Crystal structure of a thymine–thymine adduct from irradiated thymine

Abstract

Thymine irradiated in a frozen aqueous solution with ultraviolet light produces not only dimers which are cyclobutane derivatives but also a racemic thymine-thymine adduct, The structure was solved by obtaining phases directly from the structure factor magnitudes with the use of the symbolic addition procedure. Ring I has the half-chair conformation while ring II is planar. The CH3 group on C(5) and ring II on C(6) are both axial to ring I while the OH group on C(5) and the H atom on C(6) are equatorial. The dihedral angle between the planes of rings I and II is ~ 96 °. An extensive system of hydrogen bonding between the three C=O groups, the OH, two of the NH groups and the H20 molecule binds the molecules together in the crystal.

Photoinduced Damage to Cellular DNA: Direct and Photosensitized Reactions†

“…This major discovery has provided a strong impetus to the development of research activities in the field of genotoxicity and DNA repair (23,24). Another early relevant finding dealt with the isolation of a second thymine dimeric photoproduct in UVC-irradiated thymidylyl-(3¢-5¢)-thymidine (TpT) (25) that has been identified as a pyrimidine (6-4) pyrimidone photoproduct (6-4PP) (26,27). This was complemented more recently by the identification of a third class of bipyrimidine photoproducts that was also isolated upon UVC irradiation of TpT and assigned as the related Dewar valence isomer (DEW) of †This paper is part of the Special Issue in Commemoration of the 70th birthday of Dr. David R. Bickers.…”

The Photochemistry of Uracil: A Reinvestigation

“…However, this evidence does not provide a definitive indication as to which product has the structure given by IIIa (in which the hydroxyl group and the 2′‐pyrimidone ring are cis with respect to one another in their attachment to the dihydropyrimidine ring) and which corresponds to IIIb (in which the OH group and the 2′‐pyrimidone ring are trans with respect to each other). Indeed, definitive assignment of the structures must await X‐ray diffraction studies, such as was done in the assignment of the conformation of the corresponding known thymine (6‐4) adduct ( V ) to a structural type analogous to IIIa (33). One could reason from analogy that P2 corresponds to IIIa , based on the fact that it is the predominant form produced in the photoreaction and that this is the only form that, thus far, has been reported to form when thymine is irradiated under similar conditions at dry ice temperature.…”

The (5‐4) and (6‐4) Adducts of 1‐Methylthymine and Their Dewar Valence Isomers

“…It should be kept in mind, however, that while supported by quantum mechanical calculations, further experimental work ( e.g. X‐ray diffraction studies, as in [35]) remains to be done to verify these assignments. In the meanwhile, caution should be exercised in assuming that (5‐4) adducts (and (6‐4) adducts, see [19]), found as initially observable photoproducts in pyrimidine nucleobase, nucleoside and related systems, will necessarily have a configuration in which the hydroxyl and 2′‐pyrimidone moieties are cis with respect to one another.…”