RING OPENING PHOTOREACTIONS OF 5‐BROMOURACIL AND 5‐BROMO‐2′‐DEOXYURIDINE WITH SELECTED ALKYLAMINES

— Several studies in the literature indicate that histones (lysine rich proteins found associated with DNA in eukaryotic chromatin), as well as poly-L-lysine, can be photocross-linked by ultraviolet (UV) light to DNA in which 5-bromo-2′-deoxyuridine has been substituted for thymidne. To gain some insight into the possible nature of this cross-linking, we have studied the photoreactions occurring in deoxygenated aqueous solutions containing 5-bromouracil (I) (BrUra) or 5-bromo-2′-deoxyuridine (III) (BrdUrd) and ethylamine, a lysine side chain analog. In the case of I this reaction produced the ring opened compound N-(N′-ethylcarbamoyl)-3-amino-2-bromoacrylamide (Ia). A small amount of N-(N′-ethylcarbamoyl)-3-ethylamino-2-bromoacrylamide (Ic) was also isolated. It was found that purified Ia, standing in the presence of ethylamine, was gradually converted to Ic in a dark reaction. The βandαanomers of N-(N′-ethylcarbamoyl)-3-(2′ deoxyribofuranos-1′-yl)amino-2-bromoacrylamide (IIIa and IIIb respectively) were isolated as products in the photoreaction of III with ethylamine; the α anomer was produced in a dark reaction from the β anomer. The identity of these anomers was established by comparison of their proton NMR spectra with those of the four corresponding α and β furanosyl and pyranosyl isomeric nucleosides of thymine, which are presented in the Appendix. A study was also made of the reaction of I with methylamine; a ring opened product analogous to In, viz. N-(N′-methylcarbamoyl)-3-amino-2-bromoacrylamide(IIa) was formed. A similar study with 5-bromo-1-methyluracil produced N-(N′-methylcarbamoyl)-3-methylamino-2-bromoacrylamide (IIc) as a product. Likewise, the reaction of 5-chlorouracil with ethylamine was studied and N-(N′- ethylcarbamoyl)-3-amino-2-chloroacrylamide (Ie), which is analogous in structure to Ia, was found to be produced. Structural identifications were made through use of UV spectroscopy, high resolution 1H-NMR spectroscopy, mass spectrometry and, in the case of Ia and IIa, 13C-NMR spectroscopy. In the BrUra and BrdUrd reaction systems, described above, dehalogenation reactions accounted for a major portion of the products. The yields of ring opened products, determined at pH 10, ranged from a high of 10.3% in the BrUra-ethylamine system to a low of 1.7% in the MeBrUra-methylamine system. The pH profile of the relative quantum yields for production of total opened ring product (IIIa+IIIb) in the photoreaction of III with ethylamine indicates that this quantity increases rapidly over the pH range between 7.5 and 9.5 and then decreases above 10.5. An additional reaction, a photoinduced halogen exchange process, was found to occur when Ia was irradiated in sodium chloride solution; this reaction converted Ia to Ie.