Preparation and structure of photodimers of methyl orotate and orotic acid

The crystal and molecular structure of the trans–syn photodimer of methyl orotate

“…The lengthening is due to the full substitution of these carbon atoms and to the repulsion resulting from the close distances between N(I') and C(7) and between N(1) and C(7'). This weak bond accounts for the ease with which the photodimer splits into monomers by the action of radiation and/or elevated temperature (Birnbaum et al, 1971). On the other hand, the relatively strong C(5)-C(5') bond must be the reason why monomerization does not take place even more readily.…”

“…Its results revealed that the structure of methyl orotate photodimer has the trans-syn configuration (2b). The photodimer of orotic acid was shown to have the same stereochemistry (2a), since upon methylation with diazomethane it yielded a product identical to that obtained from photodimerization of methyl orotate (Birnbaum, Dunston & Szabo, 1971). This being the first crystal-structure analysis of a trans-syn pyrimidine cyclobutane dimer, it may be of interest to recall that the corresponding isomer of a thymine dimer was isolated from ultraviolet-irradiated denatured DNA (Ben-Hur & Ben-Ishai, 1968).…”

Pyrimidine Photodimers

“…In fact the rings are not flat, and the bases are typically rotated by ~ 28° with respect to each other (Camerman and Birnbaum et al (1971); Sztumpf andShugar (1965) Miskew (1970);Sztumpf and Shugar (1965) Sasson et al (1970) Camerman, 1968). The calculations are based on the assumptions that the cyclobutane ring is a square and that the bases are planar and are oriented with their N(l)-C(4) axes parallel.…”

“…The instability of dimers to ionizing radiation creates problems during x-ray analysis of crystal structures, because the crystals are destroyed during the study (see Birnbaum et al, 1971;Camerman and Camerman, 1970;Leonard et al, 1969).…”