Photolysis of the Sulfonamide Bond of Metal Complexes of N‐Dansyl‐1,4,7,10‐Tetraazacyclododecane in Aqueous Solution: A Mechanistic Study and Application to the Photorepair of cis,syn‐Cyclobutane Thymine Photodimer

Abstract

Sulfonamide constitutes a ubiquitous functional group that is frequently used in organic chemistry, analytical chemistry, and medicinal chemistry. We report herein on the photolysis of a dansylamide moiety of 1-dansyl-1,4,7,10-tetraazzacyclododecane (N-dansylcyclen, L(2)) in the presence of a zinc(II) ion in aqueous solution. By potentiometric pH titrations, the complexation constant for the 1:1 complex of L(2) and Zn(2+), log K(s)(ZnL(2)), in aqueous solution at 25 degrees C with I = 0.1 (NaNO(3)) was determined to be 6.5+/-0.1. The structure of the ZnL(2) complex was confirmed by single-crystal X-ray diffraction analysis. During fluorescence titrations of L(2) with Zn(2+) (irradiation at 308 or 350 nm) in aqueous solution at pH 7.4 (10 mM HEPES with I = 0.1 (NaNO(3))) and 25 degrees C, considerable enhancement in fluorescence emission of the Zn(2+) complex of L(2) (ZnL(2)) was observed, while metal-free L(2) exhibited only a negligible emission change upon UV irradiation. It was revealed that this emission enhancement arose from the photoinduced cleavage of a sulfonylamide moiety in ZnL(2), yielding the Zn(2+)-cyclen complex and 5-dimethylaminonaphthalene-1-sulfinic acid, which has a greater quantum yield (Phi) for fluorescence emission than that of L(2) and ZnL(2). For comparison, the photolysis of N-(1-naphthalenesulfonyl)cyclen (L(3)) and its Zn(2+) complex (ZnL(3)) under the same conditions (irradiation at 313 nm) gave the corresponding sulfonate (1-naphthylsulfonate). We also describe the results of a photoreversion reaction of cis,syn-cyclobutane thymine photodimer (T[c,s]T) utilizing the photolysis of ZnL(2) and ZnL(3).