Excited State Precursors of Thymine Photodimers

The literature pertaining to mechanisms of thymine photodimerization is reviewed along with relevant spectroscopic data on thymine and DNA. New experiments on thymine photodimerization which employed sensitizers and quenchers as well as luminescence spectroscopy are described. The systems studied include thymine in ice and in fluid water and acetoni‐trile; isolated pairs of thymine molecules in frozen glasses at 77°K; TpT; and DNA. It is concluded that, depending on the particular reaction conditions, thymine and thymine derivatives undergo photodimerization by pathways which involve the triplet state or by pathways which involve only the excited singlet state. RESUMEN Se revee la literature publicada sobre los mecanismos de fotodimerización de la timina juntamente con los interesantes datos espectroscópicos de timina y ADN. Se describen nuevos experimentos de fotodimerización de la timina empleando sensitizadores y represores. Los sistemas estudiados incluyen: timina en hielo, y en agua fluida y acetonitrilo; pares aislados de moléculas de timina en vidrios rígidos a 77°K; poli dT; y ADN. Se ha demostrado que dependiente de las condiciones particulares de la reaccion la timina y sus derivados sufren dimerización por caminos que involucran el estado triplete o el estado singulete.