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The bromination of 1,3-dimethyluracil, uracil, and thymine and of the ~lucleosides and nucleotides of the latter two compounds has been followed potentiometrically and spectrophotometrically. The results indicate that all the compounds except uracil react initially with 1 mole of bromine to form 5-bromo-6-hydroxy-hydropyrimidine derivatives. If the resulting compound still has a hydrogen a t position 5 it may undergo dehydration on heating or with acid catalysis t o form the corresponding 5-bromopyrimidine, which is then capable of further reaction with bromine. Uracil itself appears first to undergo substitution to 5-bromouracil, and then both the uracil and 5-bromouracil are brominated together t o yield ultimately 5,5-dibromo-6-hydroxyhydrouracil. INTRODUCTIONThe study of the chemical changes induced by ultraviolet light in the uracil and cytosine components of nucleic acids has led to the recognition that the initial products of reaction in aqueous systems belong to the class of 6-hydroxy-hydropyrimidines (8,9,10,12). This finding has led, in turn, to new interest in the 5-bromo-6-hydroxyhydrouracils and related compounds both as intermediates in the synthesis of the irradiation products and as chemical analogues of such products.A recent report by Wang (11) has shed a good deal of light on the bromination reactions of uracil and 1,3-dimethyluracil. Our ow11 studies reported here have included these compounds but have also been extended to the nucleosides and nucleotides of uracil and thymine.The spectrophotometric measurements were made with a Beckman Model DK2 Recording Spectrophotometer. Except where otherwise noted the spectroscopic changes during bromination were determined by adding lo-"& bromine water a few microliters a t a time to 1 0 -W pyrimidine solutions. The small dilution involved has been neglected.For the potentioinetric titrations a platinum and a saturated calomel electrode were used in conjunction with a Leeds and Northrup Student Potentiometer in the earlier experiments, and with a Radiometer Titrator in the later ones. A small beaker equipped with a magnetic stirrer and held a t room temperature (22-24' C) was charged with a measured volume of lop2 liJ solution of the pyrimidine compound in acetate buffer and, with continuous rapid stirring, the potential was measured after successive additions of standard bromine water (approximately lo-? M ) . The bromine was dispensed from a small automatic burette with blackened reservoir. T o minimize the effect of a rather rapid initial drift in potential the measurements were all made 1 minute after each addition of bromine. Corresponding measurements using the same volume of buffer in place of the pyrimidine solution were used to determine a blank correction.The uracil, uridine, uridylic acid, and thymine were obtained from Nutritional Biochemicals Corp., the thymidine from California Biochemical Research Corp. The uracil was recrystallized twice from water and dried a t room temperature in vacuo. 1,3-Dimethyluracil was prepared from it by the m...

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The course of the photochemical reactions involved in the photolysis of 0-methylcytosine is strongly affected by the presence of various anions. Similar anionic effects were noted during the photolysis of cytosine but the products of this reaction appeared to be less suitable for further investigation. Ultraviolet irradiation of 0-methylcytosine in water leads to dcstruction of the characteristic absorption bands, while irradiation in phosphate leads to the production of a new coinpound with inarked absorption a t 300 mp, and irradiation in pyrophosphate, glycerophosphate, citrate, or cacodylate leads to the production of compounds with marked absorption a t 2 i 4 mp. The phosphate-dependent photolysis of 0-methylcytosine has been studied in some detail and the product has been isolated and purified. Apparently the H2P04-ion acts as a catalyst in a reaction which involves the loss of 1 mole of ammonia and the addition of 2 moles of water to 0-methylcytositle.

The isolation, analysis, and synthesis of the principal product of the ultraviolet (2537 Å) irradiation of aqueous 1,3-dimethyluracil has made it possible to establish that the irradiation results in hydration of the 5:6 double bond of the uracil ring to form 1,3-dimethyl-6-oxy-hydrouracil. This hydration is spontaneously reversible, the dehydration being catalyzed by either acid or base. Kinetic studies of the catalyzed regeneration reactions are reported