 1 H 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(Zn 2+ −cyclen), whereas the control reaction showed 54% splitting. The kinetic and thermodynamic stability of the 1:1 m -xylyl-bis(Zn 2+ −cyclen)−d(T - pT - ) complex also accounts for the acceleration of photosplitting of T[ c,s ]T. The inhibitory effect of m -xylyl-bis(Zn 2+ −cyclen) on the photoreaction of poly(dT) was also revealed.