Structure of dl photodimer C of 1,3-dimethylthymine

Abstract

The crystal structure analysis of bis(dimethyl)thymine photodimer C, C14H20N4O4, confirms that it is a 5,6:5,6 syn stereoisomer. The cyclobutane ring is puckered, each atom lying 0.5 A out of the plane of the other three. The thymine nuclei are planar if C( 6) is omitted from the plane in each residue; the angle between these planes is 33°. The thymine residues are skewed with respect to each other, the angle of rotation being about 29°. The crystals are monoclinic, space group Cc, with cell dimensions a = 7.849, b = 15.142, and c = 13.265 Á, ß = 102.65°; Zfour dimers per cell. The structure was solved by obtaining a partial structure with phases determined by the symbolic addition procedure and the tangent formula, and the remainder with phases based on the partial structure. Photodimers of thymine were first isolated and identified by Beukers, Ijlstra, and Berends,3 as products of ultraviolet irradiation of frozen aqueous solutions of thymine and of aqueous solutions of DNA. These compounds are of considerable biological importance since there is good evidence that the formation of pyrimidine dimers is responsible for the majority of photobiological effects caused by ultraviolet irradiation of DNA; for example, it has been shown that this is the major factor in the inactivation of microorganisms by uv light.4Beukers and Berends6 suggested that the dimers were formed by linking of two thymines by a cyclobutane ring formed across their 5,6 double bonds, and Wulff and Fraenkel6 pointed out that cis linkage of the thymines to the cyclobutane ring could give rise to "our stereoisomeric dimers: 5,5:6,6 (or head-to-head) syn and anti (dimers A and B) and 5,6:5,6 (or head-to-tail) syn and anti (dimers C and D). Weinblum and Johns7 obtained four different thymine dimers, and from chemical and spectroscopic evidence tentatively assigned to these the four isomeric structures. Recently the structures of dimer A of dimethylthymine8 and of ura-cil9 and dimer D of thymine10 and of 1-methylthy-mine11 have been determined by X-ray diffraction.

A Proton Magnetic Resonance Study of the Ultraviolet Photoproduct of d(TpT) in Aqueous Solution

“…Thus some caution should be exercised in deducing configurational features in the absence of conformational information about the cyclobutane ring. In this regard it is 'Centrosymmetrically substituted cyclobutanes may be interesting to point to the absence of an observable ,J6, coupling constant in the "low-melting" DMT[ ]DMT isomer (10) which has been shown (40) to have the cis-anti geometry and to be puckered with the 6-hydrogens in a positions and the methyls in the less crowded e positions.…”

Crystal structure of a thymine–thymine adduct from irradiated thymine

“…Of the four possible cyclobutane-type dimers (Wulff & Fraenkel, 1961;Weinblum & Johns, 1966), detailed crystal structure analyses have been performed for the syn 5,5:6,6 type (Camerman & Camerman, 1968;Wei & Einstein, 1968;Adman, Gordon & Jensen, 1968), for the syn 5,6: 5,6 type (Camerman, Weinblum & Nyburg, 1968) and the anti 5,6:5,6 type (Camerman, Nyburg & Weinblum, 1967;Einstein, Hosszu, Longworth, Rahn & Wei, 1967). A different kind of thymine-thymine product has been isolated from an ultraviolet-irradiated frozen solution of thymine (Varghese & Wang, 1968).…”

A photodimer of 1,2,3,6-tetrahydro-5-methyl-2,6-dioxo-4-pyrimidinecarboxylic acid (5-methylorotic acid); crystal structure of the trihydrated barium salt

“…Of the four a priori possible structures no example of a trans-syn isomer is yet known. Two other cis-anti dimers are however known, those of 1,3-dimethylthymine (Camerman, Weinblum & Nyburg, 1969) and of uracil (Konnert & Karle, 1971). Derived bond lengths and angles are given in Fig.…”