30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 71 



Elementary analysis of the photoproduct of 1 ,3-dimethyluracil, and its 

 molecular weight of 158.3 as against 140 for dimethyluracil, indicated up- 

 take of a water molecule during irradiation; this is further supported by 

 the infrared spectrum of the photoproduct, which exhibits an intense band 

 at 2.98 n corresponding to — OH frequencies and absent in dimethyluracil. 

 The disappearance of the characteristic absorption maximum, as for irra- 

 diated uridine-3'-phosphate (Fig. 10) and dihydrouracil (Fig. 5), plus the 

 absence of the characteristic reaction of unsaturated pyrimidine rings with 

 bromine water, suggested the 4,5 double bond as the point of attachment. 

 It was therefore proposed that the photoproduct is either 5- or 4-hydroxy- 

 1 ,3-dimethylhydrouracil (VII) or (VIII). Since the properties of the known 



O O 



compound (VII) did not correspond with those of the photoproduct, it 

 was concluded that the latter must be (VIII). By analogy the corresponding 

 reversible photoproduct for uracil should be 4-hydroxyhydrouracil. Both of 

 these deductions were subsequently confirmed by direct chemical syntheses 

 of (VIII) and of 4-hydroxyhydrouracil, 143 " 146 which exhibited the properties 

 of the photoproducts of dimethyluracil and uracil, respectively, and could 

 be made to revert to the latter by elimination of a water molecule. 



It follows that the photoproducts of uridine and uridylic acid are the 

 corresponding 4-hydroxyhydro derivatives. Analogous reversible photo- 

 products are formed from isopropylideneuridine and uridine-2' ^'-phos- 

 phate 39, 68 ' 69 as well as uridine diphosphate (UDP), uridine triphosphate 

 (UTP), and UDPG. 101 However, the position of esterification of the carbo- 

 hydrate moiety markedly affects the stability of the photoproduct, i.e., 

 the rate of reversion to the original compound. The maximum stability of 

 the photoproduct of Up is at pH 5.2 135 or roughly the same as for dimethyl- 

 uracil. 142 Quantum yields for photoproduct formation, based on the loss of 

 spectral absorption, are shown for a number of derivatives in Table II. 



Prolonged irradiation results in gradual destruction of the primary rever- 

 sible photoproducts, e.g., extensive irradiation of uridine leads to the 

 appearance of uracil as well as other products. 139 A number of colored, un- 

 identified degradation products are formed from extensively irradiated 



146 A. M. Moore, Can. J. Chem. 36, 281 (1958). 



