Structure and properties of a trans–anti photodimer of 5-methylorotate

Abstract

5-Methylorotate is relatively radiation resistant in aqueous fluid medium, but readily photodimerizes in an ice matrix. Rapid formation of such a matrix made possible the preparative isolation of the photodimer in good yield. The potassium salt of the photodimer crystallizes as the hexahydrate, C12H10N4O8K2•6H2O. The crystals are triclinic with space group [Formula: see text]a = 8.139(3), b = 9.759(3), c = 7.398(3) Å, a = 100.28(7), β = 74.22(5), γ = 108.67(7)°, and V = 533.0 Å3, Z = 1. The structure was solved by Patterson and direct methods; refinement by block-diagonal least-squares converged at R = 0.041 for all 1671 observed reflections. The pyrimidine rings of the centrosymmetric photodimer are arranged in trans–anti configuration across the planar cyclobutane ring. The potassium ion is seven-coordinated. In aqueous medium the photodimer exhibits a pKa of 12.8 due to dissociation of the ring N(3) hydrogen. Irradiation in aqueous medium at 254 nm leads to quantitative regeneration of the monomer with a quantum yield of 0.8. Thermal regeneration of the monomer also occurs in neutral and acid aqueous media, but at a slower rate than for the trans–syn orotate photodimer. In contrast to the orotate photodimer, which undergoes alkali-catalyzed opening of the 3,4 bonds of the pyrimidine rings, the 5-methylorotate photodimer under these conditions dissociates to the parent monomer.