MECHANISMS OF PHOTOREDUCTIONS AND HYDROGENOLYSIS OF PYRIMIDINE NUCLEOSIDES AND THEIR PHOTODIMERS

Abstract— Photoreduction of uridine to dihydrouridine in the presence of sodium borohydride is shown to be specific for uridine in nucleic acids. This selective reaction provides a method for the production of dihydrouridine in tRNA and messenger RNA in order to investigate its physiological role. Though the photoreduction stops at the dihydrouridine stage, dihydrouridine undergoes selective hydrogenolysis (dark reaction) to ‐γ‐hydroxypropylurea; this reaction is used analytically to detect dihydrouridine in tRNA, and in sequence studies. Thymidine differs from uridine, as the photoreduction does not stop at the dihydrothymidine level, but there is a rapid hydrogenolysis and ring opening. The mechanism and stereochemistry of the hydrogenolysis of dihydrothymidine, photohydrate of thymidine, and photodimers of thymidine are described. The reaction seems capable of being developed into an analytical procedure to determine the photoproducts formed on u. v. irradiation of DN A. Se ha demostrado que la fotorreducción de la uridina a dihidrouridina en presencia de borohidruro de sodioes especifica para uridina en ácidos nucleicos. Esta reacción selectiva provee un mecanismo para la incorporación de dihidrouridina en t‐RNA y “messenger RNA. La fotorreducción se detiene a nivel de la dihidrouridina, que luego mediante una hidrogenólisis forma γ‐hidroxipropil urea, esta reacción se usa para detectar dihidrouridina en t‐RNA. La timidina difiere de la uridina en que la fotorreducción no se detiene a nivel de la dihidrotimidina, sino que sufre una rápida hidrogenólisis con apertura del anillo. Se describen los mecanismos y la estereoquímica de la hidrogenólisis de la dihidrotimidina, fotohidrato de timidina y fotodimero de timidina. La reacción es desarrollada en un procedimiento analítico para determinar los fotoproductos formados en DNA irradiado con luz u.v. Copyright © 1968, Wiley Blackwell. All rights reserved