The preparation and spectroscopic characterization of duplex decamers containing site-specific cis-syn and trans-syn thymine dimers are described. Three duplex decamers, d(CGTATTATGC).d(GCATAATACG), d(CGTAT[c,s]TATGC).d(GCATAATACG), and d(CGTAT[t,s]TATGC).d(GCATAATACG), were prepared by solid-phase phosphoramidite synthesis utilizing cis-syn and trans-syn cyclobutane thymine dimer building blocks (Taylor et al., 1987; Taylor & Brockie, 1988). NMR spectra (500 MHz 2D 1H and 202 MHz 1D 31P) were obtained in "100%" D2O at 10 degrees C, and 1D exchangeable 1H spectra were obtained in 10% D2O at 10 degrees C. 1H NMR assignments for H5, H6, H8, CH3, H1', H2', and H2" were made on the basis of standard sequential NOE assignment strategies and verified in part by DQF COSY data. Comparison of the chemical shift data suggests that the helix structure is perturbed more to the 3'-side of the cis-syn dimer and more to the 5'-side of the trans-syn dimer. Thermodynamic parameters for the helix in equilibrium coil equilibrium were obtained by two-state, all or none, analysis of the melting behavior of the duplexes. Analysis of the temperature dependence of the T5CH3 1H NMR signal gave delta H = 44 +/- 4 kcal and delta S = 132 +/- 13 eu for the trans-syn duplex. Analysis of the concentration and temperature dependence of UV spectra gave delta H = 64 +/- 6 kcal and delta S = 178 +/- 18 eu for the parent duplex and delta H = 66 +/- 7 kcal and delta S = 189 +/- 19 eu for cis-syn duplex. It was concluded that photodimerization of the dTpdT unit to give the cis-syn product causes little perturbation of the DNA whereas dimerization to give the trans-syn product causes much greater perturbation, possibly in the form of a kink or dislocation at the 5'-side of the dimer.

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The synthesis of a building block for the sequence specific introduction of the trans-syn thymine dimer into oligonucleotides via solid phase DNA synthesis technology is described. CGTAT[t,s]TATGC was synthesized in 48% overall yield by a partially automated procedure. The stepwise coupling yield for addition of the trans-syn thymine dimer building block was 58%. The dimer containing oligonucleotide was characterized by 500 MHz 1H COSY and NOESY spectroscopy and 202.5 MHz 31P NMR. The 1H chemical shifts for the trans-syn thymine dimer unit of the decamer were found to be quite similar to those found for the trans-syn thymine dimer of TpT. Upon photolysis at 254 nm, CGTAT[t,s]TATGC was converted to a major product which coeluted with authentic CGTATTATGC and a minor product which coeluted with authentic CGTAT[c,s]TATGC, further supporting the presence of an intact trans-syn thymine dimer unit.

339ChemInform Abstract The synthesis of a synthetic intermediate (VIII) for the sequence of specific incorporation of the cis-syn thymine dimer (I) into oligonucleotides via phosphoramidite-based solid-phase DNA synthesis technology is described. Photolysis of the known nucleotide (II) (mixtures of epimers at the P-atom) yields a separable mixture of the isomers (IIIa)-(IIId); (IIIc) is then converted to the required (VIII) as outlined in the scheme. For use in site directed mutagenesis studies, the tetramer (IXa) is synthesized by the solid-phase technique using (VIII) as a component. Tetrathymidylate (IXb) is prepared in a similar fashion for comparative purposes. Photolysis of (IXa) gives (IXb) as the major photoreversion product, thus establishing the integrity of the cis-syn thymine dimer unit (I). It is also established that (I) is stable to the conditions required to remove the standard amino protecting groups of adenosine, cytidine, and guanosine. This makes the building block (VIII) applicable to the synthesis of cis-syn thymine dimer containing oligonucleotides of heterogenic sequence.