EFFICIENT INHIBITION OF PHOTO[2-5')THYMIDINE AND PROMOTION OF PHOTOSPLITTING OF THE CIS-SYN-CYCLOBUTANE THYMINE DIMER BY DIMERIC ZINC(II)-CYCLEN COMPLEXES CONTAINING M-XYLYL AND P-XYLYL SPACERS(2]CYCLOADDITIONOF THYMIDILYL(3')

Monomeric and dimeric zinc(II) complexes of cyclen (cyclen = 1,4,7,10- tetraazacyclododecane) inhibited the photo[2 + 2]cycloaddition of thym idilyl(3'-5')thymidine (d(TpT)) at neutral PH in aqueous solution for a novel mechanism. Comparison of the initial rates and the product yie lds of the photodimerization of d(TpT) (in Tris buffer at pH 7.6 with I = 0.10 (NaNO3)) at 3-5 degrees C by high-pressure mercury lamp indic ates that the dimeric zinc(II) complexes, p- and m-xylyl-bis(Zn2+-cycl en) (Zn2L2 and Zn2L3), are effective inhibitors (70-85% inhibition com pared 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(-)) moietie s with two Zn2+-cyclen moieties (apparent complexation constants, log K-app (K-app = [Zn2L2 (or Zn2L3)-d(T(-)pT(-))]/[Zn2Lfree2 (or Zn2Lfree 3)][d(TPT)(free)] (M-1)), of 6.4 +/- 0.1 at PH 7.6 (50 mM HEPES, I = 0 .1 (NaNO3)) and 25 degrees C, as determined by the isothermal calorime tric titration). A major product, cis-syn-cyclobutane thymine dimer (T [c,s]T), was also found to form complexes with Zn2+-cyclens. The appar ent 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+-c yclen unit (ZnL) of m-xylyl-bis(Zn2+-cyclen), log K-app (K-app = [ZnL1 (or ZnL)-(T[c,s]T)(-)]/[ZnLfree1 (or ZnLfree)][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 (NaNO3)) and 25 degrees 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 H-1 NMR measurement showed that 78% o f 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 the rmodynamic stability of the 1:1 m-xylyl-bis(Zn2+-cyclen)- d(T-pT-) com plex 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.