Toward an artificial oxidative DNA photolyase.

The design, synthesis, structure, and binding affinity of two dioptic receptors for the selective molecular recognition of the cis,syn cyclobutane pyrimidine dimer are reported. The design is based on two 2,6-di(acetamino)pyridine recognition units that are covalently linked via triple bonds to an anthraquinone functional spacer unit. The convergent synthesis uses a modified Sonogashira reaction involving a zinc transmetalation as the key step. The crystal structure of one of the receptors reveals a supramolecular 1D polymer with strong interactions mediated by shape self-complementarity, pi-stacking, and hydrogen bonding between adjacent molecules. Hydrogen bonding between adjacent strands enforces a parallel orientation, which leads to a noncentrosymmetric crystal structure of the highly polar compound. The receptor has an association constant of K(a) = 1.0 x 10(3) M(-1) with the cis,syn pyrimidine dimer, whereas binding of the trans,syn isomer is approximately 1 order of magnitude weaker.

Unusually low barrier to carbamate C-N rotation.

The barrier for rotation about an N-alkylcarbamate C(carbonyl)-N bond is around 16 kcal/mol. In the case of an N-phenylcarbamate, the rotational barrier is lowered to 12.5 kcal/mol, but with N-(2-pyrimidyl)carbamates the barriers are so low (<9 kcal/mol) that the syn and anti rotamers cannot be observed as separate signals by 500 MHz NMR spectroscopy at 183 K. X-ray and computational data show that the N-(2-pyrimidyl) carbamates have C(carbonyl)-N bonds that are on average 0.03 A longer than for related N-phenylcarbamates. The computational results trace the origin of the effect to increased single bond character for the C(carbonyl)-N bond due to the increased electron-withdrawing ability of the pyrimidyl ring.